[R-390] R390A 2nd osc. hot R406, R407

[Charles Steinmetz]

Nelson wrote:

>Yes.  I suspect that my cap checker's leakage test may not have been 
>sensitive
>enough.  I have reinstalled the RF deck because I wanted to confirm 
>it is indeed
>generally operable after all the work done, and it is.  Badly in 
>need of a full
>alignment of course.  But there are a couple of things to take care 
>of before a
>detailed voltage check through the full range of S402 on the V401 B+ circuit.

Fingers crossed, I hope you find a definite answer.  You may need to 
warm the radio up fully, then switch through the bands slowly while 
monitoring the current through R406 and 407.  You can just measure 
the DC voltage across C411, and if it drops by 30v or more on any 
band compared to the others, consider the cap(s) associated with that 
band suspect.  (I presume you have satisfied yourself that C411 is 
not the problem?)

C413 should see a nominal 205v DC from the power supply on the RF/IF 
B+ Line (note that the V401 screen draws its DC from the 150v 
regulated supply -- that current does not flow through R406 and 
407).  I expect a 6AK5 oscillator to draw about 5mA, so the voltage 
at the plate (Pin 5) (which should be essentially the same as the 
voltage on C411) should be about 165v.  So, if the C411 voltage drops 
below, say, 135v, you probably have a fault condition.

Warm the radio up thoroughly, then switch through the bands slowly at 
first (10 or 15 seconds on each band) to see if there is (are) any 
band(s) that clearly drop the C411 voltage significantly.  If the 
C411 voltage is too low on all bands, C411 is the likely culprit.  If 
you get through all the bands and the C411 voltage stays around 165v, 
then you need to switch through the bands even more slowly (say, 30 
minutes per band) in order to give incipient leaks a chance to show 
up.  If you do that and the C411 voltage stays around 165 on all 
bands, then your problem has gone away, at least for now.  Possibly 
an intermittent  fault that is in remission, or possibly it really 
was a bad 5654/6AK5 that was replaced along the way (though I'd still 
be doubtful of that).

Happy hunting!

Best regards,

Charles