HW-16 HUM Problem - Fix

[Mark V. Johnson VE3DJU]

Hello all;

I just went through the  gruelling task of fixing an annoying HUM problem
on the receiver portion of an HW-16. Thought I'd share the details with the
group and I welcome any feedback!!

The radio worked fine when I got it except for an annoying hum at low
volume. It was not real loud but annoying and noticable when using
headphones. The real issue I had was when I turned the volume right down,ie
standby mode, you could hear the hum from 10 feet away. Drove me nuts!!!

I started with the usual fix..The power supply electrolytics. Replaced the
quad cap with a New Old Stock and I replaced the 2 single 50mfd units with
axial lead new units. No good. Thought maybe the New Old Stock cap was
leaky so I replaced it with four individual axial lead (brand new) units.
This was done without physical mods to the rig. I was able to mount all 6
axail lead caps under the chassis with terminal strips.  Kept the 3 can
style caps mounted on the chassis to retain the physical appearence. Still
no fix!!!

The replaced the 4 diodes in the voltage doubler circuit..No fix... Ready
to throw the unit out by this point!

I then found the hum was striclty in the last audio amp stage. Confirmed
this by pulling the 12AX7 Product Detector tube and still hearing the hum.
Disconnected the bias supply circuit at point E (D on the circuit board)
That was not the problem area.

The went hunting...Trial and error..found bypass capacitor C87, .005mfd
attached to control grid pin 7 of the 6HF8 was leaky! I replaced it and the
hum reduced by about 75%. I then up'd the value of C87 to .002 mfd, and the
hum is gone! Audio performance went down ever so slightly with the .002
cap, but its much much improved and back to what I think it should be.

Hope some of you get some value from my note. Thanks and All the Best of
The Season!!

Sponsored by the City of Tempe 

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