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

[Craig Heaton]

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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