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

Sat Dec 13 17:57:47 EST 2014

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

On Mon, Dec 8, 2014 at 10:09 PM, Dennis Wade <sacramento.cyclist at gmail.com>
wrote:

> Hi Craig,
>
>           To answer your question...I'm not completely sure because it was
> several years ago that the problem surfaced.    At the time I couldn't
> trouble shoot it and the radio went into storage for several years.  As
> best as I recall, it was the 1/8th amp fuse that blew.
>
>           At that time I was running the tube rectifiers. and that's what
> I kept in there till I found the short.  At that time, I swapped in some
> solid state replacements from Sigmatec that had no impact on the hum.
>
>         I will work on checking the power supply.  Thanks
>
>                 Dennis
>
> On Mon, Dec 8, 2014 at 6:26 PM, Craig Heaton <hamfish at efn.org> wrote:
>
>> Hello Dennis,
>>
>> I'd start with the power supply. First the 26Z5W's or their diode
>> replacements, check them. Then move on to L601 &L602, next stop would be
>> the
>> filter caps C606A&B. You stated the receiver was in daily use till the cap
>> failure, so this could be collateral damage. The filter caps should be
>> tested for value and leakage at rated voltage.
>>
>> Question: Which fuse blew with the cap failure?
>>
>> Craig,
>>
>> -----Original Message-----
>> From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Dennis
>> Wade
>> Sent: Monday, December 08, 2014 5:19 PM
>> To: R-390 HF Receiver List
>> Subject: [R-390] Audio HUM WAS: B+ short in RF deck
>>
>> Good evening,
>>
>>        First I want to be sure and thank everyone who contributed to the
>> previous thread regarding the shorted B+ line in my RF deck.  The shorted
>> cap was located and replaced, as well as the resistor nearby that was
>> taken
>> out when the cap shorted.
>>
>>         Reassembled the radio and powered on.  Loud 120 cycle hum with
>> some
>> signals audible underneath.  The local gain control varies the hum volume,
>> and its present in both line and local channels.
>>
>>           In looking through the Pearls, I see a couple of references to
>> the
>> 120 cycle feature being a sure sign of filter cap degradation, but the
>> writer said the hum wouldn't vary with the gain setting.  60 cycle hum was
>> felt to be associated with bad grounds, bypassing, etc.  The filter caps
>> were replaced above chassis about 10-12 yrs ago.
>>
>>            I need a few of pieces of advice..(at least :) )   First, how
>> can I systematically isolate the offending stage?  Being that the hum is
>> controled by the local gain suggests a stage before the 1st audio stage,
>> but
>> the 120 cycle part confuses me.
>>
>>           Second, I'd like to measure the ripple on the B+ line.  Is
>> there a
>> location of choice to make that measurement, and what kind of values
>> should
>> I be looking for?
>>
>>            Third, could there have been other collateral damage from the
>> shorted RF deck cap that might have triggered this him problem?
>>
>>            Roger's plan to take readings from each of the RF deck plates
>> to
>> ground was the "magic bullet" that led me to the offending cap (Thanks
>> Roger!).  I'm hoping for a similar gem here.
>>
>>                 Well I can be optimistic, can't I?  :)
>>
>>                    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
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>>
>>
>
>
> --
> "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
>

-- 
"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