[Amps] Direct rectification of AC mains to drive the amp,

[Manfred Mornhinweg]

Jim, Bill, and all,

> So, how about simply putting a small capacitor in each probe lead?
> Say about .001 uF or perhaps even smaller?  Haven't actually tried
> this but it should isolate the DC enough, I'd think.

The DC, yes, but not the low frequency AC. A .001uF capacitor at the
input of a 1 megaohm scope will give a highpass with a -3dB point at
159Hz. You would need a much smaller capacitor to block the line
frequency AC component well enough. And at that point, this capacitor
would produce a poor frequency response of the probe over the HF range,
due to the combined resistive/capacitive divisor.

> ##  Back in tech school... we would use a  hammond 120-120 vac
> isolation xfmr, to feed the 120 vac input of the scope.   Chassis of
> the scope floats. End of problem....end of story.

Jim, that doesn't solve anything! You don't need the isolation 
transformer, at least not with any scope that has a traditional old 
fashioned transformer input supply. Just cutting the ground lead in teh 
scope's power cable does the same trick. In either case, the chassis of 
the scope will be at whatever potential you connect the test lead ground 
to. If that's a directly line-connected amplifier, you have mains 
potential on the scope chassis, and its knobs, plugs, everything, 
requiring careful, non-impulsive behaviour in the lab!

> ####  Another method is to use a battery operated fluke scope meter. 

Yes. That's actually an excellent method, which I have practised 
extensively at my former job. There I ordered the Fluke Scopemeter when 
it just hit the market, many years ago. But for use at home, for my 
hobby, it's too expensive!

Manfred

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