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Re: HP200CD oscillator

 

C8 and C9 are 0.47uF paper coupling capacitors which can leak current, which will change the output tubes bias and cause them to conduct excessive current. Those mica will fail shorted, or simply explode? because of being connected from the plates to ground.
C8 and C9 should be replaced, as preventative maintenance, because if they haven't failed, they will. The insulation of high acid paper, like newsprint. It slowly deteriorates, and carbonizes which causes leakage. They are cheaper than a set of tubes, and power resistors. My military version off this had every oil filled paper capacitor leaking current, and burning resistors when a Ham gave up on it and gave it to me.


On Wed, Nov 3, 2021 at 5:40 PM Morris Odell <vilgotch1@...> wrote:

Leakage in C12 is unlikely to be a serious problem as it's a high value electrolytic in a low impedance part of the circuit and not in a position where leakage would make a difference to tube operating conditions. Leakage in C8 and C9 could make a difference but it would have to be pretty severe as they are shunted with 1Meg resistors anyway. Having said that, if they are bumble bees or black beauties they should be replaced on sight. There's no point measuring or testing them. Leakage in C10/C11 could cause excess current drain but as they are mica caps it's unlikely. I'd be changing the 6AU6s first. If the 6CW5s were weak, the 6AU6s would have had the same lifetime assuming they were all the original tubes.

Morris

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