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Improve the Squelch in Your Monitor Receiver


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last changed March 21, 2001

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        IMPROVE THE SQUELCH IN YOUR MONITOR RECEIVER

         Copyright 1988 - 2001 by Bob Parnass, AJ9S

  [NOTE: The first version of this article appeared in the
 RCMA Newsletter, December 1988.  It 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,
 sci.electronics.repair, 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/usenet-by-group/.]

 The squelch control in some FM monitor receivers is
 difficult to use because of too much hysteresis. The
 American Heritage Dictionary of the English Language
 defines hysteresis as the:

    o  "Failure of a property changed by an external agent
      to return to its original value when the cause of the
      change is removed."

 It's like having play in a car's steering wheel, or
 backlash in a gear set. Too much hysteresis in a squelch
 circuit forces one to keep the squelch at a tighter
 setting, missing weaker signals when scanning or searching.
 The squelch hysteresis problem was first addressed in a
 1984 article about the Radio Shack PRO-24 crystal
 controlled scanner.

 Why do engineers design these circuits with any hysteresis?
 Basically, hysteresis prevents weak signals from "popping"
 the squelch open and closed rapidly, which would be very
 annoying to the listener. A squelch circuit with hysteresis
 requires a stronger signal to open the squelch than it does
 to keep it open.

 Models cursed with too much hysteresis include the
 Uniden/Bearcat 800XLT and 760XLT, Radio Shack PRO-2003,
 PRO-2004, and others.  Don't despair, the fix is simple for
 most radios.  I've successfully reduced the squelch
 hysteresis by replacing a single resistor.

 Hysteresis is usually implemented by providing feedback
 from the squelch gate input to output, through a resistor.
 The higher the resistor value, the less feedback there is,
 and less hysteresis. My modification involves replacing the
 "stock" resistor with a higher value resistor, as
 identified in the table below.

 This technique does not apply to some of the newer Uniden
 models, because the squelch hysteresis is partially
 determined by firmware inside the processor IC.

 This article cannot provide step-by-step instructions for
 each radio. I always encourage radio hobbyists to purchase
 the service manuals for their equipment. The most useful
 sections of the manual are the schematics and printed
 circuit diagrams. Good manuals will also describe the
 theory of operation and alignment procedures, and contain a
 complete parts list.

 If your radio has too much hysteresis, and is not listed in
 the table, study the service manual.  Motorola MC3357,
 MC3359, and MC3361 integrated circuits, which contain IF
 and squelch circuitry, are popular in scanners. In these
 receivers, look for the hysteresis resistor to be connected
 to these ICs.


                            Notes


   1.  The author cannot furnish modification information
       for other radios.

   2.  Don't attempt this, or any other internal
       modification, unless you are skilled in electronic
       servicing.

   3.  Make sure your receiver is disconnected from any
       power source before opening the cabinet.

   4.  If space permits, you can replace the stock resistor
       with a higher value variable resistor ("trimpot") and
       adjust to suit.


Table 1.  Squelch Hysteresis Resistor In Selected Receivers


________________________________________________________________
|      Squelch Hysteresis Resistor In Selected Receivers       |
|                                                              |
|                        Res-                                  |
|Receiver                istor   Stock                         |
|Model                   No.     Value   Comments              |
|_____________________|________________________________________|
|Radio Shack PRO-24   |  R35       33K   replace with 220K.    |
|                     |                  Located between       |
|                     |                  pins 12 and 14        |
|                     |                  of MC3357 (IC1)       |
|Radio Shack PRO-2002 |  R197      47K   try 220K.             |
|Radio Shack PRO-2004 |  R148      47K   remove R148           |
|                     |                  or replace with 220K. |
|                     |                  On linear board,      |
|                     |                  between pins 12 & 14  |
|                     |                  of TK10420 (IC2)      |
|Radio Shack PRO-2006 |  R152      33K   try 100K or higher.   |
|Radio Shack PRO-2022 |  R91       33K   replace 33K           |
|                     |                  with 220K.            |
|                     |                  Between pins 12 & 14  |
|                     |                  on IC-1 MC3361        |
|Radio Shack PRO-2035 |           100K   replace 100K          |
|                     |                  with 220K.            |
|                     |                  Between pins 12 & 14  |
|                     |                  of TK10420 (IC-2)     |
|Electra/Bearcat 220  |  R75      2.7M   may be ok as is       |
|Electra/Bearcat 20/20|  R75      2.7M   may be ok as is       |
|Electra/Bearcat 250  |  R66      2.2M   may be ok as is       |
|Uniden/Bearcat 760XLT|  R58       22K   replace with 6.8K     |
|                     |                  or other value.       |
|                     |                  (select value for     |
|                     |                  the particular radio.)|
|                     |                  Located on pin 14     |
|                     |                  of MC3359P IC         |
|Uniden/Bearcat 800XLT|  R91      860K   replace with 2.2M.    |
|                     |                  Located on pin 14     |
|                     |                  of MC3359P IC         |
|Plectron P1          |  R81      180K   may be ok as is       |
|Plectron 700         |  R96      180K   replace with 560K     |
|_____________________|________________________________________|



                         References

   1.   "The Radio Shack PRO-24 Scanner," by Bob Parnass,
       excerpted in the "Technical Topics" column, RCMA
       Newsletter, February 1984.

   2.   "The Bearcat 800XLT ... an Owner's Report," by Bob
       Parnass, in Monitoring Times, March 1986.  Published
       by Grove Enterprises, http://www.grove-ent.com

   3.   "Product Review: The Radio Shack PRO-2004
       Programmable Scanner," by Bob Parnass AJ9S, in The
       Radio Enthusiast, February 1987, and Monitoring
       Times, March 1987.

   4.  US patent 4,947,456, "Scanning Radio Receiver,"
       granted August 7, 1990 to Noel D. Atkinson, William
       B. Ahlemeyer, and Ben F. McCormick, inventors.
       Assigned to Uniden America Corporation.

   5.   "Product Review: The Radio Shack PRO-2035 Scanner --
       How Does It Compare with the PRO-2006?," by Bob
       Parnass AJ9S, Monitoring Times, January 1995.

-- 
==============================================================================
Bob Parnass, AJ9S                                        parnass@bell-labs.com

User Contributions:

Nick
Report this comment as inappropriate
Jan 30, 2012 @ 9:21 pm
Bob; Is there a way to fix my quite old Realistic Pro 2001 scanner. The squelch suddenly just seems to be stuck or shot that is produces nothing but white noise static no mater where the knob is turned to any frequency. Can a new squelch be found and replaced or a new resistor soldered by me to fix. Anything to try. Thanks for your help. Nick

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