Newsgroups: alt.radio.scanner
From: parnass@cbnewse.cb.att.com (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: <1992Dec2.011535.16065@cbnewse.cb.att.com>
Keywords: scanner radio, audio, modification, midband sensitivity
Lines: 45

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	      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  -  parnass@ihlpm.att.com - (708)979-5414