[R-390] RF Deck B+ short (390A)

[Roger Ruszkowski]

Dennis,
 
Run through the band switch and see if the short exist on all octaves.
B+ runs through several of the RF tuning circuits and some times a bit of fuss will get lodged in the RF transformers and give you a short.
Also pull all the tubes and check the resistance again.
 
Measure from the tube plate pins to ground.
You should see the plate resistor value.
If you see more then the short is not in that B+ leg.
 If you see less then look for the short between the tube socket plate pin and the plate resistor.

Just some idea on how to probe the circuit and divide the problem down.

Roger AI4NI
 
 
-----Original Message-----
From: Bill Hawkins <bill at iaxs.net>
To: 'R-390 HF Receiver List' <r-390 at mailman.qth.net>
Sent: Wed, Nov 19, 2014 2:09 am
Subject: Re: [R-390] RF Deck B+ short (390A)


Problems like that are best solved by successively cutting them in half.

Pick a place on the B+ line about halfway along the schematic and
unsolder it. Now you only have half the RF deck to search, so you 
cut that one in half, and so on until you can't miss it.

Bill Hawkins


-----Original Message-----
From: Dennis Wade
Sent: Tuesday, November 18, 2014 11:15 PM
To: R-390 HF Receiver List
Subject: [R-390] RF Deck B+ short (390A)

Good evening,

      I've finally pulled the RF deck out of my Motorola 390a in an
attempt to track down a short on the B+ line.  My initial resistance
measurement suggested a dead short which lead me to C 308.
Unfortunately, removing
C308 left the short happily intact.

      Paying a little closer attention this time, the actual resistance
to ground from Pin A, J208 is 60 ohms, not a dead short.  This confuses
me...I'd think I'd be seeing R212 at 2.2K if C280 was shorted...but the
60 ohms is throwing me.  But, its not immediately obvious what else it
could be.  Could R212 have changed to 60 ohms?  (it doesn't look
abused).

        Thoughts?

           Thanks,

              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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