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Add a Plectron or Motorola Alert
Desktop Monitor Receiver
to Your Scanner Collection
by Bob Parnass, AJ9S
[NOTE: This article may not be reproduced in whole or in
part on CDROMS, in bulletin boards, networks, or
publications which charge for service without permission of
the author. It is posted twice monthly on the USENET
groups rec.radio.scanner, alt.radio.scanner, and
rec.radio.info. It is also available electronically from
the rec.radio.scanner ftp archive on the official USENET
FAQ library ftp://rtfm.mit.edu/pub/
group/rec.radio.scanner. The author writes a monthly
"Scanner Equipment" column for Monitoring Times magazine,
published by Grove Enterprises, http://www.grove-ent.com
but views expressed in this article are his own.]
In the 1960s and 1970s, thousands of firemen and ambulance
squad members came to rely on their crystal controlled,
transistor Plectron and Motorola desktop monitor receivers.
They have now been replaced by battery operated portable
pagers, and the desktop models are often available at
hamfests in the $5 - $25 range. Since many are in rough
condition and need repair, a hamfest special is better
suited for hobbyists who like to fix their own radios.
Plectron and Motorola crystal controlled receivers are
excellent for dedicated monitoring of local frequencies --
a task for which you wouldn't want to tie up your 400
channel programmable scanner. They are desktop models
which can be powered from 117VAC or 12VDC with the proper
mobile cord. The audio quality of a Plectron P1, Plectron
700 series, and Motorola Alert Monitor is far better than
any consumer grade scanner and the sensitivity, image
rejection, and intermod immunity is outstanding when
aligned properly.
Models and Frequency Coverage
The Plectron and Motorola Alert monitors are single band
receivers: VHF-low, VHF-high, or UHF. No single Plectron
or Motorola Alert monitor can cover the entire 30-50 MHz
band, so there were versions optimized for each portion of
the VHF-low band. There were different versions to cover
low and high "splits" in the VHF-high band, too. For
example, low band R8000s come in 30 - 35, 35 - 39, 39 - 49,
and 49 - 54 MHz versions. There are two VHF high band
versions of the 700 and R8000: 148 - 158 and 158 - 175 MHz.
VHF-low band Motorola Alert monitors come in two splits: 30
- 42 and 42 - 54 MHz.
Although UHF versions of the 700 series were made, they are
somewhat rare and coverage is limited to 470 MHz and below.
Plectron made several models with dozens of different
options. Both the original P1 and later 700 series are
suitable for restoration, although I prefer the newer 700
models. The 8000 series were even newer and very
desireable, but they are difficult to find at hamfests.
The Plectron 500 "economy" series is less desirable.
The P1 series have black cabinets with a blue-green and
silver color scheme used on the front panel. They were
made in the Chief (tone decoder & carrier squelch), Patrol
(carrier squelch only), and Sentry (tone only) models. The
Sentry models lack a carrier squelch and therefore are of
little use to hobbyists except for scavenging parts. Model
number and frequency information appears on a gummed label
on the rear panel:
Band (MHz) Chief Patrol Sentry
____________________________________
25-54 R19 R15 R17
148-174 R20 R16 R18
450-470 R23 R21 R22
Table 1. Plectron P1 models
The R700 series have black cabinets with brown front panels
and are slightly smaller than the P1 receivers. R700s were
made in the Chief (tone decoder & carrier squelch) and
Patrol (carrier squelch only) models:
Band (MHz) Chief Patrol
___________________________
25-54 R719 R715
148-174 R720 R716
450-470 R723 R721
Table 2. Plectron 700 models
The R8000 series are much smaller than the P1 and R700 and
are housed in metal cabinets of various colors. The R8000s
use the same MC3357 IF/detector/squelch IC used in crystal
scanners, but their crystals are not interchangeable with
ordinary scanners. R8000s were made in the Chief (tone
decoder & carrier squelch) and Patrol (carrier squelch
only) models:
Band (MHz) Chief Patrol
___________________________
30-54 R8190 R8150
148-174 R8200 R8160
Table 3. Plectron 8000 models
The Plectron FM Receiver/Recorder is a collector's item.
It is essentially a 700 series Chief receiver with a built
in cassette tape recorder. The recorder is carrier
activated and can tape transmissions while the receiver is
unattended. A connector on the rear panel allows for all
kinds of remote control possibilities.
The tape recorder inside the Receiver/Recorder employs two
rubber belts. The Projector Recorder Belt Company makes
replacement belts which work fine, though they may not be
the original equipment:
1. PRB SCA8.6 (218 mm)
2. PRB SCQ 2.5 (64 mm)
I use two Plectron P1s and four R700s in my living room,
seven R700s down the basement, and several Plectrons and
Alert Monitors stored in reserve. The basement Plectrons
are turned on automatically when someone walks downstairs.
Power to them is controlled by a modified Radio Shack Safe
House infrared motion detector, originally sold as an
intrusion alarm.
When a rescue squad member from Missouri advertised a
number of Plectrons on USENET, I bought them -- all 20+ of
them. I cleaned, fixed, and restored each one. After
recrystalling them on local frequencies, I distributed them
to members of my scanner club who were glad to get them.
Crystals
Both brands of receiver require special crystals. I
sometimes use Radio Shack's generic 3rd overtone scanner
crystals in the Plectrons but they oscillate on frequencies
far away from their marked frequencies. That's because the
Plectron oscillator is designed to be used with a crystal
which oscillates on its fundamental, not overtone,
frequency.
The best source for crystals is:
International Crystal Mfg Co.
11 N. Lee Ave
Oklahoma City, OK 73102
tel. 405-236-3741, 800-725-1426
You need not bother with calculating the crystal
fundamental frequency. Merely supply International with
the exact receiver model number, printed on the rear label,
and the frequency you want to monitor. They will perform
the necessary calculations.
Crystals for Plectrons
The Plectron P1 series contain two crystals. The second
local oscillator crystal is 11.155 MHz regardless of band.
On VHF high band (148 - 175 MHz), the P1 series employ this
formula for the first local oscillator crystal:
Crystal frequency (MHz)
= (station freq (MHz) + 10.7) / 4
or
Crystal frequency (MHz)
= (station freq (MHz) - 10.7) / 4
On VHF low band (25 - 54 MHz), the P1 series employ this
formula for the first local oscillator crystal:
Crystal frequency (MHz) = (station freq (MHz) + 10.7)
or
Crystal frequency (MHz) = (station freq (MHz) - 10.7)
There are two crystals (Y1, Y2) used in the Plectron R700
series, one for each local oscillator. The crystal
formulas are:
RF Range (MHz) Y1 (MHz) Y2 (MHz)
_____________________________________________________
25-29 (f+10.7)/3 11.155
29-32.6 (f+10.7)/3 11.155
32.5-35 (f-10.7)/2 10.245
35-41 (f-10.7)/2 11.155
41-47 (f-10.7)/2 11.155 (use 10.245 MHz
for Y2 on 44.62 MHz)
47-54 (f-10.7)/3 11.155
148-158 (f-10.7)/9 11.155
158-175 (f-10.7)/9 10.245
450-470 (f-10.7)/18 11.155
The economy Plectron 500 series use this scheme for the
first local oscillator crystal:
RF Range (MHz) Crystal (MHz)
______________________________
25 - 35 (f+10.7)
35 - 54 (f-10.7)
148 - 174 (f-10.7)/4
Most Plectron 500s use an 11.155 MHz second local
oscillator crystal. However, a 10.245 MHz crystal is used
instead for receivers in these ranges: 33.4 - 33.78, 44.62,
and 158.78 - 158.82 MHz.
Power Cords
AC power cords for both types of receivers use unique
connectors and are scarce. I published the pinout for both
Plectron and Motorola Alert Monitor receivers in June 1998
Monitoring Times magazine.
You can buy the proper replacement power cords but it's
much cheaper to drill a hole on the rear panel and solder
on a permanent a power cord. Make sure you use a grommet
in the hole to prevent the metal chassis from chafing the
line cord.
Many Plectrons and Alert Monitors were equipped with an
internal NiCd battery pack, intended to power the receiver
if the AC power fails. It's unusual to find a used monitor
receiver with the NiCd pack still capable of holding a
charge. More often, the batteries have died and the series
charging resistor has overheated and burned. Be sure to
remove the dead batteries as they often leak.
Squelch and Minimum Volume Setting
If you find the squelch on your Plectron 700 series
receiver has too much hysteresis, replace R96, a 180K
resistor, with a 560K resistor. On the P1, the resistor is
designated R81.
Both Plectron and Motorola Alert monitor receivers are
designed purposely to have a minimum volume setting which
is still audible. The intent was to prevent firemen from
turning the volume down completely, forgetting it was down,
then missing an important call.
This "feature" was usually implemented by having a fixed
value resistor in series between one end of the volume
control and ground. In the R15X5, R14X5, R15X5, and R16X5,
a 47 ohm resistor, designated R624, was inserted in series
with the violet wire leading from the volume control.
You could decrease the lowest volume setting by decreasing
the value of this fixed resistor. Alternatively, you could
replace the resistor with a jumper wire to completely
silence the receiver at minimum setting of the volume
control.
Tone Alert Feature
I have not used the Plectron tone alerting features so I
cannot address that except to say that the special coils
and capacitors for the dual tone decoders vary depending on
the tone frequencies and are somewhat difficult to obtain.
Repair and Replacement Parts
Repair service and replacement parts for the Plectron P1
and R700 models is offered by:
Weber Electronics
PO Box 212
5138 Laurel Ln.
Broad Run, VA 20137
telephone (540) 347-7760
email: weber@citizen.infi.net
Alert Monitor parts are available from Motorola. Phone
(800)422-4210. The part number on the service manual for
my VHF-high band monitor is 68P81048A55-G and my VHF-low
band manual bears part number 68P81050A95-D.
John Miller reports the current price is $5.70 for service
manuals. Motorola advised John that Alert Monitor parts
are scarce, with last stocked being in 1995, Knobs are no
longer available, though there are many that would fit.
Plectron SM-series Scanning Receivers
Plectron also sold an SM series of scanners, manufactured
in Japan by the same folks who made Craig (division of
Pioneer) scanners. The SM series are consumer-grade models
and are included in this article for the sake of
completeness. They are crystal controlled radios and
feature priority scan and a separate trimmer capacitor for
each channel to net the crystals on frequency.
Band Frequency Limits
___________________________
VHF-low 30 - 50
VHF-high 150 - 174
UHF 450 - 470 MHz
Table 4. Plectron SM Model Band Limits
Model UHF VHF-high VHF-low
_________________________________
SM-301 X X X
SM-302 X X
SM-303 X X
SM-304 X X
Table 5. Plectron SM Models
The crystals used in the SM series are quite different from
the crystals used in the conventional single channel
Plectron monitors. Here are the crystal formulas:
VHF low band:
crystal 3rd overtone freq = receive freq + 10.7 MHz
VHF high band:
crystal 3rd overtone freq = (receive freq - 10.7 MHz)/3
UHF band:
crystal 3rd overtone freq = (receive freq - 10.7 MHz) /
10
Other crystal specifications for the SM series are:
CR 25/U holder
3rd overtone
20 pF load capacitance
parallel resonant
max series resistance 40 ohm
max drive 2 mW
Plectron Alternatives
Too bad Plectron is out of business. TCS Communications
Corp. sells used Plectron receivers, in working condition,
for $75-100. If you require crystals or reeds, TCS may be
able to find some types/frequencies. Contact:
Terry Marengi
TCS Communications Corp.
tel. 1-800-TCS-XMIT
email: TERRYTCS@aol.com
If you wish to purchase a new receiver with "Plectron-like"
signaling capabilities, consider the receivers made by
Reach Electronics. Reach sells tone & voice pagers, Alert
monitors, and ENCODERS. They also do contract
manufacturing.
Reach Electronics
1311 West Pacific
Lexington, NE 68850
(308) 324-6661
(800) 445-0007
FAX: (308) 324-4985
Another Plectron-like receiver, "The Informer," is made by
Federal Signal and sold by Brinkley Electronics:
Brinkley Electronics
P.O. Box 502
Wallburg, NC 27373
(336)769-2902
FAX: (336)769-4029
email: sales@brinkleyelectronics.com
web page: http://www.brinkleyelectronics.com
I currently have no Plectrons or Motorola Alert Monitors
for sale. I'm hoarding them. :-)
--
==============================================================================
Bob Parnass, AJ9S parnass@bell-labs.com
User Contributions: 1 Gary Richards ⚠ Oct 11, 2021 @ 10:22 pm I have a Motororla Alert Monitor. Has 110v & 12 volt plug in back. Cord has been lost. Like to know if anyone knows the layout of the 4 pins, witch 2 are 12 volt. I can make a cord. The unit is a motorola alert monitor model # m01cnb Comment about this article, ask questions, or add new information about this topic:
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