[Heathkit] SB-104A low tx audio problem

[Jim Shorney]

Interesting, my pristine 104A came to me with a bad 3900 in the mic stage. It's
been long enough that I don't recall if the symptom was complete loss of audio,
or if a little was getting through. Odd, though, as 3900s seem to be pretty
reliable (a previous employer of mine used them in fire alarm controls). They
are also common as dirt, I had one in my junque box. Can't hurt to swap it out.


73

-Jim
NU0C

On Mon, 13 May 2013 23:11:56 -0400, Alexander N. Gerli wrote:

>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

--
“There’s something out of place – let’s go and poke it with a stick.” – The Doctor, "Amy’s Choice"