HW-16 HUM Problem - Fix

[Bob]

Mark.. Interesting and congrats on the symptomatic cure of the hum
problem. But, it doesn't sound like you found the problem. Did you try
replacing the 6HF8 ( a possible heater-cathode leakage) or checking the
grounds for high impedance. Something is getting AC into the stage.
Maybe the high impedance grid circuit is picking up a nearby heater
wire...

Jippy



"Mark V. Johnson VE3DJU" wrote:
>
> 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!!
>
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