 [40] SCANRADIO (9:1992/111)  SCANRADIO 
 Msg  : 50 of 130                                                               
 From : Roy Cloutier                1:115/881               Sat 13 Feb 93 00:35 
 To   : Gary Westfall                                                           
 Subj : r                                                                       

              REPAIR TIPS: UNIDEN/BEARCAT SCANNERS

                       Bob Parnass, AJ9S

                    Bad Solder Joints Common

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

  The BC350, BC300, BC250, BC220, BC20/20, BC211,  BC210,  and
  BC210XL  models were hand assembled, and every one I've ser-
  viced 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 cool-
  ing 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 two of the BC300 scanners I've fixed, C98, a capacitor in
  series  with  the  primary  of  the  switching  transformer1
  failed,  causing  the output of the supply to drop below the
  level needed to power the display.  The  22uF/16V  capacitor
  used  for C98 in early BC300s was marginal, and was replaced
  with a 47uF/25V capacitor in later units.

  I recently replaced the 22 uF  capacitor  in  the  switching
  power  supply stage of a BC210XL which caused the same symp-
  tom. 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 origi-
  nal BC210 is driven by a clock signal derived from a  custom
  Exar NC57902 divider integrated circuit  (IC)2.   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 fre-
  quency  displayed  on  the  readout.  This condition is tem-
  porarily "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.3

  A similar problem in the Bearcat 20/20 was discussed  previ-
  ously 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 pushbut-
  ton  switch  for  this purpose.  These scanners use a flimsy
  potentiometer4 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 pri-
  mary 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 prema-
  turely 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 key5.   Replace-
  ment  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  depres-
  sions.  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.

  __________

   1. 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.

   2. The EXAR chip is designated IC6 in the BC250 scanner.

   3. See Martin Toomajian's article, "Bearcat 250 Erratic
      Display Cure", in January 1987 Monitoring Times.

   4. Potentiometer designation R81 in BC300s.

   5. The MANUAL key usually fails first.


--- Echodor 3.11
 * Origin: JIM'S FlyInn BBS! Chicago Il - 312-881-1142 - (1:115/881.0)

