Uniden Bearcat BC-760XLT Squelch Repair and Modification

Uniden Bearcat BC-760XLT Squelch Repair and Modification

Post by Uniden BC-760X » Fri, 28 Jun 1991 22:54:48

The Uniden BC-760XLT non-functioning squelch has been repaired.
The problem was fixed by adjusting the VR1 22K trimmer
potentiometer located on the main board. Apparently, the 760s leave the
factory properly adjusted, but then component aging sets in to
disturb the correct alignment.

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. This resistor is found by locating the FM IF detector IC,
NJM3359D-A (MC3359) on the main board, and turning the board over.
The resistor R35 is a surface mount technology device located
immediately adjacent to pins 12 and 13 of the detector IC. This
330K resistor may be replaced with a 1M or greater resistor. A standard
1/4 or 1/8 watt resistor may be used through the judicious bending
of the leads (R35 actually connects pins 12 and 13 together, so
no need to worry about shorts).

 
 
 

Uniden Bearcat BC-760XLT Squelch Repair and Modification

Post by Bob Parnass, AJ » Sat, 29 Jun 1991 10:47:56




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

Some people are alive only because it would be against the law to kill them