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Repair Tips for Bearcat Scanners


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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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