SB-301 bfo adjustment

[Kenneth G. Gordon]

On 16 Nov 2006 at 19:50, Dave Edwards wrote:

> I'd be curious to hear from Heath owner's who have done these mods.

I have done them to a couple of HW-101s and to an SB-101.

> I
> know my Heath stuff always had a thin, tinny sound on receive. Back in
> '75, that is why I traded mine in towards a TS520. The SB's and HW's
> always had a sound that gets tiring to listen to after a fairly long
> time.

Yes. After making the mods shown in that article, plus another one I 
have used with good effect over the course of 40 years, the audio 
output of my modified HW/SB rigs is quite pleasant to listen to for me, 
and I am a muscian.

The added BFO injection mod is especially important, although the 
other audio mods certainly make the audio much more pleasant to 
listen to.

My OTHER mod, one I got from an article in an old Hint and Kinks, is to 
remove the large capacitor which is mounted under the audio circuit 
board, which connects pin 7 of the 6GW8 audio output tube to the 
UPPER connection (as shown on the schematic) of the audio output 
transformer. I cannot read the "C" number on the schematic I have at 
the moment, but I believe it may be C-912, or possibly C-913, 0.05 mfd.

This capacitor adds an unnecessarily large amount of negative 
feedback and makes it necessary to run the audio gain control near the 
top of its travel in order to get enough output.

After removing this cap, I run my audio gain control in the bottom 1/3 of 
its travel, and if I need to crank it up, I get more than enough audio 
output.

Another extremely simple mod I can recommend is to replace V-1, a 
6EA8, with a 6GH8A. This increases the mic gain quite a bit with every 
mic I have tried, and, again, I can now set my CW/MIC LEVEL control 
in the bottom 1/3 to 1/2 range, instead of almost fully open.

Ken Gordon W7EKB

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