[R-390] R390-URR BFO Problem

[David Wise]

I restored my fading-out BFO by augmenting the valve with
a transistor, an emitter follower driven by the cathode.
Works great.

Dave Wise 

>-----Original Message-----
>From: r-390-bounces at mailman.qth.net 
>[mailto:r-390-bounces at mailman.qth.net] On Behalf Of 
>g4gjl at btopenworld.com
>Sent: Wednesday, June 09, 2010 4:50 PM
>To: R-390 at mailman.qth.net
>Subject: [R-390] R390-URR BFO Problem
>
>Im looking for group wisdom and experience with an 
>intermittent BFO in my R390-URR.
>
>The set is in average condition but is very hot, much better 
>than either of my R390A-URRs.
>
>The problem is that the BFO is intermittent.
>
>The failure mode is a slow fade out of BFO signal, though no 
>audible change in BFO frequency, until just before it drops 
>out completely. This occurs about 30 mins to 1 hour after 
>initial warm-up. I have found that removal of the tube will 
>re-start the oscillator. Power-cycling the BFO switch will not 
>re-start the oscillator.
>
>My actions thus far are as follows:
>1 Changed valve (tube) for a NOS British equivalent. Result:  
>No change in nature of fault.
>2 Inspection of valve holder reveals no visible sign of tracking.
>3 Inspection of valve holder reveals no obvious dry joints.
>4 Inspection of valve holder reveals no obvious bent pins or 
>loose pin sockets.
>5 Last night I replaced all three HT feed resistors. Result no 
>change in the nature of the fault.
>6 Today I removed the BFO PTO and built a new oscillator to 
>test the PTO on a separate chassis. I used the same valve but 
>all new resistors and capacitors. I ran the oscillator into my 
>HP frequency counter and it remained on and stable at 455.03 
>kHz  for 30 mins., it then 'failed'. Interestingly the 
>'failure' resulted in the oscillator squegging at about 84MHz. 
>So I guess that is exactly what is happening when the PTO is 
>installed in the IF strip. 
>One faulty BFO  PTO......
>
>Now the questions.
>Does anyone have similar experience with this PTO, or any 
>suggestions as to how they might fail?
>
> How I should set about opening and repairing the PTO?
>
> Does anyone have any pictures or text description of the 
>opening procedure?
>
>Looking forward to replies and thanks in advance for any help.
>
>Pete
>G4GJL
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