HW-16 HUM Problem - Fix

[Ed Richards]

Mark;
Glad you are happy now. It should not be necessary to modify the circuit
to correct the problem. Sounds like you may have a ground loop. Try
disconnecting all grounds and connections to other equipment  (antenna,
tuner, VFO, etc. If you have a 3 wire power cord on it, use a
ground-breaker and have it sitting on an insulated surface , isolated
from everything else. If the hum goes away you have a ground loop. If it
persists, you have a broke radio. Good luck and Merry Christmas.
Ed Richards

On Fri, 24 Dec 1999 13:48:25 -0500 "Mark V. Johnson VE3DJU"
<mjohnson at CA.IBM.COM> writes:
> 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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