[R-390] R390 B+ Expected Tolerance

[Barry]

Hi Jacques,

Still unable to post to the list...

Yesterday, I experimented with some shunt resistors across R626.  100k had almost no effect; however, I worked my way down to another 2.7k across it (making the total about 1.3k) and that finally brought the regulated voltage up to 180v  Since 1.3k is obviously well out of spec, then I'm wondering what's causing that to work and I'm thinking it may be possible that the 5651s are not drawing the correct current when they fire.

R624 checks high (a little over 600k) but shunting that down a bit doesn't help much at all.

If I'm not mistaken, my Hickok 752A seemed to indicate the proper voltage and current when testing them so not sure if that could be it; however, I did notice that one out of the five tubes I have did yield a much higher voltage than the other four combinations so that might be the case.

Thanks,
Barry - N4BUQ

> 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