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

[Dennis Wade]

Good evening all.  I hope everyone had a wonderful holiday.

Well it took a little longer to get back to this than I thought, but I did
pull the tubes and looked at the resistances from plates to ground.  V201
shows a dead short, and the plates of V202, 203 and 204 show about 2.4K
ohms to ground instead of the 20K nominal the manual specifies.  I get
infinite resistance from V205A and B, 206 and 207, but the deck is out of
the mainframe.

Looking at the schematic, my untrained eye sees that C248 (5000 pf to
ground) is a likely suspect, with C254 and C276 being less likely.  The
2.4K I'm seeing on the other plates is likely R205.

Thoughts?  Think I'm getting warm?  :)

           Dennis

On Wed, Nov 19, 2014 at 12:03 PM, Dennis Wade <sacramento.cyclist at gmail.com>
wrote:

> Ah, ok..great info about L209.  I'll keep that in mind.
>
> I will certainly keep all informed...but maybe not till Thurs night.
>
>        Thanks again.  D.
>
> On Wed, Nov 19, 2014 at 12:01 PM, Grayson Evans <wa4gvm at gmail.com> wrote:
>
>> A little more info for you.
>> My RF deck is on the bench, tubes out, everything disconnected including
>> the coils
>> I measure 125K from pin A to ground.  This high value is probably due to
>> the coils being unplugged.
>> It is very hard (for me) to trace the B+ line under all that circuitry,
>> so it might be hard to disconnect someplace convenient.
>>
>> Notice that L209, the RF choke that is attached to pin A is 25 ohms, so
>> your short is really 60-25 or 35 ohms.
>>
>> With the tubes removed, try measuring from the tube plate pins on the
>> sockets to ground and see if you see anything weird.
>>
>> You got me interested, so let me/us know what you find.
>>
>> Grayson
>>
>>
>> Grayson
>> TA2ZGE - Ankara, Turkey
>> KJ7UM
>> *Follow the Hollow-State Design Blog*
>> *hollowstatedesign.tumblr.com <http://hollowstatedesign.tumblr.com>*
>>
>>
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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