[Heath] SB-104A low tx audio problem

[Alexander N. Gerli]

Hi Bill,  thanks SO much for that tip. I'm going to get that done ASAP. I
have a whole stash of caps like that. Should do the trick.
73, Sandy AC1Y

On Tuesday, May 14, 2013, William Houghton wrote:

> 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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-- 
God Bless You,

Alexander N. (Sandy) Gerli, Sr.
548 Allred Mill Road
Mount Airy, NC 27030-2202

Scotsmen walk about 900 miles each year.
A Scotsman drinks around 45 gallons of Whisky a year.
Scotsmen get about 20 Miles Per Gallon.
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