Category : Tutorials + Patches
Archive   : BEARCAT.ZIP
Filename : BEARCAT.FIX

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.