[Heathkit] Fixed: was: HW-16 audio troubles

[Tom NØJMY - AAR7FV]

Thanks to all that responded.  For anyone interested, my problem was an 
unacceptable level of hum in the speaker when the AF gain was turned 
down.  This seems to be a problem with lots of HW-16's I've had over the 
years.

I tried the old trick of un-grounding the shields of the audio cables at 
one end -- to no avail.

In the end the problem was cured, rightly or wrongly, by replacing and 
changing the value of C87, which which bypasses the control grid of V6B, 
the audio power amp half of the 6FH8.  This was originally a .001 uf. 
disk cap.  I replaced it with a .01 uf.

I experimented with various values using a capacitor substitution box, 
although it and the attendant hook-up wires exacerbate the problem 
somewhat themselves.  Generally, the larger the value of the cap the 
greater the hum reduction (through about .047 uf. where it was 
imperceptible).  However, the gain of the amp started to take a hit 
above about .0033.  I settled on .01 as a good compromise, as the 
decrease in gain was barely noticeable, and once the jumper wires were 
removed and the installation was made permanent, the hum virtually 
disappeared.  Maybe a .0068 would have sufficed.

I recapped the power supply prior to this, BTW.  It's possible that this 
might be an uncouth "brute force" approach to a problem whose origins 
are elsewhere in the rig, but hey, that's what God created us uncouth 
brutes for.

Interested in any other comments you might have,
Tom