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?

Hi...

Worked on lots of those sets; wish I had a nickel for
everytime I burned my fingers pulling a 5U4

Anyway, can't help asking... why the fireworks possibility?

All of those old sets had a power transformer connected
straight to the line (except power switch), and nothing
else connected to the line, so....

Take care.

Ken

Nomen Nescio wrote:
On Fri, 29 Jul 2005 23:36:46 GMT, Ken Weitzel wrote:


Anyway, can't help asking... why the fireworks possibility?
All of those old sets had a power transformer connected
straight to the line (except power switch), and nothing
else connected to the line, so....


Truthfully, I don't quite understand why the need for an isolation
transformer. But I've just read the need for such a thing often enough in
many places that I got one. Sam Goldwasser mentions it in his FAQ and that
was good enough for me:

http://repairfaq.ece.drexel.edu/sam/tvfaq.htm#tvihw

Hi Nomen...

Aha! There what's happened...

First, I'm getting pretty old, (you knew that if I had worked
on those sets to pay for my education, right? and am
easily confused and forgetful - if that's not enough, a stroke
victim too.

So, wait until someone else jumps in and corrects me or confirms
this, but meantime...

Virtually, if not all of todays sets, and for many years back,
are transformer-less. The chassis is connected to one side of
the AC line. Hence the need for polarized plugs (a bit of
safety while the set is in its case) and the desire for an
isolation transformer while its apart with the chassis exposed.

So if you work on something current, then either an isolation
transformer or great, great care is needed while you work on it.

But the set you're now involved with has a transformer in it,
so it's redundant to put a second (isolation) transformer in
series with it. Can't hurt anything, but nor will it add
anything.

And just to be double-sure. The transformer issue is only
*one* of the safety issues. With it or without, the set will
still do you great harm if you aren't very careful.

Now wait for a second opinion 'fore you dig in

Take care.

Ken

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

-- Use this address until 12/31/2005

-- Use this address after 12/31/2005

"Nomen Nescio" wrote in message
...
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?


Very likely the voltages you see are just capacitive coupling from
Primary to secondary.
The impedance of a Fluke 77 is so high it will see all sorts of stray
voltages.
The only simple way to test the isolation is to measure the resistance
between the primary and secondary with a high voltage Megger or similar
insulation tester.
--
John G

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