SB-102 Help, please! Mic output way low

[Kenneth G. Gordon]

On 22 Mar 2009 at 13:58, Alexander N. Gerli Sr. wrote:

> Hi, Gang!
> 
> I'm cleaning up my SB-102 and I have a nagging problem with very low
> mic output. There's some output, but like 5W or so on peaks
> 
> Puts out full power on Tune and CW. And my mic's OK with my SB-101.
> 
> So, something in the mic amp chain ain't right. Can someone give me an
> idea what to chase down?
> 
> Sure appreciate it!

First, check the voltages around V-1, 6EA8. In an HW-101 I 
have here, the resistor feeding voltage to the screen was 
open.

BTW, some of the CORRECT voltages around that tube 
may appear to be rather strange, but they are correct. 
Check your manual. For instance, the plate voltage is lower 
than the screen voltage.

There is also a cathode bypass capacitor, a small 
electrolytic, in that circuit that can open, and reduce mic 
gain.

Secondly, check the diodes in the balanced modulator. 
Those are originally 1N34s, which tend to deteriorate over 
time. I have also found one or more of those installed 
backwards, open, or shorted.

They really should be replaced with Schottky diodes, 
available from Mouser.

Thirdly, check ALL the tubes between the mic and the 
balanced modulator, including those involved with the VOX 
circuit. ALL of those can cause problems with low audio.

After you have done these things, get back with us.

Ken Gordon W7EKB

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