08-28-2009, 06:03 PM
Old carbon resistors can be really cantankerous. Even out of circuit they test OK, but once they warm up, they can go way up in resistance. Best defense it to replace all the hard working ones, you know the cathode resistors in the output, all of 'em in the voltage dividers, and if it's not drifting that is happening, just about all of 'em after the detector stage.
About the only other thing to be on the lookout is micas with mica disease. In the early stages, they usually cause no signal or weak signal. In the later stages, they can produce distortion. Wait until the condition occurs, and insert a clean audio signal to rule this stage out, then an IF signal, and see if it holds. It is a process of elimination.
It's a rare tube that craps out after 15 minutes, unless there is a problem with the connection with the socket or an arc trace on the base.
This is an interesting one, keep us posted.
About the only other thing to be on the lookout is micas with mica disease. In the early stages, they usually cause no signal or weak signal. In the later stages, they can produce distortion. Wait until the condition occurs, and insert a clean audio signal to rule this stage out, then an IF signal, and see if it holds. It is a process of elimination.
It's a rare tube that craps out after 15 minutes, unless there is a problem with the connection with the socket or an arc trace on the base.
This is an interesting one, keep us posted.
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