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

[Larry H]

Hope everyone had a nice Thanksgiving.

Dennis,  Looks like good progress.  Yes, C248 is a good possibility.  If not, your problem is probably in the plate coils for V201 - if any 1 of the coupling caps C249 through C254 short, all bands will be shorted to ground through the coils Z201-1 through Z206-1.  An easy way to track down which one would be to use an ohm meter that measures very low resistance fairly accurately and measure each one.  If that is not feasible, measure across C248 and pull each tuning assemblies just mentioned one by one.  You'll find it.

Regards, Larry


________________________________
 From: Dennis Wade <sacramento.cyclist at gmail.com>
To: R-390 HF Receiver List <r-390 at mailman.qth.net> 
Sent: Saturday, November 29, 2014 5:13 PM
Subject: Re: [R-390] RF Deck B+ short (390A)
 

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