[Heathkit] SB-104A low tx audio problem

[William Houghton]

Sandy,

I had the exact same problem with my first SB-104A. No problem tuning up, a 
whistle blow would drive it to full power, but for normal speech I had to turn 
the mic gain up to the point where the audio amp broke into oscillation. After 
struggling with it for a long time I decide to connect an audio generator to the 
mic input. What I found was that it took a much larger signal to drive it at 
300Hz than it did at 1000Hz. From about 1kHz and up the drive required was 
fairly constant. 


I solved this problem by replacing C204, 0.01uF cap, with a 0.1uF cap. I now 
make this change to every "B" board that comes across my bench.

73,

Bill




________________________________
From: Alexander N. Gerli <ac1wye at gmail.com>
To: "heath at puck.nether.net" <heath at puck.nether.net>; heathkit at mailman.qth.net
Sent: Mon, May 13, 2013 8:12:09 PM
Subject: [Heathkit] SB-104A low tx audio problem

Hi, gang!

Well, now that the rx is working a lot better, I have a new problem - LOW
tx audio. I can get full power in tuneup, that's fine. With the mic level
set at 50%, I can get full-power with a whistle-up. But, nearly nothing
when speaking. I'm using a good Heathkit mobile mic I've had for years, the
Turner 350 that was made for Heath.

Looking at the boards, I suspect something is amiss on Board B, the
Transmit Audio Board. Looking at the schematic, I'm wondering if the Q201,
the quad OP-AMP could be the culprit? This provides the two stages of
amplification. LM3900N chips are pretty common and available. Before I head
to the 'Bay and get one (or maybe my local RS store might have one on a
blister-card), does anyone on the list have any similar experience like
this, and what did you discover?

When I got this 104A three years ago, it ran great and has been a solid
rig. But, as with anything like this, past history is, well, just that.

Thanks tons for your kind help, folks!

73,

Sandy AC1Y
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