On Sat, 30 Jul 2005 01:10:05 +0200 (CEST), Nomen Nescio
wrote:

I just got an isolation transformer for restoration of my 1950's RCA Victor
TV. The power cord is non-polarized. Up until now, I've just been using a
Fluke 77, so I didn't bother with the isolation. But I'm now moving into a
phase where I need to view waveforms on an oscilloscope. And from what
I've read, without an isolation transformer, fireworks will be the result
of the first probe. Now that I've explained the scenario, can anybody
explain how I can test the isolation transformer so that I know it is doing
the job it was designed to do? I took some measurements with the Fluke and
this is what I got:

wall outlet: 118VAC
iso: 118VAC
Hot_wall - Hot_iso: 67.5VAC
Neutral_wall - Neutral_iso: 60VAC
Hot_wall - Neutral_iso: 162VAC
Neutral_wall - Hot_iso: 34VAC

With this information, can it be concluded that the isolation transformer
is working as designed?


The important thing is the resistance from the primary to secondary.
It should measure open circuit from primary to secondary. If it's
less, the transformer is leaky, or shorted (I've never seen this
happen).

As others have said, if the set uses a power transformer you don't
need an isolation transformer. If it's transformerless you need to
use an isolation transformer. There will be voltage on the chassis of
a transformerless set no matter which way it's plugged in.
Andy Cuffe

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