Tek 7904A rapid ticking - no HT...


Bad day at the office (shack) - two failure in one day ;-(

I recently turned on my trusty 7904A after perhaps 2 years of non-use...

It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A), and then the traces dissapeared after about 10 seconds and the scope is issuing a rapid "ticking" at about 200 Hz - more of a sharp-edged buzz really. Now when I turn it on, I only get the buzz/tick.

Removing all the plugins makes no difference...

I'm not a HT person. Ideas? If it helps, I'm in the Uk.

Many thanks

Reply to
Mike Deblis
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Sounds like bad filter caps in pwr supply

Reply to

The 7904 has an overcurrent protection circuit, which will cause the ticking. This is a good thing.

First step, remove all the plug-ins. Still get the ticking? If yes, you can isolate boards by removing connectors, one board at a time.

I had one of these fail that turned out to be a bad diode on the Z-axis board.

Most likely problem: a bad cap on one of the boards. The tantalums go bad and Tek used them by the truck load.

Good luck.


Steven D. Swift, novatech@eskimo.com, http://www.novatech-instr.com
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Reply to
Steven Swift

It could also be that one of the high voltage power supplies arching to ground.

Check the HV wires for breaks in the insulation.

The post deflection accelerator is very high voltage so try not to get killed.

Reply to
Keyser Soze

I've had that one, too.

A good plan can be to remove each receptacle, one at a time from the PSU output cable socket molding to isolate each rail. Pull the sensing feedback plug first, though, or you can find that a shorted rail will fry one of the sensing bypass resistors. The PSU will regulate, albeit a bit poorly, with the sensing disconnected, because of those same bypass resistors.

The PSU won't run properly with no load at all, BTW.

There are some 100uF (??) electrolytics on the 15V rails on the interface PCB that can go short under operating conditions, but appear OK on low voltage. They are a bitch to get at.

Five'll get you ten that the PSU itself is OK.

"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
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Reply to
Fred Abse



Is Jim Yanik (sp?) still around?


Reply to
Mike Deblis

"Mike Deblis" wrote in news:dpmrhd$6n5$ snipped-for-privacy@nwrdmz01.dmz.ncs.ea.ibs-infra.bt.com:

The fast ticking,IIRC,indicates a failure in the PS itself,like a shorted diode,bad electrolytic,open switcher xstr(s).Probably a primary-side failure. I strongly advise reading the serivce manual's circuit description on the PS. If you don't have a svc manual,then I would not attempt repair;it's a very complex series-resonant switching supply.There are under/overvoltage lockout circuits,several current and voltage feedback loops.

Jim Yanik
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Jim Yanik

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