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The neon functioning is acting normal for operation of the PSU. The service manual calls it out as a "safe handling" indicator IE don’t handle the supply while it is still flashing. The supply has bleeder resisters for the larger caps so it sounds like they are functioning. That is what is helping with the quick drain off.
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In my case I tested the caps out side of the supply with an SMU so I am 100% sure the fault is internal to the 980uF 200VDC though yours are not indicating like mine did one was reading higher then expected capacitance 1086uF with a climbing ESR 0.06 retest to 0.56 How do the spark gaps look? There are some glass tubes that looks like small fuses but are designed to arc over if they need to. If any are carboned up or dirty that will lower their arc over voltage. If we are sure it is not the HV section then I would look for an arcing component. I have seen tants die that way when they cracked open some times you can even see the static. Or some for of contaminants. If you have a fast enough meter are any of the power rails jumping around in time with the crackle? If it is true arcing then an AM radio will be sensitive as a detector as well. Louder = closer Absolute worst-case would-be breakdown in the transformer. Zen -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of James via groups.io Sent: Sunday, January 5, 2025 1:13 PM To: [email protected] Subject: Re: [TekScopes] 7904 pre-260000 PSU crackling Hi all and thanks for the replies and help. I'm pretty sure it's in the PSU but I will reconnect it all (again), fire it up, and try the trick with the microphone in a plastic tube. Trouble is, I'm 90% certain it comes from inside the PSU and when I had the scope turned on in a darkened room I couldn't see any corona. My sense of smell isn't the best but I detected no ozone. But I will try again and report back. I did quite thoroughly clean the HV lead and the connector originally, and cleaned the cap multiplier lead when I had the PSU apart. They were totally blackened with this oily grime, as were all the wiring looms. I imagined that this could lead to arcing and other unwanted stuff, so it seemed a good way to start. It's quite astonishing that the thing worked at all! What is the dog house hole? Is it the slot in the middle of the PSU panel where the HV lead and other wiring comes out? Once I have that confirmed I might try getting another plastic tube and seeing if I can sniff ozone through that? Re bumping the HV lead, yes, I have tried it. No change, sad to report. Currently waiting on a supply of tantalums and other bits to replace those present on the other boards, also a tool for inserting the connectors into new molex connector plugs before the connectors totally fall apart (the blue ones are already partially absent, I'm concerned for shorts). |
