Newsgroups: alt.radio.scanner
From:
[email protected] (Bob Parnass, AJ9S)
Subject: Improved audio for the PRO-43 portable scanner
Organization: AT&T Bell Laboratories, Naperville, IL
Date: Wed, 2 Dec 1992 01:15:35 GMT
Message-ID: <
[email protected]>
Keywords: scanner radio, audio, modification, midband sensitivity
Lines: 45
x
IMPROVED AUDIO FOR THE PRO-43 SCANNER
by Bob Parnass, AJ9S
The Radio Shack PRO-43 scanner audio is too bassy.1 The
lack of treble makes it difficult to hear the PRO-43 in
noisy situations, especially while listening in a car
or truck. Louis Shirley sent me a schematic and sug-
gested I remove C341, a tiny 0.015 ufd surface mount
capacitor.
I'm glad to report that removing C341 made a great
improvement. The audio is now much "crisper," more
like the Uniden 200xlt, although still not quite as
loud. Turning the volume control up still overloads
the PRO-43's small, internal speaker, but there's less
need to do that once C341 is removed.
The PRO-43 contains 3 printed circuit boards, and the
middle (second) board contains the audio circuitry.
Finding C341 is difficult, as it is neither marked with
a value nor a component designation. It is located on
the foil side of the middle board, under IC304, an
LM386 amplifier IC. C341 is in parallel with, and phy-
sically next to, R350 (33,000 ohm). R350 is slightly
larger than C341 and is marked 333. Both R350 and C341
are connected between pin 2 of the LM386 and ground.
A Note About 50-88 MHz Reception
Now that I have a schematic, I see that the European
version of the PRO-43 has different coils and capaci-
tors in the low (mid) band front end filter. That
explains why the 75 MHz sensitivity isn't stellar after
adding diode D3 to enable 30-88 MHz coverage.
__________
1. See "PRO-43 Product Review," by Bob Parnass, AJ9S, in
the November 1992 RCMA Journal.
--
==============================================================================
Bob Parnass, AJ9S - AT&T Bell Labs -
[email protected] - (708)979-5414
