[R-390] R390-URR BFO Problem

Sun Jun 13 20:12:50 EDT 2010

Jon, Thanks for your help and especially the pictures you sent. They gave me the confidence I needed to open up the unit and fix it!

 I now have the BFO going and I have tested 
it several times today for up to 3 hours continuous.. The signal is 
steady and clean and has not failed.

The problem turned out to be a dry joint in the BFO PTO can. I cannot 
identify exactly which joint, as I re-soldered them all before putting 
the PTO back into my makeshift test oscillator on Friday. The BFO 
performed fine ever since, so I transplanted it back into the RX today.

I might get one approved by Art Collins from Fair Radio, and 
thanks for that tip, but as the set is ok now, I think Im happy it is 
fixed for the next 58 years or so!

Thanks once again.
Pete
G4GJL

________________________________
From: "g4gjl at btopenworld.com" <g4gjl at btopenworld.com>
To: Jon Schlegel <ews265 at rochester.rr.com>
Cc: R-390 at mailman.qth.net
Sent: Thursday, 10 June, 2010 8:51:19
Subject: Re: [R-390] R390-URR BFO Problem

Thank you for this, Jon.

 If it was easy for you to take a few pics of the interior of the PTO would be very helpful. I feared that the problem lies within the unit after all the HT feed resistors were changed.
Those N400s sound a bit of a nightmare, but the failure (major change of frequency) is not consistent with failure of these rather small value capacitors...well Im banking on that! 

Thank you for the Fair Radio tip. I might get one as a safeguard. Do you happen to know if the R390 BFO PTO is the same as the R390A one?

Pete
G4GJL

________________________________
From: Jon Schlegel <ews265 at rochester.rr.com>
To: g4gjl at btopenworld.com
Sent: Thursday, 10 June, 2010 2:17:03
Subject: Re: [R-390] R390-URR BFO Problem

Hi Pete,

I have an opened BFO PTO that I can photograph if that would help.  Please let me know.

L508 or L509 could be opening up.  When it goes into squegg mode, check for continuity from BFO PTO pin 1 to earth.  I'm reading virtually a dead short on my unit so anything above that is likely a problem.

As you know, there is not much inside the PTO that can go bad.  Unfortunately the parts are somewhat specialized and C555 and C556 are are selected for temperature compensation of the unit.  The tempco values for the capacitors chosen for my particular BFO PTO are both N400 (negative 400 ppm/degree C).  No doubt this is why the unit is "sealed".  These capacitors are rather large thin wafers of delicate looking ceramic.  I had to remove the slug tuned coil (L509) on mine to repair it and working around these fragile capacitors was a little nerve wracking.

"Fair Radio" in Lima, Ohio here in the states usually has these little PTO units if it comes to that.

Best regards,
Jon Schlegel
WA3MVM

At 11:49 PM 6/9/2010 +0000, g4gjl at btopenworld.com wrote:
> 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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