[Heath] SB-104-A TX weak audio

[JF]

On 10/2/2013 8:01 PM, Alexander N. Gerli wrote:
> Got a TX audio gain problem. And it's not my mic, as it happens 
> with several known good mics.I get perfect 100W out tuning up on 
> High Power, and 10W on Low Power. So, the finals and CW are 
> fine.But I have to run the audio gain wide open to be heard. Only 
> way I can get full-power on SSB is with a whistle into the mic. 
> Speech drops way off. and it's not a matter of meter damping in my 
> wattmeter; I've discounted that. This is a recent problem.Are there 
> any known mods to the speech amplifier (don't remember which board 
> that is, my manual is downstairs in the shack)? If there are any 
> good ones to share please e-mail me.

The SB104-A may not work well with many good mics, new or old, simply 
because they may be the wrong impedance and/or don't put out enough 
audio. The radio requires a high impedance (~50 K) magnetic mic with 
a lot of output like the old unamplified Shure 444, which I measured 
as putting out 8 times the peak-to-peak audio voltage of a Kenwood 
MC-50 even though they have about the same impedance at the hi-z 
setting.  I had to build an external mic preamp to get the MC-50 to 
work with it; some mics with built-in adjustable preamps might also 
work if they can get DC power from an independent source.

If a mic like the Shure 444 can't drive your SB-104A to full power at 
about half audio gain setting, then there is a defect somewhere in 
the internal mic audio chain.

73
John  KE5ZB
.