From: Bob Parnass, AJ9S
Subject: Squelch Improvements for Scanners
IMPROVE THE SQUELCH IN YOUR MONITOR RECEIVER
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 ShackO PRO-24 crystal controlled scanner.
1. 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 Bearcat 800XLT2, Radio Shack PRO-2003, PRO-20043, and others.
Don't despair, the fix is simple for most radios.
Unfortunately, the Uniden Bearcat BC600XLT and BC760XLT series use tiny surface mount components, which makes modification very difficult.
I've successfully reduced the squelch hysteresis on other models by replacing a single resistor.
_________
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.
START OF PART 02:
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 and MC3359 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.
START OF PART 03:
_______________________________________________________________________
| Squelch Hysteresis Resistor In Selected Receivers |
| |
|Receiver Resistor Factory |
|Model Designation 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 and 14 |
| | of TK10420 (IC2) |
|Bearcat 220 | R75 2.7M may be ok as is |
|Bearcat 20/20 | R75 2.7M may be ok as is |
|Bearcat 250 | R66 2.2M may be ok as is |
|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 |
|_____________________|_______________________________________________|
|
