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

[Dennis Wade]

Thank you all very much.  I've already isolated the short to the RF deck
with the unplugging modules technique.  I will pull the tubes and do the
measurements possibly tonight, more likely tomorrow night.

       Thanks again....I love this list.

                Dennis

On Wed, Nov 19, 2014 at 10:07 AM, Roger Ruszkowski <
flowertime01 at wmconnect.com> wrote:

>
> 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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-- 
"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