HW-16 HUM Problem - Fix

[Mark V. Johnson VE3DJU]

Hello wolfbob at csnsys.com

Thanks for the reply Bob...Appreciate the feedback..I've found a
"discussion' often leads to ideas....Yes I did replace the audio output
tube and got the exact same problem...Also, I checked grounding on the
circuit board and even tried probing at ground with a jumper attached to
GND. That is, with one end attached to GND, I touched the other end to
various gnd spots on teh circuit board..no change...I also thought I had a
cathode to htr short in the other tubes...I pulled ALL tubes except the
6HF6 and still got hum....I then checked out the htr wiring circuit and
made sure it was grounded OK..No fix....

I tried putting grounded tube sheilds around the tubes..No change...I tried
using a sheilded pwr lead from the pwr supply to the connection pt on the
circuit board..No change. I traced the audio cables back to the AF gain
control...All gnds OK, any attempts to jumper past..No change.......A very
HAIRY BUG!..Thanks again for the ideas Bob!

Bob <wolfbob at csnsys.com> on 12/24/99 03:52:33 PM

Please respond to wolfbob at csnsys.com

To:   wolfbob at csnsys.com
cc:
Subject:  Re: HW-16 HUM Problem - Fix




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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