[R-390] R390 B+ Expected Tolerance

[Jim Whartenby]

Barry
I don't think that there is a real problem here.  There is no regulated voltage adjustment so whatever the tolerances are will set the voltage.
The 5651 has an average regulated voltage of 87 volts +/- 5 volts, per the datasheet.  
With two in series, I would expect that the uncertainty would be 174 volts +/- 7 volts.  
R624 and R625 are 10% resistors, I didn't find R626 in the parts list but I assume that it is also 10%
The preliminary R-390 manual calls out 180 volts +/- 5 volts on page 169 (pdf page 192) when measured with a TS-505.  
The TS-505 has a +/- 4% FSD specification so it's measurement accuracy is +/- 8 volts on the 200 volt range.
Considering all of the above tolerances, 175vdc does not sound unreasonable, IMHO.
JimLogic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

-----Original Message-----
From: Barry <n4buq at knology.net>
To: R-390 Mailing List <r-390 at mailman.qth.net>
Sent: Fri, Nov 18, 2022 10:11 am
Subject: Re: [R-390] R390 B+ Expected Tolerance

I replaced C606 and C608 with new paper caps but still only getting 175V instead of 180V.

David Medley's page on the VR circuit states that once the VREF tubes have ignited brightly, their brightness should reduce somewhat after a short while.  I don't think I'm seeing that behavior and am wondering whether I still might have a problem with the reference voltage to V607.

I've tested the 6082s on my (uncalibrated) Hickok 752A and, while the readings there could be off, they don't show any obvious signs of problems and I haven't found any other resistors in/around that part of the regulator to be out of spec far enough to make this difference.

Any other suggestions as to what to look for here?  

Thanks,
Barry - N4BUQ

> Jacques, Jim,
> 
> Thanks for the replies.
> 
> I didn't check C606 or C608 so I have something I need to do now!  Hopefully
> that'll get the regulator back up to 180v.
> 
> Thanks again,
> Barry - N4BUQ
> 
>> I discovered that the regulated B+ is running about 175v instead of 180v in my
>> R390.  I found R625 had drifted from 1k to 1.2k and, since I think that was
>> originally a 1%, I replaced it.  I now get very close to 3.7k at the cathode of
>> V609; however, that didn't change the regulated voltage.
>> 
>> I have five 5651s and except for one of them, virtually any two will produce
>> 175v.  If that fifth tube is in place, I get about 185v.  According to my
>> Hickok 752A, the fifth one appears to have a different current draw at its
>> regulating voltage and that might account for the difference.
>> 
>> I have a spare 6082 that I've tried swapping in either socket with no
>> differences there either.
>> 
>> Running at 115VAC input and getting close to 300v unregulated input to the
>> regulator.
>> 
>> It leaves me curious as to how close to 180v is within specification.  I've
>> searched the manuals and I can't seem to find a tolerance for that.  Anyone
>> know?
>> 
>> Thanks,
> > Barry - N4BUQ
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