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

[Dennis Wade]

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