Quote:
Originally Posted by joe111671
I think I understand, and I do have a HV probe. Correct me if I got it wrong, but what I'm checking is the focus voltage to the CRT on the black wire. If it drops when the picture goes out of focus, it's either the CRT itself, or the socket, and I'm going to be finding whether it's one or the other by these tests.
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Yes.
Quote:
Originally Posted by joe111671
But if it's the rectifier stick - will the voltage just come up to a certain point and stay there without dropping? That's the only thing I'm unsure of right now, could a bad rectifier stick produce the higher initial voltage and then drop?
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If the stick is bad, the voltage will be low. I have to admit that the initial higher voltage followed by a drop off isn't the main symptom of the stick being bad, but it's still a real possibility, hence the tests. Most sticks just gave bad focus, but I do remember a few drifting as the TV ran, yes.
There were some chassis designs (including Zenith) where the focus voltage was sourced from a divider off the second anode that could cause excessive (high) focus voltage, but yours is not like that. For your situation, we're assuming that when the TV goes out of focus, the voltage is dropping.
So, if you remove the black wire and the voltage still drops, the stick or the focus pot itself is bad (internal leakage - often visible as small purple arc spots in the dark). We changed tons of those sticks back when those TVs were in every day use, and it's a good probability here. By disconnecting the black lead, we're making sure the focus voltage isn't being pulled down by the CRT socket (fairly common), some corrosion under the CRT guide pin (really uncommon unless some liquid got spilled on it), or internal problems in the CRT socket (really really uncommon).
But us old guys aren't parts tossers, which is why we're giving you specific tests. When we had to get five to ten of these out the door a day, we had to be reasonably sure what part was the problem.
John