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

[Charles Steinmetz]

>Charles wrote:
>
>>Leaky capacitor(s) attached to S402 (C417 through 431E).  More 
>>likely one or more of the fixed capacitors (C417 through C428), but 
>>the variable trimmers are suspect, too.
>
>When I did the leakage test, I connected to the osc. B+ line with 
>the circuit intact, then cycled the band switch.  If any of the 
>above was leaking it should have showed up, no?

Maybe or maybe not.  What do you mean by "showed up"?  Were you 
monitoring the B+ current at the time, and looking for relatively 
small differences?  Resistors cooked as you describe were most likely 
only moderately overloaded, over a long period of time, so it might 
not be much of a leak as of yet.  It also may take a while to develop 
(minutes or even hours) if there is a bad cap that is only 
conditionally leaky at this point -- say, after the radio gets hot 
and/or the cap has had B+ across it for a while.  So at this point it 
might take careful observation and some time to tell, as opposed to 
"I switched through all the positions and the resistors didn't 
immediately go up in smoke."

I'm still not inclined to think it was V401 drawing too much current 
(for example, if the tube suffered from grid emission and ran 
away).  I could be wrong about how much cathode current a 6AK5 could 
draw in extremis, but even so it is extremely unlikely that the tube 
would suffer a high-current fault (grid emission is the only thing I 
can think of that could cause it, and I've never seen that in a 
small-signal pentode; or physical contact between the grid and the 
screen, which is even less likely).  So, I just don't see V401 as 
likely to be the fault that caused damage to R406 and 407.  Leaky 
mica capacitors, on the other hand....

Best regards,

Charles