Re: 577 AC collector supply not reading zero on display

Bob Koller
 

I don't think the variac is bad, it just isn't going all the way to zero. You can also measure the voltage between C-E on the test fixture with the Polarity set to AC.
Have you removed it from the instrument for inspection? It is possible that the carbon brush has worn to the point where it has too wide a contact area to go fully to zero. In that case I guess the variac has "gone bad" You may be able to carefully reshape the brush to a narrower contact area, should be about 1-1.5mm. If not, replacement is the answer.

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