[R-390] Audio HUM WAS: B+ short in RF deck

[Dennis Wade]

I found a 10 uf cap at an appropriate voltage and hung it across the B+
line.  Dramatic reduction in hum.  So I'll be ordering some caps.
Hopefully that will let me get on with an alignment.

The thing is, these aren't the original caps!  I replaced those a while
ago, and really didn't expect to have to do it again. That's what was
confusing me (in part).C606 and 603 were down on the suspect list since the
original parts were replaced before.

 Lesson learned I suppose.

For those who might be curious, the caps I used previously (and apparently
failed) are "Xicon" 33 uf (or 45 uf) at 350 V axials mounted on octal
headers.

Thank you all for your learned advice.  :)

              Dennis

On Sat, Dec 13, 2014 at 3:25 PM, John Kidd <u453902 at outlook.com> wrote:
>
> Hi,
>
> In my day we got a pigtail cap of around 45uf, handheld grounded one end
> and
> touched B+ to check if original caps were doing their job. If hum went the
> caps
> were the problem. Be careful where you put the test cap when you have
> finished.
> I would just short it, but then I'm rough.
>
> John
>
> > Did more troubleshooting last night and this morning.
> >
> > To briefly review, unit was blowing fuses caused by shorted C248 which
> > caused R205 to roast. Both replaced.
> >
> > Unit now has strong 120 cycle hum. Hum is controlled by local AF gain.
> > Present in both audio channels.
> >
> > Can see with the scope very large 120 cycle ripple on RF/IF B+ line at
> the
> > B+ fuse and plate of V201 as well as the plate of the 1st audio amp.
> > Didn't check other plates. V601 and 602 were both swapped out for new
> > tubes with no change.
> >
> > Checked L601, 602 and 603 for DC resistance with a VTVM, they are within
> > spec. Can't check the inductance.
> >
> > Caps C603 and 606 were previously replaced by axial caps installed on
> octal
> > headers and plugged into the original sockets. 33uf/350 and 45uf/350 caps
> > were used. They were checked with the VTVM and do not appear shorted or
> > open. Can't test for leakage at rated voltage.
> >
> > I'm at a bit of a loss now as to what to do next. I have a feeling its
> > something obvious that I'm missing....
> >
> > Thoughts?
> >
> > Thank you,
> >
> > Dennis
>
>

-- 
"If they trust you, it is an extraordinary privilege, and you simply can't
abuse it."
      - A. Alan Post  1914-2011.  California Legislative Analyst 1949-1977.

-------------------------
Dennis L. Wade
KG6ZI
Carmichael, CA