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7904 CRT problems

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Joined: July 3rd, 2021, 11:18 pm
Country: New Zealand

7904 CRT problems

Post by valvehead003 » July 3rd, 2021, 11:41 pm

Hi all. Long time tek scope user but first time poster.
I have a 7904 that I have used for the last 10 years or so and apart from some power supply issues it has been fantastic. I'm hoping that someone here can help with a problem that has developed over a period of a year or so.
At first it started with the power supply taking a few 'bursts' to get going. This got longer and longer but I am sure it's not a psu problem. After a while a hissing noise was audible. Finally it now refuses to get going and there is a strong arc from the anode connector on the crt to the chassis. I have removed the crt and thoroughly cleaned the entire area around the connection and scraped a little of the HV silicone back. I then coated with many layers of Corona dope to try and break the arc. So far nothing has helped. Any advice on how to solve this problem received with thanks.

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Joined: June 7th, 2021, 11:06 am
Country: United States

Re: 7904 CRT problems

Post by daveca » August 31st, 2021, 8:47 am

Youre BEGGING for a disaster by continuing to use that unit without recapping it.

Failing to do so results in the PS hand-grenading and that may have already happened from the comment about "have to turn it on several times"

Youre going to DESTROY that priceless antique for lack of $100 in caps. Or have already.

That SMPS generates the DC supplies, but also generates the 25 Kc drive to the HV board.

The SMPS may have lost regulation and increased the HV to the CRT... excessive voltage would correlate with excess arcing. (10Kv / inch in free air)

Once that HV insulation has arced, its ruined, Dont waste time with Band Aids like Corona Dope. Clearly that didnt work!

Its next to impossible for a CRT cap which is properly sealed on a clean CRT surface to arc from the HV well out to chassis, the path is too long. Its more likely plastic on the wire insulation has broken down. The HV wire is excessively long in that unit.

Continuing to operate it while arcing WILL destroy the HVM, if its not already happened.

1. Replace ALL the electrolytics in the SMPS AND HV BOARD. There are two on the HV board severely under-rated and have probably leaked (160V caps on a 130V line, in my 7904, they had leaked on the PWB)
2. Remove the HV cup, inspect the wire and cup with a magnifying glass for pin-holes and burning. REPLACE damaged parts. INspect the CRT HV well for burning.
3. Plug it back in, check the PS voltages. HV is dependent on PS regulation.

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