> Improve the Squelch in Your Monitor Receiver

Improve the Squelch in Your Monitor Receiver

June 11, 1998.

Copyright (C) 1988,1998
Bob Parnass, AJ9S (parnass@bell-labs.com)
--- All Rights Reserved ---

NOTICE: 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/rec.radio.scanner.

Introduction

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:

"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

The author cannot furnish modification information for other radios.
Don't attempt this, or any other internal modification, unless you are skilled in electronic servicing.
Make sure your receiver is disconnected from any power source before opening the cabinet.
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

"The Radio Shack PRO-24 Scanner," by Bob Parnass, excerpted in the "Technical Topics" column, RCMA Newsletter, February 1984.
"The Bearcat 800XLT ... an Owner's Report," by Bob Parnass, in Monitoring Times, March 1986.
"Product Review: The Radio Shack PRO-2004 Programmable Scanner," by Bob Parnass AJ9S, in The Radio Enthusiast, February 1987, and Monitoring Times, March 1987.
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.
"Product Review: The Radio Shack PRO-2035 Scanner -- How Does It Compare with the PRO-2006?," by Bob Parnass AJ9S, Monitoring Times, January 1995.