Re: Help with Tektronixs 492 S.A. PSU fault.

geoff wiggins

I had a similar type of fault on my Tek 466 storage scope. The supply rail was down to 4.7v approx and that was the clue.

I found a huge current drain and the fault was a short cct 2.2uF capacitor fed from a 4.7v zener which was supposed to feed 10v or so to the trigger pots. Believe it or not this little 400mW zener was taking all that current and didn't blow. The capacitor in question was a small yellow type about the size of the zener. Just follow the track around looking for a short to deck.

Good hunting.

73 Geoff. G4XMJ

On 15/01/2021 18:55, Nick Gregory G0HIK via wrote:
I came to switching on one time and it light up, but nothing on the display.

The +15 v rail is 4.7v
The -15 v rail is -3.1v.

I think the minus rail uses the plus rail as a reference so it may well be just the + 15v rail faulty.

I've disconnected all cards and connectors and the volts are still low, so I dont think is a card fault and it's probably a PSU fault. The 0.3 ohm resistor in the +15v line says its has a draw of 1.0 amps with seems a lot at 4 volts though.

the +/- 17v rails that feed the +/- 15v are fine.

I would have suspected s/c Tants, but there does not seem to be any tants.

Any help welcomed please.

I've included a part of the circuit with some voltages on.

Thanks Nick G0HIK

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