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Posted-By: auto-faq
Archive-name: antiques/radio+phono/faq/part7

See reader questions & answers on this topic! - Help others by sharing your knowledge Frequently Asked Questions (part 7)

Revision  Date			Notes

1.1	Oct. 24, 94	Revised and reordered as part 5.  
1.2	Dec. 12, 94	Minor edits, added new material on caps and tv
1.3     Jan. 8, 95      More stuff on caps.  
1.4     March 3, '95	Include Dan Schoo's writeup on paper caps.
2.0     Nov. 19, '95	Move from part 5 to part 7

Part 7 - Radio and electronic phono technical questions.
FAQ editor: Hank van Cleef.  Email,

This is a regular posting of frequently-asked questions (FAQ) about 
antique radios and phonographs.  It is intended to summarize some common
questions on old home entertainment audio equipment and provide answers
to these questions.  

Vacuum tube electronics runs on much higher voltages than transistor or
solid state devices.  These sets were generally not provided with
interlocks or power fuses.  In certain designs, the power line may be
connected directly to the chassis.  Many home-entertainment electronic
devices had 250 volts or higher as a standard operating voltages, and
voltages as high as 750 or 800 volts, may be present in some circuits.
Fault conditions may cause HIGH VOLTAGE to be present ANYWHERE, even
after the set it turned off and disconnected from the power line
(mains).  Use a grounding lead to assure that no voltage is present
before working on a set.  

There is sufficient power to overheat components to the point that they
will catch fire, and many components used in old electronics will
support combustion.  While addition of a fuse can reduce fire hazards,
it is not a sure and complete protection against overloads which may be
adequate to overheat components, but inadequate to blow the fuse.  In
addition, soldering irons operate at temperatures of 400-500C (approx
700-900F), and are hot enough to ignite many flammable materials such as
paper and cloth.  

Several of the CHEMICALS and PROCESSES discussed in the newsgroup, and
in this FAQ, present safety hazards of one type or another.  Fire
hazards are common, and many chemicals and processes require substantial
ventilation as well.  Read manufacturers' labels and follow all 
instructions for safe handling closely.  Above all, do not store
or use chemicals with food or food preparation items.  

Small children (and some not so small)---if you have some of these
around, take some precautions to make sure their inquisitiveness does
not get them into something that will hurt them, or damage anything.
Old electronic equipment is full of bright colors that will attract
small fingers.  The best thing to do with children is introduce them to
radio.  Don't just tell them "no, don't touch," etc.  It's amazing how
quickly, diligently, and thoroughly a child will learn mathematics
and physics, with the help of an old radio and someone who will take the
time to explain it to them.  Noxious chemicals and children don't

Do not attempt any process unless you know exactly what you are doing,
have evaluated the risks, and have taken safety precautions.  Many of 
the regular contributors to have been formally 
trained in chemistry and physics laboratory procedures, and use
chemicals and processes professionally. They may discuss techniques that
require substantial safety precautions without noting the hazards

If there is the slightest doubt in your mind about the safety of any
process or material, don't charge off and "just do it" because others
say "it works."  Ask questions.  There is no substitute for learning
under supervision.  Many community colleges and high schools offer
courses open to adults, including courses in laboratory sciences and
shop practices.  

Q.  I've got a very nice Philco tombstone radio that is only a
decoration because it doesn't play.  What can I do to get it to play.

A.  This section of the FAQ addresses getting them to play as nicely as
they look.  While not intended to be a comprehensive primer, this
section covers many questions that come up regularly.  The topics 
discussed in this section of the FAQ presume that you have a working 
knowledge of vacuum tube circuits.  

Q.  Why does a 35Z5 or 35W4 rectifier have a number 40 or 47 bulb
connected across part of the heater?  

A.  The heater serves as a voltage divider.  Resistance of cold
filaments is much lower than when they are hot, and connecting a bulb in
series will put almost the whole 110 VAC across it until the heaters
warm up.  The plate current flows through the bulb/heater to balance the
current once the tubes are warmed up.  Note that this also applies to
ballast tube setups---the ballast resistance is designed to increase as
the set warms up.  It's a way of putting a cheap light bulb in a cheap
radio.  (Historical note: This is an interview question I used to use
when interviewing engineering applicants in the fifties and sixties).  

Q.  I just found a (very old tube) radio in a (barn, attic, junk
sale, etc.).  It's complete.  Can I plug it in and see if it works?

A.  If you didn't hear the radio playing, it would be very wise to do
some resistance checking first.  
	a.  What is the condition of the line cord?  Replace it if it is
frayed or the rubber is petrified.
	b.  Condition of filter capacitors.  Wet electrolytics, which
were used in the 1930's, should be replaced without question before
applying any power.  These are identifiable by the metal cans with vent
holes on them.  Dry electrolytics (which aren't really dry inside) can
also lose their film and be low resistance.  If DC resistance between
the B+ line and circuit ground (this may not be chassis ground) is not
500K or more, find out why.  Make sure the speaker is included in this
check if it has a field coil or has the output transformer mounted on it.
With electrolytics and any voltage divider resistors out of the B+
circuit, DC resistance should be several meghohms.  
	c.  If it's an AC-DC set, check to see if one side of the line is 
wired to the chassis.  Many of them were.  If so, keep the set away from 
any metal objects to avoid shock hazard.  Some of the early AC-DC sets 
would hum like crazy if they were plugged in with the chassis "hot."  
	d.  If it's an AC set, consider installing a fuse in the line
circuit.  2 amps 250 volts for sets with 80/5Y3, 4 amps 250 volts for
sets with 5Z3/5U4.  
	e.  Do a cosmetic inspection.  You'll want to vacuum off any old
dust, dirt, cobwebs, etc. first.  Look for things like charred
resistors, melted wax from capacitors, coils, and transformers, and any
indications that the radio go put in the (barn, attic, etc.) because
something was wrong with it.  
	f.  Take a look at the bias circuit for the power output stage.
See below for discussion of typical bias circuits.  If there is an
electrolytic in the circuit, make sure it isn't "low ohms."  If your
output stage is 6L6's, or if it is filament tubes like 2A3, 6A3, 45, or
47, take a very good look at things.  
	g.  Condition of old wiring is important.  Don't fool around
with petrified insulation that is breaking off the wires.  

A few hours spent doing a good visual inspection and some ohmmeter
checks can pay off handsomely.  If you've got to replace a charred
resistor, find out what burned it out and fix that too, before applying
power to the set.  

Remember that 99% of vacuum tube failures are due to open heaters or
filaments.  The other 1% are due to gas or interelectrode shorts.  This
leaves the item that tube testers have a big BAD-?-GOOD meter to
measure, emission, down in the mud as a tube fault that makes a radio
play poorly.  Except for rectifier tubes that have been "sucked dry" by
a gassy output tube or shorted filter cap, most of the tubes I have
diagnosed as causing problems because of low emission would not exhibit
that low emission in a tube tester.  Example: a 6SQ7 diode that quit
conducting after 15-20 minutes of playing.   Diagnosis was confirmed by
soldering a 1N34 diode across the terminals.   

If the getter material (you can see it on glass tubes) is white instead of
silver, the tube is probably gassy----most common on power output and
rectifier tubes.  A few sets used gas-filled rectifiers.  The 0Z4 is
most common in auto radios, but you may find and old set with an 82 or 83
mercury vapor rectifier.  

Also remember that with tube equipment, DANGER, HIGH VOLTAGE. applies.
In home entertainment transformer sets, we are talking about as much as
500 volts, and most smaller transformer sets used somewhere between 250
and 350 volts as the main B+ voltage.  The transformerless sets
generally provide 135 volts, and have the mains power (to use the British
term) hooked directly to various circuits and often the chassis as

Q.  The chassis of my radio is covered with a thick layer of dust, fine
dirt, and underneath is a film of brown crud.  How can I clean this
thing up without damaging it?

A.  This particular topic gets a lot of discussion and advice, some of
it very bad.   Your radio has some irreplaceable components, and if you
use the wrong methods, you can make a junker out of a restorable set in
a hurry.  There are some things to keep in mind:
	a.  The chassis is probably cadmium-plated steel.  Some radios
were made with nickel-plated steel (looks green when corroded),
copper-plated steel, or chromium-plated steel.  A few chassis were made
of aluminum.  If it is a dull silver color, check with a magnet.   An
aluminum chassis is non-magnetic, all of the steel chassis are magnetic.
	b.  The dial face may be a water-soluble paint or a decal.  
	c.  Colored knob markings (lines and dots, as well as letters
filled with color) may be water-soluble. 
	d.  Any silk-screened surface markings may come right off.
These include tube layout information on the chassis, inspector's marks,
and other printing.  
	e.  The tuning mechanism may be stiff because of petrified
lubricant in various shafts and rotating elements.
	f.  Coils, IF transformers, and tuning condensers may be
difficult or impossible to replace if you damage one.  
	g.  If the radio is complete, tubes in place, the crud and dirt
is on top of everything, not in the electronics.  You want to get it off
the radio, not melt it down so that it flows into the working parts.   

You can remove the tubes.  Make sure that the tubes are clearly marked
as to tube type, and make sure you have an accurate diagram so that you
can replace the tubes in the same sockets you removed them from.  Get a
pencil and piece of paper and make notes about things you move,
disconnect, or take apart, so that you can get everything back together
the way it was originally.  Begin by vacuum cleaning the set, and use a 
soft brush to loosen dirt while keeping the vacuum nozzle near the brush 
so that it will pick up loosened dirt.  If you find mouse droppings, be 
prepared to examine the set closely for damage from mouse pee.  Gently 
brush off the tuning condenser, being careful not to bend the plates.  
Once the surface dirt is off, you can begin to consider how best to 
remove the crud, and how far to go with the cleanup.  

There are two things that are very poor to use around electronics: steel
wool and soap-type detergents.  Steel wool will shed little particles
and raise havoc.  Soaps and liquid detergents leave residues that can be
hard to remove.   Liquid detergents also do a fabulous job of softening
and removing silk screen inks, water soluble dial markings, and tube
markings, even those that may be safely soaked in water for a few

Start on the chassis crud by using a damp rag moistened with plain
water.  Don't slosh water onto things.  Most tap water is safe to use
around electronics, and is an excellent solvent.  I note that I have
refurbished electronics that have been immersed for days in fresh water
after they have been allowed to dry out, and found very little damage,
mostly to capacitors.  If the crud comes off with water alone, continue
with the damp cloth treatment.  It may be slow, but it will leave a
clean surface with little residue.  Finish the job with moistened Q-tips
to get into various nooks and crannies.  Be careful that you don't
remove marking inks and paints.  

A stronger alkiline solvent is clear household ammonia.  This also
evaporates without leaving a residue.  If water is not melting the crud,
try a little ammonia on a Q-tip.  Use the ammonia straight, and if it
gets results, use it on a damp rag to moisten the chassis.  Generally,
once ammonia-sensitive crud has been melted, it will come right off
using a rag dampened with water.  Be careful not to get ammonia on a
shellac wood finish---it will cut the shellac and leave marks.  

If this doesn't get results, try a mild acid---clear cider vinegar.  Use
the same methods as with ammonia, finishing with a rag dampened with

By this time, you should have most of the removable crud off the
chassis.  Some other solvents to try---only in small areas with Q-tips:

Isopropyl alcohol.  This dissolves a great many things, including flux
rosin, some marking inks, etc.  

Trichloroethane (GC Electronics "Chloro-Kleen").  Also dissolves many
things.  Don't use on plastics until you have checked to make sure it is
safe.  Chloro-Kleen works very well on phenolic and ceramic-mounted
switches such as bandswitches and pushbutton switches.  

Lacquer thinner.  This is a "court of the last resort."  It is a
powerful solvent that will damage many plastics, remove a lot of marking
inks in a jiffy, and generally raise merry hell if you get it in the
wrong place.    Use on metal parts only.  

Also pay attention to the various warnings about flammability and use
only in well-ventilated areas.  

Corrosion on cadmium-plated chassis generally does not respond very well
to anything.  You can use Naval Jelly to improve the situation,
particularly if there is visible rust.  Light fingerprints often will
respond to automobile polish (Dupont No. 7 is good).  This treatment
(followed by an application of Simoniz paste wax) will make many
lightly-scratched plastics look like new.  

The best solvent for use with petrified lubricants in tuning mechanisms
is diesel fuel.  If there are separately-mounted shafts or gear
mechanisms, you can often take them off----just make sure you can get
them back on again, and positioned properly.  Watch for spring-loaded
double gears in gear mechanisms that need to be preloaded when you
assemble them.  Shafts should be relubricated with a light grease like
white Lubriplate---use only enough to leave a film on the parts needing
lubrication, and wipe off the rest.  Gear trains generally work well
with a little 3-in-1 oil on axle pivots and a film of lubriplate on the
gear teeth.  A stiff volume or tone control will generally respond to a
drop of 3-in-1 at the end of the bushing---use only a drop, and wipe it
off after about 5 minutes.  

Tube washing gets a lot of attention.  Keep in mind that washing most
tubes won't make them work any better.  Before you start, make sure that
the tubes are clearly marked as to what they are.  While there is no
mistaking a 6A7, a T-9 beam power pentode with no markings may be a 
a 6W6, a 25L6, a 35L6, a 60L6, or a 6V6.  A 50L6 plugged into a 25L6 or
35L6 socket can produce interesting symptoms that can be very hard to
diagnose.  Contrary to popular opinion, tube markings on glass will come
off, some more easily than others.  During the 1950's and 60's, tubes
were specifically marked with easily removable markings in an attempt to
thwart a grey market in used tubes being washed, reboxed, and sold as
new.  Generally, just holding the tube under flowing water will rinse
off most of the dirt--- a little help from rubbing the surface with a
thumb where it is not marked generally gets fine results.  Use a china
marker to marked the type on any tube that isn't clearly identified, and
let them dry thoroughly before reinstalling.  Tubes that are loose in
their base, or have a loose top cap, respond to squirting a little
superglue into the gap.  Make sure, in the case of a loose base, that
the leads aren't twisted (and shorted).  

Q.  What about AF power amplifier bias circuits?  

A. You can do a little inspecting to see what your radio uses.
	a.  By far, the most common circuit is to use a cathode resistor
with an electrolytic capacitor for AC bypass.  This is what you will
find in all of the transformerless sets.  AC bypass is less critical in
push-pull output stages, although most of them operate class AB (i.e,
both tubes biassed near cutoff).  If the capacitor is shorted, the output
tubes will over-dissipate and their plates will glow red in a few
minutes.  If the capacitor is open, audio output will be low and
	b.  Back bias.  I was somewhat surprised in checking Terman
"Radio and Electronics Engineering" 4th edition (1955) not to find this
circuit.  It uses a power resistor in the B- return to develop a bias
voltage, typically 10-30 volts, and may be used in conjunction with the
cathode resistor self-bias circuit.  The center tap of the power
transformer will be connected to one end of the power resistor and B-
circuits will be connected to the other end.  On sets using filament
power tubes, the filament supply may be connected here, and the power
tube grids returned to the power transformer center tap.  Most of the
bias voltage is developed by output tube plate current.  If there is a
leaky electrolytic here, it will generally overstress this resistor and
burn it out.  
	c.  Separate "C" bias supply.  In this case, the set will have
second rectifier tube, filter, etc.  These are not common in home
entertainment equipment, much more likely to be found in theater and
public address amplifiers.  

Q.  OK, I've checked that the tube heaters are continuous, that the 
filters are OK, and generally walked through and done the visual and 
ohmmeter inspection.  I want to plug it in.  What do I look for?

A. This is the moment of truth, even for an old grey-hairs.  Fortunately,
tubes will take abuse that transistors won't tolerate.  But you want to
have your eyes and ears wide open, and be prepared to shut the thing
back off instantly.  Some people like bringing them up on a Variac,
which is an expensive piece of equipment unless you are in the
restoration business.  So I'll assume you are going to plug the thing
into the 100 volt line, turn it on, and see what happens.  Make sure you
have some sort of antenna connected on sets without a built-in loop.

a.  On AC-DC sets, turn it on.  The tubes should light up, and in 10-15
seconds (when the rectifier and power tube heaters warm up) you should
hear 60 cycle hum in the loudspeaker.  Indeed, hum is a built-in feature
of these sets.  If it is overwhelming, you've got a bad filter cap.
Check for smoke signals and signs of overheating.  If you can tune in a
station, you are probably in business.  On 35Z5/35W4-type radios, if the
pilot lamp burns out after the set warms up, you've got a short in B+
somewhere---probably a shorted filter cap.  Turn the set off and find
the problem---if you've got a short, the rectifier heater will take the
load and burn out after a while.  

b.  On transformer sets, I like to connected a 600 volt DC meter across
B+, preferably in the supply to the IF screen grid or plate.  If the
rectifier is a filament type (80, 5Y3, 5U4, etc.) you'll see full B+ a
couple or three seconds after turning the set on, and it should drop to
about 100 volts on the IF screen when the cathode tubes warm up (around
10 seconds).  Check for smoke signals, burning, and that all the heaters
glow.  A low level of 120 cycle hum is to be expected, though a really
fancy set will give almost no hum at all.  Once again, if you can tune in
a station, you are probably in business.  

Watch in particular for a violent purple glow in tubes, particularly the
power output and rectifier, plates beginning to glow red, and other
signs that there is a short circuit.  If the radio doesn't play, keep a
close watch on things, although if you have good B+, no gassy tubes, and
no red plates, and things are OK after five or ten minutes, you are
probably safe in continuing on to do trouble-shooting.  A few sets use
tubes with mercury vapor in them, which normally glow purple between
the elements.  Typical are the 83 (not 83-V) and 0Z4 rectifiers, and 
the gas-discharge VR tubes (0A2,0C3, etc.).  

Trouble-shooting.  If all the tubes light up, you've got B+, and no
smoke signals, you can begin your walk through the radio.  If the radio
is completely dead---no stations, no static---try rocking the bandswitch
if the radio has one.  Also, the volume control, any tone controls, etc.
I've found that on ancient sets, it's a good idea to walk right through
and do voltage checks everywhere, no matter how well the radio seems to
play.  If you have a schematic with voltages marked on it, so much the
better, although some of the voltages given by manufacturers can
disagree rather markedly from actuals that can be figured by
reverse-engineering the design.  

a.  Power output stage:  Check screen and plate voltages.  These should
be close to B+ at the rectifier.  Check for positive bias voltage at the
cathode on self-bias circuits or negative voltage at the grids if
separate bias.  

b.  Audio amplifier.  Usually a triode.  If the 6SQ7 diode-amplifier
type, the only thing to check is plate voltage, which should show a drop
across the plate resistor.  On resistance-coupled output circuits, make
sure the coupling cap is not leaking current to the output tube grid
circuit, which will pull up the grid voltage and make the output tube
plate(s) glow red.  Probing the AF amplifier grid generally shouldn't
show any voltage, but should make plenty of noise in the speaker.  

c.  IF amplifier.  Check for screen voltage.  If you don't have any,
you've got a shorted bypass cap and a dead radio.  Plate voltage should
be near the supply voltage (generally fed by a blue wire to the 2nd IF
transformer).  Cathode should show some bias being developed (i.e.,
plate current through the tube).   The grid will generally show the AVC
voltage, though your meter will shunt a lot of it, unless it is a
high-impedance type, such as a VTVM.  

d.  Mixer.  If the pentagrid type, tetrode, or pentode, check screen
voltage.   Check for proper bias voltage on the cathode.    

On voltage checks: if you have a schematic and voltages, these can be a
general guide to the voltages you should see on the tube elements.  If
you don't have voltage measurement data, most of the tube manuals give
standard values for circuit DC levels as  "typical operation."  Most
designers used these "typical operation" values in circuit design.  

Q.  I have a nice old fifteen tube radio.  It's got problems with
insulation falling of the wiring, and a couple of repairs that were
badly done.  There is a lot of dirt in the coil boxes and bandswitch,
and I can't get at them to clean them up.  All of the paper capacitors I
checked were leaking electrically, and several resistors have drifted
way out of tolerance.  What can I do with this set to get it working 

A.  There is a point where the best thing to do with an old radio is to
take the thing completely apart, clean up everything, and build it up as
a new radio.  While this may seem like a lot of work, it actually is
easier than a major piecemeal restoration.  For one thing, taking major
components off the chassis will open up areas and make the rest  of the
set easier to work on.  You will need an accurate schematic for the set,
and you will need to make copious notes as you take it apart.  Note how
the bandswitch and other tap switches are wired, and identify the
connections on the schematic.  Make notes on what components are where,
the hardware used to mount them.  You will want a bunch of containers,
typically one for each type, to keep parts in.  Mark the
containers---don't rely on memory for anything.  Your notes are going to
be "kit building instructions" for putting it back together.  Clean off
all the old solder to make it easy  to install  components.  

The results can be little short of astounding.  You can start with a dog
that has parasitics, won't align properly, and has been butchered by
hackers who fixed everything except what was wrong, and end up with a
brand new radio with superb performance.  Work?  Yes.  But take it one
tube circuit at a time, one subassembly at a time, etc., and you'll be
surprised and pleased with the results.  

Q. What about electrolytic capacitors?  Can they be re-formed?

A.  Roy Morgan sent me a drill for re-forming old caps.  Keep in mind
that some caps won't come back to life.  The "wets" from the early
thirties generally have internal problems and corrosion, and a lot of
the axials have dried out internally.  Note that a "dry" electrolytic
has a moist gauze with electrolyte inside---what makes them "dry" is
that the electrolyte doesn't slosh around.  "Dry" can types, like the
Mallory FP series, often will come back to life with a re-forming.  I
used the procedure that follows on a 20/20/15 mike 450V Mallory FP with
a date code of June 1945 that probably last saw power in the 1960's,
and the cap came back to usable condition.  Here's Roy's procedure:

To Re-form electrolytic capacitors:

With the "patient" set off, set the external supply at the rated voltage 
of the cap(s), and feed the old set at the input to it's B+ filter through 
a 100K, 2W resistor.  The old caps will slowly come up to voltage as 
their elecrolytic layer re-forms after long storage.  You may want to 
unhook bleeders or screen voltage dividers if present in order to get no 
dc load other than the caps.  Once re-formed up to nearly the cap rating, 
increase the external supply voltage to the point where increased voltage 
only increases the current drawn (the electrolytics begin to "leak".)  You 
can vary the series resistor depending on the voltage of the cap you're 
trying to reform.

If the final cap(s) voltage is high enough, it doesn't need to be 
replaced.  If it's too low, put new one(s) in (leave any original cans in 
place for appearance, and substitute new axial lead ones under the 

Some caps take only a few minutes to re-form.  Some take a day or so!  Be 
patient.  Your Adjusta-Volt or Variac can be well-used for this if your 
external supply is solid state, or has a separate hv supply transformer.  
I have one good for 900 volts no-load having 5R4's and separate filament 
transformers.  This lets me re-form 500 volt electrolytics if I need to.

With a 500 volt supply, and a number of 100k or 200k resistors, you can 
re-form a number of caps all at once.  Measure the voltage on the caps as 
time goes on with a high-input-resistance meter (VTVM or solid state 
DVM).  Allowing an electrolytic to idle with a small leakage current of 1 
to 5 ma won't hurt it, so if the thing re-forms to it's limit during the 
night after you've left it on the re-former, no harm is done.

Most electrolytics in good health will leak at a voltage from 125 to 200 
percent of the continuous rating.  If the leakage voltage is only a little 
above the needed circuit voltage, or is below about 110 percent of the 
cap's rating, then you can excpect it to not live too long.  New axial 
lead caps are fairly cheap, and are good peace of mind in my opinion.

(I didn't have a separate power supply.  What I did was disconnect B+
from the caps and feed the rectifier output through 100K resistors to
each section.  With a 670VCT plate winding, and only a few ma. current
draw, an 80 will come very close to delivering 500 volts peak (1.41*370
is a little over 500.  Once the caps settled down, I put 20K's in the
circuit to pull them up even further---they had about 480 volts on them
at the end).  

Q.  What about testing other caps?
A.  This is also from Roy Morgan.

Test interstage coupling caps (e.g. from an audio driver tube to the grid 
of the output amp tube) by measuring the dc voltage at the grid  (across 
the grid resistor if it's not going to ground).  Use a high-impedance 
voltmeter like a VTVM or DMM.  If it's above zero, you need a new cap!  
The vast majority of paper caps from the 30's through the 60's are at 
least moderately leaky now.  Your tubes will thank you with long life for 
replacing these caps.  Ceramic caps have indefinite life expectancy, as do 
good quality modern film caps.

You can do this kind of testing while you are re-forming the filter caps 
in-circuit.  The tubes are off, and will not be harmed by excessive plate 
current while you find all those leaky paper caps.  The voltages across 
them will be higher than  normal running conditions, because the driving 
stage is not drawing any plate current.


With B+ applied and the tube pulled or set off, the voltage at the screen, 
again measured with a high-impedance voltmeter, should be the full B+ or 
value at the other end of the dropping resistor.  If not, the cap is 


Set your high-impedance voltmeter to a high-enough range and clip one end 
of the cap to the DC probe and connect (carefully) the other end to a B+ 
supply corresponding to the rating of the cap.  The meter will jump up 
briefly and then settle down toward zero.  Analog meters (VTVM's) are good 
for this because you can watch the movement of the needle.  Once the 
reading settles, any indication much above zero indicates leakage.  A 
quick ohms-law estimate with the input resistance of you meter will give 
you a value for the leakage.  DVM's are often 10 megohms.

Q.  I looked under my radio and there are a lot of parts with several
color markings on them but no printing.  What does this mean?

A.  There has been a color code for marking part values since the early
1930s.  The numbers are always the same:
Black	= 0
Brown	= 1
Red	= 2
Orange	= 3
Yellow	= 4
Green	= 5
Blue	= 6
Violet	= 7
Grey	= 8
White	= 9
There are several mnemonic sentences for remembering this series, some
lewd, some not.  "Bad Boys Ruin Our Young Girls Behind Victory Garden
Walls" is one of the politer versions.  
Resistor markings:  early-mid 30's was "body-end-dot" where the
resistor body was the first significant digit, one end was the second
digit, and a dot in the center of the body was the multiplier.  After
about 1935, resistors were marked with color bands; the first
significant digit is the band nearest one end.  Silver is used to
indicate 10% tolerance; Gold, 5%.  These are either on the other end of
a body-end-dot resistor or a fourth band on band-marked resistors.  The
scheme is simple to decipher: a resistor marked yellow-violet-green is
47 mulplied by 10 to the 5th (100,000), or 4.7 megohms.  

Mica and molded paper capacitors, in little rectangular plastic
packages, used the same color values, but there were about twenty
different schemes for locating the dots, and most of them use six dots,
with three or four giving the value.  These can be a nightmare to
decipher.  Generally, either the first or second dot in the top row is
the first significant figure, and either the rightmost dot in the top
row or the rightmost dot in the bottom row is the multiplier.  The size
of the capacitor (bigger values are physically bigger) and the circuit
application will give a clue as to the approximate value.  The left
bottom dot generally gives the voltage rating in 100s of volts (red is
200; green, 500), and the center bottom dot generally gives the
temperature characteristic.  The left top dot may be a significant
figure or may be a type indicator.  Some types have six dot positions,
but one or more with no marking, which may mean "not used" or "brown." 

Knowing the series of standard values for resistors and capacitors can
help in deciphering color codes.  These were changed during WW II.
Prewar 20% resistors (no tolerance color) were 1000, 1500, 2000, 2500
ohms, etc.  Postwar were 1200, 1800, 2700, 3300, 3900, etc., replacing
the old 0/5 scheme with approximately 20% jumps in value.  Mica
capacitors in old radios were generally 50, 100, 150, 200
"micromicrofarads" (picofarads---term did not come into use until the
early 1960's in the US).  Molded paper capacitors are generally in the
1000 pf. (0.001 microfarad) to 10000 pf. range, with 0/5 as second
figures.  Postwar production switched to 12, 18, 22, 27, 33, 39 as
significant figures, although the old scheme was still commonly used.  

Wattage ratings of resistors in different package sizes have been
revised several times, always increasing the rating for a given package
size.  When replacing resistors, modern 1-watt metal film resistors
generally are about the right physical dimensions for older 1/4, 1/3,
and 1/2 watt resistors.  Values should be derated 50%; that is a 1 watt
resistor should calculate to a dissipation of 1/2 watt or less in a

An overstressed resistor will overheat, and discolor its color bands,
sometimes very deceptively.  In particular, the red and orange
multipliers may look brown, and it may require inspection with a
magnifier to see that the center of the resistor is charred.  Any
resistor that looks as though it has been heated to the point of
charring or discoloring its markings should be replaced .  Also, some
compositions used for composition resistors were unstable over time,
and a resistor that looks perfectly good and is in a circuit location
where overstress is nearly impossible may be wildly out of tolerance.  
Use an ohmmeter to check, but check your ohmmeter against some
known-good new resistors of similar value.  

Typical resistor failures:
240 ohm 1 watt cathode resistor for a 7C5---looks like it might have
gotten warm, colors still normal, actually is 150 ohms.  Inspection
with a magnifying glass after removal found more signs of overheating
on a side that was not visible with the resistor soldered in place.  
Failing "low" like this is not common, and generally comes from using a
resistor with too low a wattage rating for the application.  The
coupling capacitor to the 7C5 grid was leaking, pulling the grid up
enough to over-dissipate the resistor.  Oddly enough, the tube survived.

240 ohm 1/2 watt screen resistor for a 6K7.  This was found on visual
inspection, connected to a replacement bypass capacitor in a
suspicious-looking repair.  Ohmmeter showed about 10K ohms, and the
circuit location should have a 2K ohm resistor.  Closer
inspection after removal disclosed a charred center which had turned
the red multiplier brown.  This resistor was originally 2400 ohms, used
to replace a 2K.  

33K 2 watt screen resistor for a 6BA6.  The screen bypass capacitor was
shorted, "killing" the set.  Ohmmeter showed about 250K.  This resistor
showed no signs of distress.  A shorted bypass capacitor often takes
out the resistor in the circuit, but a further check in this radio
showed about 2/3rds of the resistors over 20% high, some as much as
twice the value, even though they were not discolored.  It got 100%
resistor replacement.  

A resistor that is physically broken generally has been subjected to a
short circuit condition that overheated it until it exploded.  Look for
a hard short in the circuit.  

Q. My old radio has a lot of tubes covered with wax, and some of the wax
has melted out and is on the bottom of the cabinet.  What should I do
about this.
	A.  These are inexpensive wax-impregnated paper-dielectric
capacitors.  They were notorious, even when fairly new, for developing
opens, shorts, intermittents, high dissipation, and tend to be rather
fragile as well, particularly when soldering around them.  Melted-out
wax is common, and may be only the result of heat developed under a
chassis in normal operation.  From reliability and other engineering
points of view, replacing all of them with newer capacitors of other
types is part of a refurbishment/overhaul.  Some collectors feel that
40-60 year old capacitors are "survivors," that wholesale replacement is
unwarranted.  Also, there are two schools of thought on replacing
components with others that are very dissimilar-looking, even in areas
that are not normally visible when a radio is installed in its cabinet.
A few restorers go so far as to melt the wax out of old capacitors,
remove the foil-paper "innards," install a new capacitor, and refill the
body with wax.  Other restorers feel just as strongly that consistent
appearance is more important, and that 100% replacement with no attempt
to disguise the appearance of new components is to be preferred.  Alfred
Ghirardi, in "Radio Physics Course," has a lengthy discussion of
failure modes of these capacitors, and states an expected service life
of 10,000 operating hours.  
	Whether to do a wholesale replacement or not is a decision
you'll have to make yourself, and whether to use modern radial-lead
components or to try to find lookalike replacements or disguise the new
ones, also has no uniform consensus.  Your radio may not give you much
choice about wholesale replacement.  If you find more than one or two
bad ones, or if the set has mysterious ills, parasitics, or poor
performance, or is intermittent, 100% replacement is indicated.  If the
item you are repairing is "blue collar" or "high tech," 100% replacement
with obviously new good-quality components seems to be preferable.  By
"blue collar," I refer to test equipment and items such as Hammond
organs and studio equipment that worked for a living.  By "high tech," I
mean good communications receivers and genuine high-fidelity equipment.
Many of these items used higher quality components originally.  
	One item that has complete consensus is quality of workmanship.
You will want to learn how to remove component leads completely, clean
up old terminals, and make neat new solder joints.  

Q.  I found an RCA model 630 ten inch TV set at a flea market.  The
power cord is shot, and when I pulled the chassis out, I found the wires
to the switch appeared to have had the insulation burned off.  I found
that the 5U4 plates were melted together.  I put in a new 5U4 and
plugged the set in, but it doesn't do anything---no picture, no sound.
What should I do now?

A.  First of all, a TV set draws substantially more power than a
radio.  Do yourself a favor and install a fuse in the primary power
wiring to the switch.  Use a slow-blow fuse rated at about 150-200% of
the set's power consumption.  For a set drawing 250 watts, a 4 amp
should give reasonable protection.  
	On a 630, there is a black box mounted on the left rear of the
set, with some power resistors inside.  Open the box and check the
resistors.  These are back-bias resistors, in the B- circuit.  If they
are open, check all the filter caps.  Replace the resistors, if
	Bringing up an old TV takes some care, and the order in which
you check things out is important.  As with all old electronics, assume
that it has several things wrong with it.  Check that the CRT heater is
continuous (ohmmeter)---you should be able to see it glow when you turn
the set on.  The first thing to fix is the power supply.  Once you have 
good B+, and all the tubes are lit up, do you have a raster?  If not, 
check the horizontal oscillator and amplifier.  Note that the horizontal 
amplifier has very high voltages in it, and that some faults may cause 
these high voltages to appear where they shouldn't be.  Don't go probing 
around in the horizontal circuit with the set turned on.  Horizontal
amplifiers on magnetic deflection sets ran with voltage and current
levels appropriate for a transmitter, and several postwar sets continued
to use the 807 beam tetrode as a horizontal amplifier tube, rather than
one of the purpose-built tubes.  Shut the set off, connect your probes, 
then turn the set on, take your readings, then shut the set back off again.
Don't touch anything in the set without first assuring that it is shut
off, then touch an insulated probe connected through a 1K resistor to
ground to all of the terminals in the circuit to assure that there isn't
a high voltage charge somewhere.  If the horizontal circuits are OK, the
1B3 high voltage rectifier filament will glow.  Make sure that the high
voltage cable isn't shorted somewhere, and that there isn't a lot of
dust or crud to bleed off the high voltage---problems here are usually
pretty obvious in the dark, where you can see corona discharges, arcing,
and other leakage problems.  Unless you have equipment of measuring
10KV, you can't measure the high voltage directly, but if the 1B3
filament is lighting, and the flyback plate winding to the 1B3 is not
open, you probably have high voltage.  If you have high voltage, and the
tube does not show any light (this may be a spot or a line, rather than
a raster), check the CRT grid-cathode bias voltage---once again, keeping
hands completely away from the CRT socket unless the set is turned off
and you've grounded terminals through 1K.  The brightness control should
be able to swing the voltage back and forth from about -20 to -60 volts.
Check grid 2 voltage---should be around 250.  
	If you have a horizontal line on the CRT, you are not getting
vertical deflection.  Check that the oscillator is oscillating, that the
output stage is operating.
	Once you have a raster, then you can start debugging any
problems in the video and audio circuits.  Prewar and early postwar TV
sets trapped the audio right behind the tuner and used separate IF
strips for video and audio.  Later sets use "intercarrier" IF's, with
one IF strip and a sound trap at the end of the IF chain.  In either
case, "raster, no picture, no sound" means that the problem is between
the tuner and the sound trap.  "Picture, no sound," or "sound, no
picture" means the problem is after the sound trap.  Don't fuss with the
tweaks on the IF strip (strips) unless you have the proper equipment and
instructions for doing an alignment.  Unlike most radios, these are
stagger-tuned, and you don't just "tweak them up" for best performance.
The video comes from a conventional AM detector and a "high fidelity"
voltage amplifier, connected to the CRT cathode.  Note that the bandpass
of the video amplifier is very wide, and the term "video amplifier" has
become a generic term from a wideband untuned amplifier.  The audio is
through a conventional ratio detector and single-ended audio amplifier
to a (incredibly cheap setup for something that cost $400 in '46) small 
	One fairly standard complaint is loss of raster sync.  If the
tubes are OK, this is generally the paper capacitor bugaboo at work.
Loss of both horizontal and vertical means that the coupling out of the
video amp has a problem.  Horizontal sync comes from differentiating the
video signal, and vertical sync from integrating the double-speed
interlace "trick" pulses that ride on the "pedestal" portion of the
video signal (the vertical sync portion).  
	These are some basic things about forties TV sets.  Note that
the CRT's on early magnetic deflection sets had offset guns and "ion
trap" magnets.  This was to prevent burning a spot in the center of the
CRT.  Around 1948, the aluminized phosphor coating, which was impervious
to ion burns, went into production, eliminating the need for offset
guns.  If the ion trap is misadjusted, the electron beam won't be aimed
at the phosphor screen properly, so the raster will be dim or
nonexistent, or have "neck shadows" at the edges.  This, like the IF
tweaks, another "if it's working, don't fix it."  Electrostatic
deflection sets that used tubes like the 7JP4, did not have ion burn
problems, so are mounted with nothing on their necks.  These sets also
did not require transmitter-like power for horizontal deflection, so did
not have high voltage derived from the horizontal circuit.  Instead, a
separate RF oscillator was used.  CRT circuits in electrostatic
deflection sets are quite similar to oscilloscope CRT circuits.  
	There are several books on servicing television sets that
generally apply to forties sets, although they are generally oriented
toward later sets.  Compared to later sets, most forties TV sets were
powered through transformer supplies, did not have any tricks like B+

Q.  My radio plays, but the audio is distorted.  Announcers sound like
mush-mouths, and music sounds as though gravel is rattling in the
instruments.  I checked it with another speaker, and it sounds just as

A.  The most common causes of distortion are in the power amplifier
(Note that in the following I am assuming class A or AB1 operation,
where tubes do not draw grid current.  If the grids of your power
amplifier tubes are driven by power tubes, such as a 6N7 or 6V6's, most
of the following applies to operation at low output).  

	1.  Check that the coupling cap (or caps, in the case of
push-pull) are not leaking DC from the preceding stage and pulling the
output tube grid high.  Most circuits use a 180K to 500K grid leak to
ground and a .05 or .1 microfarad coupling cap.  At low-moderate audio
output, there should be no measureable DC voltage across the grid leak
resistor.  Check the grid leaks themselves for proper value and good
connections (typically to ground).  Wax paper coupling caps here are
notorious for giving problems, and are candidates for replacement even
if they appear to be good. The tube itself may be developing excess gas
current in the grid circuit.  Disconnect the coupling cap, turn the set
on, and make sure there is no voltage developed across the grid leak.  If
there is, replace the tube.  Note that most tube testers won't disclose
this problem.   With larger tubes (6V6, 6L6), replacement tubes made
after the mid-70's often had poor gas current characteristics, and some
designs were built with higher-value grid leak resistors than specified
by the manufacturers on the assumption that replacements would "never be
that bad."  Most beam tubes specify a maximum impedance in the grid
circuit of 500K for cathode bias, 100K for fixed bias operation.  

	2.  Check the value of the cathode resistor.  Be careful here,
because a resistor that has overheated may not only have changed value,
but have charred the color bands so that they look like a very different
value resistor.  If the circuit uses a cathode bypass capacitor (usually
an electrolytic, 20 mfd. 25 volt typical), check that it isn't leaking
current, and check that it has capacitance.   

	3.  Check grid bias with the set running.  Proper bias for
various tubes can be estimated from tabular data in tube manuals, and
ranges from around -7.5 volts for a small high-gain beam pentode like a
50L6 to around -60 volts for a large low-gain triode like a 2A3.  

	4.  On a push-pull output stage, check that both sides are
operating.  An easy check is to jumper across the grid leak resistor
with a clip lead, and see if things change.  If jumpering one input
kills the audio, the other side is inoperative.  Prime things to suspect
if one side is dead are the power tube on that side, open transformer
plate winding (no B+ on that tube), open coupling cap, or problems in
the voltage amplifier ahead of the output stage.  

	5.  If you haven't found the problem yet, check the quality of
the audio coming out of the preamplifier stages.  

	6.  DC imbalance can cause problems in push-pull circuits.  Most
old radios don't have any place to measure this.  You can wire 100 ohm
resistors into the plate circuits, in series with the output
transformer, and measure the quiescent DC voltage across them.  For most
old radios, a 20% imbalance is tolerable.   Keep in mind that the
voltage developed across a cathode resistor is total cathode current,
both screen and plate, and that a common cathode resistor in a push-pull
circuit is looking at the effects of two tubes simultaneously.   

Q.  I've got an "All American Five" 50L6 radio that has new filter caps,
but the hum that comes out of the speaker is really out of sight.  I can
hear it in the next room when the volume is so low I can't really hear
the station it's tuned to.  I know these sets hum, but should it be that
bad?  All the tubes test good on a mutual conductance tube tester.  
	A.  No---you've probably got a very common tube fault that a
tube tester doesn't detect, heater-cathode leakage, probably in the
50L6.  In these sets, the low end of the 50L6 heater is about 38 VAC 
above ground, and the high end, up at 88 volts.  What you are getting is
AC on the cathode, and the only real solution is a 50L6 that doesn't
have heater-cathode leakage.  12SQ7's can also have this problem,
although they are always wired at the ground end of the heater string.
The only real diagnostic is to scope the cathodes of both tubes.  
	One item that aggravates this situation is that many "All
American Five" sets had no bypass capacitor across the power amplifier
cathode bias resistor.   Hanging a 50 mfd. 50 volt cap here often will
improve set performance and reduce hum, although it won't solve a
serious case of leakage.  
	Before trying to diagnose hum problems, particularly in a series
string set, try turning the plug to the wall socket around the other
way, to reverse the polarity of the chassis.  Many of the older 300 ma.
series string sets were very sensitive to primary power polarity, and
would have very loud hum if the power plug were connected the wrong

Q.  What sort of tools and test equipment do I need.  

A.  A 20,000 ohms/volt multimeter is indispensible. They are relatively
inexpensive, and modern multimeters have protection circuits in them.
You can trouble-shoot and fix almost anything discussed in this
newsgroup with a multimeter and some knowledge of circuit theory.  Many
prefer an analog meter with a needle over digital.  You can watch the
needle move and see what's happening.   

While not "test equipment," tools for unsoldering and soldering
components and wire are also mandatory.  Soldering is discussed in
another FAQ question.

Other small hand tools include screwdrivers, allen wrenches (for knobs
with setscrews), nut drivers, and small diagonal cutters and
needle-nosed pliers.  There is only one kind of tool, a good quality
tool.  Buy the best.  They'll last forever, and do their jobs well.
Don't buy cheap knucklebusters.  They are hard to use, will make
scratches, bend, and break, and scar up the work.  Buy the best---many
of the good tool manufactures have sold the same tools for over fifty
years, and many of us use tools that old today.  

Beyond the basics are the following:

a.  Oscilloscope.  This has become the primary instrument for use in
electronics work of all sorts.  While they were not commonly used for
radio repair in the 1930's and '40's.  There are a great variety of
scopes, ranging from the old relaxation oscillator sweep type used in
the thirties (and sold by Heath as late as the 70's) to the very latest
solid state scopes with triggered delaying sweep and multiple trace
vertical inputs.  Almost any scope that works is fine for working on old
radios and vacuum tube amplifiers.  While you can get old vacuum tube
scopes for very low prices, keep in mind that you may find yourself
trouble-shooting and fixing it.  

b.  RF signal generator.  Once again, these come in many sizes and
shapes.  These are used for aligning tuned circuits (RF and IF
amplifiers).  For an AM-shortwave radio, you need 100 kc. to around
15-20 mc, with AM modulation capability, and for FM, you should have
88-108 capability as well.  A sweep signal generator (i.e., able to
swing the frequency back-and-forth over a small range electronically,
with a voltage output to drive an oscilloscope horizontal amplifier) and
a suitable scope are very nice to have but not mandatory.  

c.  Tube tester.  The value of tube testers as a primary diagnostic tool
tends to be overrated, but a good mutual conductance tester (Hickock
made several) can be of value if it is used appropriately.  Cheap "tube
checkers" will test filaments (an ohmmeter will do as well) and whether
the tube conducts or not, and may detect hard short circuits (these do
happen).  A Tektronix 570 curve tracer (a specialty oscilloscope that
gives graphic displays of tube characteristics) is the ultimate in test
devices.  However, the ultimate "tube tester" is the equipment in which
the tube is used.  The function of tube testers, more than anything
else, was to sell replacement vacuum tubes.  And many really nasty
tube-related problems will only show up in the socket in the equipment
where they are supposed to function properly.  

If you have a good scope, multimeter, and signal generator, and know how
to use them, you have all the tools you need for radio work.  Here are
some other items, some of which were popular as radio shop tools, and
some of which aren't primarily test equipment.   

d.  Signal analyzer, signal tracer.  These were very popular in radio 
shops.  They are an AF amplifier, small speaker, and a diode detector 
that can be switched in and out of the probe circuit---in essence, a
small radio without any tuned circuits.  If signal is getting into the
antenna, you can probe each stage and hear it, and quickly locate a
"dead" or "distorted" stage.  

e.  Condenser tester.  Also "radio shop" stuff from the 1930-50 era.  
An inexpensive L-R-C bridge with an electronic oscillator.  Used
properly, it can be a handy tool.  I use the term "condensor" because
it was the standard term for a "capacitor" in the US until the late

f.  VTVM (stands for "Vacuum Tube Voltmeter").  The virtue of these is
the high input impedance (generally megohms) and their ability to
measure resistances into the megohms range.  Largely supplanted by
oscilloscopes, which draw a picture of the signal, but of value today
for their ability to measure high resistance.  

g.  Grid dip meter.  This is a small oscillator that comes with a set of
plug-in oscillator coils that can be poked into tuned circuits.  They
rely on the fact that a resonant circuit near the oscillator coil will
cause the grid current of the oscillator tube to drop, hence "grid-dip."
A very simple and handy little device, though generally used with things
like transmitters that have to be tuned before power is applied.  Since
they oscillate, they are also a fine "poor man's signal generator."  

There were several specialty houses in the US in the 1930-50 era that
built very good measurement equipment.  I'll mention them by name:

Boonton Radio, built Q-meters and R-X bridges.  These measure the
inductance and other characteristics of RF coils and tuned circuits.  
Generally used to support coil design efforts.  The British Marconi
Q-meters are excellent as well.  

Measurements Corp.  Built very nice signal generators, much higher
quality than those from repair equipment manufacturers like Hickock.  

General Radio (Cambridge, Mass.).  This company moved to the suburbs in
the late 1950's and is now known as Genrad.  Their 650 impedance bridge
was the general use DC/400 cps L-R-C bridge.  It used a small battery
and a 400 cps "hummer" (a small vibrator) to generate AC for measuring
impedance of things like audio transformers.  Over the years, General
Radio built a broad line of devices, primarily for engineering use, only
some of which are applicable to radio electronics.  

Guildline of Canada.  I mention them because they built some of the very
best calibration standards.  Their potentiometers and other products are
not only "not test equipment" but can easily be damaged if used for
testing things.  The proper use of such equipment is calibration of
working equipment, and the appropriate place for it is a calibration

While I mention equipment common in the US, I am familiar with products
of Marconi in England, who built engineering support products similar to
the Boonton, Measurements, and General Radio products.  I believe that
Telefunken, Phillips, and Thompson-CSF (spelling?---French company) also
built and sold similar equipment.  The US stuff often shows up at things
like ham swapfests, and is bought and sold by several companies, notably
Tucker, of Dallas, Texas.  

Q.  My radio is supposed to have 295 volts on the screen of the 6L6
amplifiers.  I read 303.5 on my digital voltmeter.  Is something wrong?

A.  Yes, both your expectation that the screens are supposed to read 295
volts, not 295 +/- 20%, and that your DVM is precise just because it
gives you a lot of digits.  When was that DVM last calibrated (or was it
ever calibrated) against a known standard of some sort?  

Most shop test equipment is wildly inaccurate to begin with, and has had
enough use and abuse (and time) since last checked that you can't trust
the readings at all.  At best, they will tell you "around 300 volts" or
"around 455 Khz" unless you have some way to check against standards.
Don't trust anything to be telling you other than "approximately" unless
you have had it checked against standards recently, know what accuracies
you can expect, and things that can affect accuracy.  Most major cities
have services which have standards against which to check test
equipment, and if you have something like a GR 650 bridge that is
working properly, it may be worth the tariff to have it's calibration
checked by one of these shops.  

When selecting test equipment, keep in mind that that nice old Tek scope
may have 35 or 30 tubes and 50 adjustments, and pose much more of a
maintenance problem than any radio.  

Q.  I don't trust the calibration of my instruments?  What can I use to
check them?

A.  There is a good frequency standard available for free: WWV, which
broadcasts on 5, 10, and 15 Mhz.  If you have a signal generator with a
crystal calibration oscillator, you can tune in WWV on a shortwave
receiver, tweak the crystal tank circuit, and have a fairly good
reference to WWV for other frequencies----though it's a long stretch
from 5Mhz to 455 Khz.  Fresh dry batteries generally are fairly close to
their nominal voltages, and an automobile battery that is fully charged
is a first cut "standard" 12.6 volts.  Accurate voltages above that are
hard to find in the basement workshop.    Ohmmeters tend to be wildly
inaccurate, but you can measure a bunch of resistors of different values
to get "somewhere near."  
(Faq editor note:  other countries have frequency-standard time
stations; if someone familiar with them could E-mail me the information,
I will include it here).

The rule of thumb is that two-figure accuracy is readily achievable, and
more than what is needed for service work.  However, if you are using
flea-market test equipment, it may have been discarded or surplussed
because it could not be calibrated, or may not have been checked and
calibrated for thirty or forty years.  

Q.  I tried to use a Tek scope to trouble-shoot my AC-DC set, but when I
connected the probe ground, I got sparks and burned out the wire.
What's wrong?

A.  US AC-DC sets typically have one side of the line connected directly
to chassis ground.  Some European sets may also have a direct connection
between one side of the supply mains and the chassis.  Virtually all US
test equipment built over the last 40 years uses a three-prong plug with
a direct connection between the ground prong and the test equipment
chassis.  What happened here is that the radio was plugged in with the
high side of the line connected to its ground, and you connected the
ground strap across the line voltage.  While in US power distribution
systems, the "neutral" wire is connected to earth ground at the
distribution panel, grounding the line neutral at the radio may cause
currents to circulate in the neutral-ground circuits (ground loop).  

The best way to avoid a shock hazard with an AC-DC set is to use an
isolation transformer.  It is possible, but not recommended, to "float"
the test equipment ground by using a two-prong "cheater," but this may
cause other problems.  Plugging the set in so that the grounded side is
neutral may also work, particularly if you use a .01 mfd or larger cap
in the ground circuit to the scope.  However, with any method other than
an isolation transformer, the scope and the radio may have some voltage
between them, posing a shock hazard as well as problems making

AC-only sets were often connected with a .02 mfd cap from each side of
the AC line to the chassis to provide an AC reference ground between the
chassis and the AC line.  If either of these capacitors is shorted, the
chassis is directly connected to one side of the line.  Find these caps
and check them before doing any trouble-shooting.  

Q.  I want to fix my old radio myself, and have never used a soldering
iron before.  What do I need to do?  

A.  Soldering equipment for radio work is discussed in section 8 of the

Q.  Replacing all those capacitors is a lot of work.  Somebody told me
that I could just clip the leads and solder new caps to the old leads.
That sounds a lot easier.  Should I do that?

A.  Going back to Frye's "Mac's Service Shop," a column that appeared in
the old "Radio News" in the 1940's, a proper repair is to make the radio
"like new," using the methods that were used to build it originally.
The Yiddish term, "schlock," was invented for folks who do things like
clip out old parts and solder new ones to the leads.  Yes, removing
solder from terminals and prying the ends of tightly-wrapped leads open
so that you can remove an old part is hard work, and it will take a
while to learn to do it with any ease.  Take those old components
completely out, clean off the terminals, and install the new components
neatly.  In many cases, particularly if you are replacing wax paper
capacitors with axial-lead mylars, you will find the old leads bent
around quite tightly to connect one end of the capacitor to the nearest
ground that could be reached.  The new capacitors are much smaller, and
may install much more neatly, particularly if an appropriate ground
point is nearby for bypass caps.  

If you take pride in good workmanship, you'll end up with a set that
works well, isn't a fire hazard, and doesn't have mysterious squawks and
squeals.  Sloppy workmanship is a red flag to anyone who looks at the
radio---it says that there is probably extra trouble installed by
whoever did the poor work.  And, most of the time, investigation shows
miswires, wrong-value components, and a host of other problems.  

Q. I have an early 1930's radio and want to replace the wax paper
capacitors, but want to keep the chassis looking original.  Where can I
get look-alike wax paper caps?

A.  The manufacturers who made these discontinued them years ago.  You
will probably find repairs from the 1946-70 era in old radios using
paper capacitors molded in plastic, and even these are difficult to find
nowadays.  While recent axial lead caps with mylar and other plastic
dielectrics work well, as long as the voltage rating is adequate.  
Generally, 400 volt caps will work, but 600 or 630 volt caps are safe
in any set with an 80 or 5Z3 rectifier.  However, they don't look
anything like the old axial lead capacitors.  

It is possible to melt the wax out of old capacitors, salvage the
cardboard sleeves, and install new axial lead caps inside.  While
axial-lead caps are somewhat hard to find, and tend to be expensive,
they are still made, and are usually small enough to fit in the old
cardboard sleeve.  

Dan Schoo, who does this type of restoration regularly, kindly wrote up
a procedure for salvaging the old sleeves and putting new caps inside,
and made it available for inclusion in the faq.  Here it is:

Rebuilding Wax Filled Paper Capacitors
by Daniel Schoo
From:  (Dans Cockatoo Ranch)

A paper capacitor is a type of capacitor that was used
extensively in radios from the thirties through the fifties. They
are made of wax impregnated kraft paper and two thin metal foils
cut into long narrow strips. The foils were placed one on each
side of the kraft paper and rolled up along the long dimension
into a rod shaped assembly. The foils were skewed such that they
extended a little past the paper at the ends of the rod, one on
each end. This provided an electrical connection point to each
foil over it's entire length. The voltage rating of the capacitor
was controlled by the thickness of the paper. Thicker paper could
hold off a higher voltage. The capacity was controlled by the
surface area of the foils. Longer wider foil wraps would have
higher capacity. This is why higher voltage and/or higher
capacitance values would require a larger size for the capacitor.
The lead wires were attached to the foils extending out from the
ends of the rod. The entire assembly was then slipped into a
cardboard sleeve and the sleeve was filled with wax. Later types
were molded into plastic shells and had paper labels attached or
were printed with colored bands or text to indicate the values.
The black band around one end of the sleeve and the words
'outside foil' indicate the lead that is attached to the foil
strip wound on the outside of the kraft paper. This is important
in some applications and tells the assembler which lead to use
during construction. Paper capacitors were used for higher
voltages at medium to small capacities. The voltage ranges are
usually from 100 to 600 volts and from .0001 to 1 microfarad in
capacity. They typically fail by becoming leaky and allowing DC
current to pass.

The purpose of rebuilding an old type paper capacitor is for
appearance only. When restoring an old radio to operable
condition, some owners desire to keep the appearance of the
components under the chassis as close to original as possible.
When certain components fail such as capacitors, it is not
possible or even desirable to replace them with original types.
To keep the original appearance, the old component is taken apart
and a new one is hidden inside the old shell.

After you have determined that a capacitor is bad, or if you just
want to replace one because you have a basic dislike for them,
remove it from the radio. Begin by melting out the wax potting.
Wear eye protection and use a heat gun, blow drier or small torch
with a hot air attachment like a Master Ultratorch. Do not use a
torch or other open flame on the capacitor as this will apply too
much heat in a small area and probably cause it to burn. Not much
heat is required to melt the wax but it has to be steady and even
to heat up the entire body of the capacitor. Hold the capacitor
sleeve with a long nose pliers and heat it slowly until all the
wax has dripped out. Discard the old wax. Some paper capacitors
have cardboard end disks. For these, the ends of the sleeve are
rolled inward to retain the disks. Unroll the end crimps and
smooth them out. Remove the end disks with a small screwdriver.
After the end disks are out continue to heat the capacitor until
the rest of the wax is out. After most of the wax has run out,
hold the capacitor with an insulated pad, grab a lead with a
pliers and pull the insides out. If the wires come off, push the
insides out with a small screwdriver. If the insides are stuck in
the cardboard sleeve, it may be necessary to drill them out. Pull
out the wire leads and drill a small pilot hole down through the
center of the capacitor. Drill another larger hole about half the
diameter of the sleeve. You should be able to dig out the rest of
the insides with a small screwdriver. Be careful not to puncture
the sleeve.

Once the sleeve is cleaned out you can install the new modern
capacitor. The most difficult part of this is to find a suitable
capacitor that will fit into the paper shell. You can substitute
a new capacitor with an equal or higher voltage rating than the
old one but try to get as close as possible to the original
capacity. Fortunately many of the modern capacitors are much more
compact than the old paper ones. Modern capacitors use plastic
films like polycarbonate, polypropylene, polystyrene, and
polyester which is also known as Mylar. The most common is Mylar
and is suitable for many replacement applications. These do not
degrade with time like paper does and should give years of good
service. The popular Sprague "Orange Drop" is a Mylar capacitor.
These are not suitable for use in this application because they
are designed for printed circuit mounting and the leads are
radial. This means that they extend out the side of the capacitor
at a right angle. You must use an axial lead capacitor with the
leads extending out in line with the capacitor body.

Slide the new capacitor inside the old sleeve and center it. If
there is a lot of space around the new capacitor such that it is
loose you can wrap a few turns of plastic tape around it to build
it up. Slit the tape down to about a quarter of an inch wide and
wrap it in a band around the middle of the capacitor until it
fits snugly in the sleeve. After the new capacitor is centered in
the sleeve you can fill the ends with wax. You can get beeswax at
any well stocked hardware store. It comes in small tan blocks
about three inches square and one inch thick. Cut off a small
chunk and place it in a small metal can. Prepare the can by
bending a pour spout into the top edge and make sure it is clean
and dry. Wear proper eye and skin protection when heating the wax
just in case it spatters. Heat up the wax slowly with a heat gun,
a hair dryer or small torch. Remove the heat when all of the wax
has melted and be careful not to overheat it. If it begins to
smoke remove the heat immediately. Support the capacitor in a
vice or tape it to the edge of a table top. When the wax has
melted thoroughly, pour it slowly into the end of the sleeve just
up to the edge. If the capacitor had end caps leave enough room
to reinstall them. When the wax has cooled sufficiently, flip
over the capacitor and fill the other end. Allow the capacitor to
cool completely before installing it in the radio.

Daniel Schoo                             (o o)
Electronics Design Engineer             (  V  )
Fermilab, Batavia, Illinois, USA   .......m.m......Dan's Cockatoo Ranch

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