On Thursday, January 2, 2020 at 12:17:25 PM UTC-5, wrote:

Yeah, it seems odd to have an isolated auto transformer. But it is.
The auto transformer is isolated by another transformer all in the
same case.


This gets down to basic bench safety. I do have some "expensive" instruments, but my very first line of 'defense' is a metered iso-variac - the Heath IP-5220 pictured. On first introduction, any new-to-me item goes through it for preliminary screening and testing. And after many years of using this device, and finding all sorts of potentially fatal and/or dangerous faults thereby, I purchased a second unit for my traveling kit.

As a diagnostic tool, it is quite useful - I can "see" B+ at onset, and I can measure current draw as compared to what it should be in theory. For the most part, those things we see and service are simple devices with a fairly limited set of common failure modes. Getting past these conditions allows us to get to the heart of the matter much faster and with much more confidence, ultimately leading to a better (and safer) outcome. No wonky power-supply as one example disguising a more critical condition down-line.

Peter Wieck
Melrose Park, PA