Isolation Transformer DIY

[Videng]

Hello-
Just starting a new hobby of restoring a few vintage Sony and Teac RR tape decks as well as a Marantz receiver and amp. I’ve been slowly assembling some test gear and wanted to include an isolation transformer. I came across one that turned out to be homemade and upon testing doesn’t seem to be that isolated. The supply side has the ground attached to the metal enclosure and passes through to the outlet. Measuring the voltage on a wall outlet is 122v. Measuring the two legs of the secondary side of the transformer is 135v, then from each leg back to the wall outlet ground is 92v and 15v. I’ll pick up a Variac to adjust the output down to 120v but again wasn’t too sure how isolated and safe a DUT connected (only using a two prong non-grounded plug on these older pieces of gear) to the isolated outlet would be if I go probing with an oscilloscope. I was planning to rebuild it, hoping to salvage the transformer and incorporate a volt /current meter with a non grounded outlet. Question I had, is this transformer suitable to still use and any other tips for doing a DIY isolation transformer?
Thanks

[wasedadoc]

You don't really need an isolation transformer when working on the items you mention.  Equipment with "old fashioned" 50 or 60Hz transformers  is already isolated after the transformer.  There is rarely any need to look at the powered-on primary side with any test equipment other than an AC voltmeter.

Yes, a fault could theoretically leak from the mains input to the circuitry on the secondary side but that should be one of the very first tests you do.

Having said all that, using an isolation transformer shouldn't do any harm but might lull you into a false sense of security.

[Videng]

Thanks for the quick reply. I noticed the original design had both H1-H3 and H2-H4 tied together, perhaps start by separating those. So, "one of the very first steps to do", ohm out between pri and sec sides? And in theory, after all this is electrical, when measuring voltage from sec sides to house ground should be 0v or is a little leakage acceptable? Appreciate the advice and avoiding the sparks. Thanks

[soldar]

primaries are in parallel for 115 volts and in series for 230 volts.

I have several transformers like that.