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