Subject: Re: BC250 problem, any fixes out there?

From: [email-masked] (Bob Parnass, AJ9S)
Date: Tue, 21 Jan 1992 14:08:32 GMT

In article <[email-masked]>, [email-masked] (Dan Schein) writes:
 
>    My old stand by scanner (Bearcat 250) has developed a problem....
>  all 50 channels have the same freq (9xx.xxx) with delay
>  any lockout set. When I try to reprogram a channel it goes right back to
>  the (bogus) 900 frequency.

            REPAIR TIPS: UNIDEN/BEARCAT SCANNERS

                     Bob Parnass, AJ9S

                Bearcat Schematics Available

Schematic diagrams may  be  obtained  from  Uniden/Bearcat's
parts  department,  (800)428-5455.   Don't be discouraged if
the line is busy, keep trying.  Product information  may  be
obtained by calling (800)722-6637, although it might be dif-
ficult to get through on the first call.

                  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.

-- 
===============================================================================
Bob Parnass, AJ9S                                             att!ihlpm!parnass
Distinguished Member of the Technical Staff                       (708)979-5414
AT&T Bell Laboratories                         Knife collector, treasure hunter