HW-16 HUM Problem - Fix

[Brian]

I have just bought an HW-8 and was wondering if anyone on the list could
fill me in on common problems that this radio has. Or, any mods which will
make it a better radio to use!

                                    Thank you!


----- Original Message -----
From: "Mark V. Johnson VE3DJU" <mjohnson at CA.IBM.COM>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Friday, December 24, 1999 5:07 PM
Subject: Re: HW-16 HUM Problem - Fix


> 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!!
> >
> > Sponsored by the City of Tempe
> >
> > Listserver Submissions:  heath at listserv.tempe.gov
> > Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath
> 'name' 'call'"
> > Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"
>
> Sponsored by the City of Tempe
>
> Listserver Submissions:  heath at listserv.tempe.gov
> Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath
> 'name' 'call'"
> Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"
>
> Sponsored by the City of Tempe
>
> Listserver Submissions:  heath at listserv.tempe.gov
> Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath
'name' 'call'"
> Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"
>

Sponsored by the City of Tempe 

Listserver Submissions:  heath at listserv.tempe.gov
Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"