Newsgroups: alt.radio.scanner

From: parnass
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
Keywords:scanner radio, audio, modification, midband sensitivity

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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 whilelistening in  a car

     or truck.  LouisShirleysent mea schematic andsug-

     gested I remove C341, a tiny 0.015 ufd  surface  mount

     capacitor.



     I'm glad to reportthat  removingC341  made  a  great

     improvement.   The audiois  nowmuch "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'sless

     need to dothat once C341 is removed.



     The PRO-43contains 3 printed circuit boards,  and the

     middle  (second)  board  containsthe audio circuitry.

     Finding C341 is difficult,as it is neither markedwith

     avalue 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, andphy-

     sically next to, R350 (33,000 ohm).  R350is  slightly

     larger than C341 and is marked 333.  Both R350 andC341

     are connected between pin 2 of theLM386 and ground.



 A Note About 50-88 MHz Reception



     Now that Ihave 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 toenable 30-88 MHz coverage.



__________



 1. See"PRO-43ProductReview," by BobParnass, AJ9S, in

    theNovember 1992 RCMA Journal.



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Bob Parnass, AJ9S  -  AT&T Bell Labs  -  parnass