last changed December 21, 1994                               |

        IMPROVE THE SQUELCH IN YOUR MONITOR RECEIVER

                    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.]

 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 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.


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-2022 |            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.

   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.


-- 

==============================================================================
                       Copyright 1995,  Bob Parnass, AJ9S
           AT&T Bell Laboratories  -  parnass@att.com - (708)979-5414