[R-390] R390 B+ Expected Tolerance

[Barry]

Sheepishly, I'm going to admit that the reason I was seeing very different voltages at V607 is that I was not measuring the pins of V607 but was looking at V604!  The labeling on the top of the AF deck confused me a bit and I was off by one tube location.

The voltages are still off just a bit for the real V607 but not nearly as much as I thought.  I might have spotted a few things with the new measurements so have more work to do.

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "n4buq" <n4buq at knology.net>
> To: "Jacques Fortin" <jacques.f at videotron.ca>
> Cc: "R-390 Mailing List" <r-390 at mailman.qth.net>
> Sent: Friday, November 18, 2022 1:58:48 PM
> Subject: Re: [R-390] R390 B+ Expected Tolerance

> Hi Jaques,
> 
> I should've said film caps, not paper.
> 
> Vishay MKT1813422254
> and
> Vishay MKT1813410255
> 
> Honestly, I'm tempted to put the original parts back in.  They're probably still
> good and may last even longer than these replacements.
> 
> I'm seeing some very different voltages at V607 and am going to pull the AF deck
> once again to see if I can determine why those are the way they are.  I do note
> that I'm not seeing 182V on the cathodes of the regulators and, with the 1.5V -
> 2.0V drop across the load balancing resistors (which, BTW, are running right
> around 50-ohms), accounts for a bit more of the "under-voltage".
> 
> As David Wise commented, 175V is probably well within tolerance and I'm picking
> at nothing but, just the same, it bugs me...
> 
> Yes - I have adjusted the HUM BALANCE per the manual.
> 
> Thanks,
> Barry - N4BUQ
> 
> Subject: RE: [R-390] R390 B+ Expected Tolerance
> 
>> Hello Barry,
>> 
>> When you mention "new paper caps" did you really mean paper-in-oil caps ?
>> I do not trust any paper cap for a long time, except the sealed ones
>> (Sprague Vitamin Q, for ex.)
>> All "real", not vacuum sealed PIO caps develops leakage with time.
>> Better to use Mylar (polyethylene) or polypropylene ones, especially for
>> C608.
>> 
>> There are other causes that can make the output a little low in this
>> circuit:
>> R623 can be of an higher value than the tolerance range.
>> One of the two voltage divider resistors R616 and R617 can be out of their
>> tolerance range, or both.
>> V607 can have a higher transconductance than the "mean" value.
>> The sum of the voltage drops of V608 and V609 should be 170V: if it is
>> lower, the output of the regulator will be lower as well.
>> 
>> Just curious: what is the brand of the 5651 you have there ?
>> They vary by few volts from one maker to the other and when they age also
>> (they tend to go up when they age).
>> 
>> If nothing wrong can be found in the resistor values and you are still
>> unhappy with the 175V (which is not a bad result at all, IMHO), try to lower
>> the value of R626.
>> A 100K resistor bridged across the actual R626 should raise the output by
>> few volts...
>> Adjust value in consequence of the measured result.
>> 
>> Last comment: the HUM BAL. adjustment can "move" the output voltage also.
>> Be sure to adjust for minimum AC voltage at the test point J601.
>> 
> > 73, Jacques, VE2JFE in Montreal