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TUCoPS :: Radio :: bcrepair.txt

Repair Tips: Bearcat Scanner Radios by Bob Parnass





 Last changed: January 4, 1995.   Lines  changed  since  the  |
 previous  issue  are marked with a | character in the right  |
 margin.

             REPAIR TIPS: BEARCAT SCANNER RADIOS

       Copyright 1993, 1994, 1995 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.  Free distribution is encouraged.]


                        Introduction

 The original  Bearcat  scanner  line  was  manufactured  by
 Electra  of  Cumberland, Indiana.  In the mid 1980s, Uniden
 bought out the Bearcat  scanner  line  and  Uniden's  first
 Bearcat scanner was the 800XLT model.

 Most  of  the  models  discussed  in   this   article   are
 base/mobile  units  made  by  Electra  during the 1970s and
 1980s.   Electra  stamped  all  of  its  scanners  with   a
 manufacturing  date  code  on  the rear of the cabinet. The
 code is comprised of a single character  (C  =  Cumberland,
 Indiana, P = Puerto Rico), followed by four digits denoting
 year and week the radio was  built.  For  example,  "P8422"
 denotes  the  radio  was  made  in  the Puerto Rico factory
 during the 22nd week of 1984.


               Schematics and Parts Available

 Schematic diagrams may be  obtained  from  Uniden/Bearcat's
 parts department, (317)842-2483.

 G & G Communications (telephone 716-768-8151) is  a  family
 owned  company  which repairs scanners and stocks parts for
 several  older  models,  especially   Electra/Bearcat   and
 Regency  brands.   They are located at 9247 Glenwood Drive,
 LeRoy, NY 14482.

 Electronic  Repair  Centers  in  Franklin  Park,   Illinois  |
 (telephone   708-455-5105)   has   been  repairing  Bearcat
 scanners for several years.  They charge a flat $45  repair
 rate  for  programmable  and  $30 for crystal scanners, and
 shipping is extra.   Electronic  Repair  Centers  will  fix
 Regency scanners if they can obtain the parts.


                  Bad Solder Joints Common

 Before addressing specific symptoms, circuit boards in  the
 malfunctioning  scanner should be inspected for poor solder
 joints.

 The Electra/Bearcat BC350, BC300,  BC250,  BC220,  BC20/20,
 BC211,  BC210,  and BC210XL models were hand assembled, and
 every one I've serviced had several connections  that  were
 either soldered poorly, or not soldered at all.

 Resoldering joints on the ribbon cable  connecting  the  RF
 and keyboard logic circuit boards in a BC250 attenuated the
 microprocessor/synthesizer hash noise noticeably.

 A Bearcat 20/20 was experiencing periodic loss of memory on
 some, but not all channels.  When the problem occurred, the
 frequencies on some channels would be  completely  changed.
 On other channels, the frequency would still be intact, but
 the channel would be locked out, and the delay toggled from
 "on" to "off".  Some channels were not affected.

 The 2 "AA"  memory  backup  batteries,  and  their  holder,
 tested  good.   Much  time was spent tracing logic, heating
 and cooling components, and making voltage measurements.

 One of the secondary leads from the power  transformer  was
 connected  to the main circuit board through a hole drilled
 through foil traces on both the top and bottom sides of the
 board.   A  close  examination  revealed that this lead had
 been soldered only on the top of the  board  -  the  bottom
 side had never been soldered.

 Soldering the lead on both sides of the  board  solved  the
 memory loss problem.


                   Symptom: Blank Display

 The BC300 scanner, and several other Bearcat models, employ
 a  switching  type  power supply stage to generate plus and
 minus voltages  in  excess  of  20  volts  DC.   When  this
 switcher fails to function, the display goes blank, but the
 squelch control appears to work, and  white  noise  can  be
 heard in the speaker.

 In almost a dozen of the BC300 scanners I've fixed, C98,  a  |
 capacitor  in  series  with  the  primary  of the switching
 transformer failed, causing the output  of  the  supply  to
 drop  below  the  level  needed  to power the display.  The
 switching transformer is mounted on the RF  circuit  board,
 and  is much, much smaller than the main power transformer,
 which is  usually  fastened  to  the  metal  chassis.   The
 22uF/16V  capacitor  used  for  C98  in  early  BC300s  was
 marginal, and was replaced with  a  47uF/25V  capacitor  in
 later units.

 I replaced the 22  uF  capacitor  in  the  switching  power  |
 supply  stage  of  a BC210XL which caused the same symptom.
 Other capacitors in the switcher stage have failed.   C114,
 a  4.7  uF/35V  tantalum  capacitor  failed in at least one
 BC250, causing the display to blank.

 A more sinister problem affects  the  switcher  in  earlier
 models.   The  switching  supply  stage  in  the  BC250 and
 original BC210 is driven by a clock signal derived  from  a
 custom  Exar NC57902 divider integrated circuit (designated
 IC6 in the BC250 scanner).  I've seen this divider IC  fail
 in  several  BC250s,  causing a blank display (except for a
 decimal point in the BC250's rightmost digit).  This custom
 IC is no longer available from Uniden.


            Symptom: Invalid Frequency Displayed

 A common Bearcat 250 malady is  manifested  by  an  invalid
 frequency  displayed  on  the  readout.  This  condition is
 temporarily "cured" by unplugging the AC line cord from the
 wall, then replugging it.  This condition is symptomatic of
 a power supply problem in which Q204, a  Texas  Instruments
 TIP-29 located on the feature circuit board, fails.

 A Philips ECG291 will work as a substitute for the  TIP-29.
 Don't  try a Radio Shack substitute, it hasn't worked.  See
 Martin Toomajian's article, "Bearcat  250  Erratic  Display
 Cure", in January 1987 Monitoring Times.

 A similar  problem  in  the  Bearcat  20/20  was  discussed
 previously in the section on bad solder joints.


        Symptom: Squelch Won't Eliminate White Noise

 Most Uniden/Bearcat base/mobile scanners  feature  an  AUTO
 squelch  position,  actuated  by  rotating the squelch knob
 fully  counter  clockwise.   The  BC350  used  a   separate  |
 pushbutton  switch  for this purpose.  These scanners use a
 flimsy potentiometer (designated R81 in BC300s)  internally
 mounted on the RF circuit board, to set the level of signal
 required to open the squelch when  in  the  AUTO  position.
 This  pot  also  has an effect on the squelch action in the
 non-AUTO mode, and determines at which  point  the  squelch
 knob must be positioned in order to silence the radio.

 Although the potentiometer  is  adjusted  at  the  factory,
 changes  in component values due to aging often necessitate
 readjustment of this internal pot.  Misadjustment  of  this
 pot  has  been  the cause of "no squelch" complaints in two
 BC300s and a BC250 I fixed.

 Another squelch failure is due to a blown  transistor  that
 acts  as  the  electronic switch in the squelch circuit.  I
 replaced this transistor in only one BC300, so I don't know
 if this is a common problem.


              Symptom: Scanner Completely Dead

 In Bearcat scanners using an internal power  supply  (e.g.,
 BC350,   BC250,   etc.),  the  main  power  transformer  is
 connected directly to the AC line.  Since the on/off switch
 is  on the secondary side of the transformer, current flows
 in the primary as long as the AC line cord is plugged  into
 an   active  AC  outlet.   These  transformers  contain  an
 internal circuit breaker,  not  visible  without  unwinding
 (destroying)  the  transformer.   The  internal  breaker is
 known to fail prematurely  in  a  batch  of  Bearcat  power
 transformers.

 If your scanner is completely dead, check  the  primary  of
 this transformer for an open circuit condition.


                  Symptom: Keyboard Bounce

 After  much  use,  the  Chromerics  keyboards  in   Bearcat
 scanners  start  to wear out.  The first sign of trouble is
 usually keyboard bounce on the most  frequently  used  key,
 e.g.,  the  MANUAL  key.  Replacement keyboards are usually
 available from UNIDEN, but replacement requires  dexterity,
 as  one must take care not to tear the flat, flexible strip
 connecting the keyboard to the logic board.


          Symptom: Keyboard Completely Unresponsive

 The keyboard matrix is  "scanned"  by  the  microprocessor.
 Another problem is when none of the keys seems to function;
 the  receiver  just  keeps  scanning  in   spite   of   key
 depressions.   I  found this condition in a BC210XL scanner
 owned by a heavy smoker.  Perhaps  nicotine  smoke  was  to
 blame, as the resistance between two input port pins on the
 microprocessor was down to about  1000  ohms,  fooling  the
 microprocessor  into  believing that a key was stuck in the
 "down" position.  Scraping the circuit  board  between  the
 two pins with an X-Acto knife fixed the problem.


                       Other Problems

 Complaints of low audio output and occasional  microphonics  |
 in  two  Uniden/Bearcat 800XLTs were caused by a bad 47 ufd
 electrolytic  capacitor  in  series  between  the  external
 speaker jack and audio amplifier.
--

==============================================================================
                       Copyright 1995,  Bob Parnass, AJ9S
           AT&T Bell Laboratories  -  parnass@att.com - (708)979-5414



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