[R-390] R390 B+ Current?

Sat Jun 1 15:56:58 EDT 2019

 John,
Thank you for the taking time to document this problem and of course your story on the hunt.
This is how you find problems and fix them.
But, you show once again that we can keep these radios operating.
Respectfully,
Roger Ruszkowski

-----Original Message-----
From: jgedde <jgedde at optonline.net>
To: 'Jacques Fortin' <jacques.f at videotron.ca>; 'r390a' <r-390 at mailman.qth.net>
Sent: Sat, Jun 1, 2019 4:33 am
Subject: Re: [R-390] R390 B+ Current?

Good morning folks,

I thought I had found it since the radio behaved well afterwards, but alas it was not to be.  I found that after the radio ran for a while (45 minutes at least), suddenly the hum would return.  When the radio cooled off, all would be well again.  

Putting a desk fan next to the radio delayed the onset, but it didn't stop it.  So, clearly I had some kind of thermal issue.  I was thinking a cracked resistor.  

Inspection showed nothing obvious so I used my SMT hot air rework iron to heat components one by one after the radio had cooled.  Lo and behold, the hum returned almost immediately.  So, a thermal issue was confirmed.  Problem is, after heating the parts, all I did was make the hum become persistent - even when cold.  But what did I heat that caused it?  After heating so many individual parts, inevitably all in the area got hot.  Except now when they cooled off they didn't become good again.

Still thinking a resistor issue, rather than go through the trouble of multiple surgeries replacing resistors one by one, I just replaced them all (in the regulator circuit).  This made no difference whatsoever!  Damn!  I tried different tubes (6082's, 6BH6 and 5651's), it made no difference.  So, not a tube, and not a resistor and not a capacitor (or so I thought since I had already replaced the two cans).  

Staring at the schematic yielded no good thoughts.  Staring at the underside of the af deck, I noticed something I hadn't noticed before: a SM polka dot box capacitor buried down on the floor (or is it really the ceiling?) between pin 5 of the 6BH6 and ground.  What is that doing there?  I didn't see it on the schematic - but it's there...  Off to the right of the 6082's - between the grids of the 6082's and ground - C607.  

This morning I dug it out of there after removing two layers of resistors and replaced it with a CD dipped silver mica.  (Before I pulled it, I clipped one end as a test and powered up the radio to see if it did anything profound - hummed really bad!)  

Now the radio is all put back together and hum free!  The B+ supply only has about 25 mV of ripple after adjusting the hum balance.  All's finally well I think.  So, all in all, there were multiple problems with my B+: leaky oil/paper caps a bad resistor, two bad tubes, charred cathode resistors in the reference tube string. 

To make a long story short, If you ask me in simple terms why the radio hummed, I'll answer "because it doesn't know the words..."  😊

John

-----Original Message-----
From: jgedde at optonline.net <jgedde at optonline.net> 
Sent: Sunday, May 26, 2019 4:28 PM
To: 'Jacques Fortin' <jacques.f at videotron.ca>; 'r390a' <r-390 at mailman.qth.net>
Subject: RE: [R-390] R390 B+ Current?

I found the problem and fixed it.  Apart from the two charred resistors in the cathodes of the 5651's, it was a slightly leaky C608 and one other bad resistor (R616).  That one was tough to find as it tested at 86k (versus 82k it should be) when it was cool.  As the radio heated up, it changed in value - a lot.  I should have changed it from the start because I didn't like the look of it.  It was dull in appearance not shiny like the rest of the resistors.  Weird.

I also brought my AF deck up to MOD 4.

I now have a B+ supply of 181.7V with 160 mV p-p of ripple (and a room heater to boot.)

John

-----Original Message-----
From: Jacques Fortin <jacques.f at videotron.ca> 
Sent: Sunday, May 26, 2019 10:01 AM
To: jgedde at optonline.net; 'r390a' <r-390 at mailman.qth.net>
Subject: RE: [R-390] R390 B+ Current?

Hello John,

>From what you describe (the hum balance drifts as the whole radio is heating
up) I guess that it is the 6BH6 transconductance that changes when it is heating up, which is more or less an indication of an end-of-life tube.
The "gain" of the 6BH6 should change as it is heating up: the hum balance adj. role is to vary the open-loop gain of the whole regulator, balancing the hum part of the voltage feeding the 6BH6 plate with the amplified version of the output hum coupled by the C608 to the grid.
Maybe it is the time to find a "real" tube tester of the mutual conductance type, at least.
Or better, a parametric tester.
Or to find someone nearby that can test the tubes for you.
It can be a leakage internal to the 6BH6 also, and tubes testers are not all equal at detecting that type of defect.

73, Jacques, VE2JFE

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