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last changed December ,
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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 - [email protected] - (708)979-5414
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