Quote:> ...Also, the scanner has quite a bit of hysteresis in the squelch
> control, which manifests itself by much "slop" in it's operation.
> The squelch action is much improved by replacing a single resistor R35...
I decreased the hysteresis in my Uniden/Bearcat 760xlt
squelch circuit like I did in the 800xlt. I replaced R58, a
22,000 surface mount resistor, with a 1 megohm resistor.
One side of R58 connects to pin 14 of IC2, the same 3359
chip used int the Uniden 800xlt. The other side of R58
connects to both the wiper of the squelch pot (VR551) and
VR1, the board-mounted squelch limit adjustment pot.
Here's an updated copy of an older article.
IMPROVE THE SQUELCH IN YOUR MONITOR RECEIVER
by Bob Parnass, AJ9S
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 Shackr 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 600XLT and 760XLT 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.
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. Motorolar 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.
TABLE 1. Squelch Hysteresis Resistor In Selected Receivers
_______________________________________________________________________
| Squelch Hysteresis Resistor In Selected Receivers |
| |
|Receiver Resistor Factory |
|______________________________________________________________________|
|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) |
|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 760XLT | R58 22K replace with 1M. |
| | 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 |
|______________________|_______________________________________________|
__________
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.
--
============================================================================
Bob Parnass, AJ9S - AT&T Bell Laboratories - att!ihlpy!parnass (708)979-5414
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