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Power transformer testing - mark123 - 08-08-2017

Hello -

Recently acquired a 46-1226 parts chassis, thinking that I could harvest a few components for other radio projects.

Completely removed the power transformer from the chassis and began checking each set of windings for continuity, using the ohms function of my multimeter. Found the high-voltage secondary winding (I think, yellow wire to yellow wire) to be open, no continuity. All other windings tested good for continuity.

My question: Is there a need to test further, such as apply voltage to the primary and measure what I get at the high-voltage secondary winding or is the ohm/continuity check a pretty good indicator that the transformer is worthless?

RE: Power transformer testing - morzh - 08-08-2017

Continuity is a good test, that is if there is no continuity (provided you have your leads right) - the winding is bad.
I would stop right there.
PS. Make sure you do have your leads correct AND they are not oxidized etc so the meter did in fact have good connection when the continuity was being measured.

RE: Power transformer testing - Radioroslyn - 08-08-2017

Check to see if you have any resistance from either of the yellow wire to the green wire. It's the center point between the yellow wires.

A couple of things to look for on a pt are: Do you see a blob wax in the cabinet under the pt? Does it smell like it was on fire (burnt smell). These are signs overheating which can cause a transformer fail.

RE: Power transformer testing - mark123 - 08-09-2017

Thanks for the input gents.

Regarding Terry's comments, I am able to measure resistance from the green wire (center tap) to each yellow wire. I get 90 ohms on one and 97 on the other, which is fairly close to what is shown on the schematic. No surprise, I get 187 ohms yellow to yellow. However, no continuity from yellow to yellow, or green (center tap) to either yellow.

I can't check the cabinet for a blob of wax, since this was a chassis-only acquisition. I did take the covers off of the transformer just to peak at the wires to see if anything obvious stood out. Nada. Nor did it appear like any waxy substance or anything for that matter dripped out. It doesn't have that acrid, sulfur-like, nasty burned-electronics smell that is hard to forget. Just that warm, nostalgic, old-tube-equipment smell from the vaporized dust, oils, maybe even the varnish from the core plates? "Eau de old electronics" if you will.

RE: Power transformer testing - morzh - 08-09-2017

Mark

What do you mean by continuity? You need finite resistance which you have. or do you want a short ciruit?
You probably have a good transformer.

RE: Power transformer testing - mark123 - 08-09-2017

What I meant by "continuity" is just the simple go/no-go feature of my multimeter. With the selector knob in the ohms measurement position, I have the option of two "modes" with a selector button. One mode displays the actual measured resistance in ohms on the meter readout, the other mode simply displays "open" or "shrt" on the readout, with a short-circuit indication accompanied by an audible beep tone.

Is this a bad evaluation technique? Should I stick solely to the ohms readout mode or should I apply voltage to the transformer (variac) and measure the output voltage, which was my original question.

RE: Power transformer testing - morzh - 08-09-2017

You do not use Go-No go with anything other than wires.

Transformer has a finite resistance that could be from units of Ohms to a kilo-Ohm or so. The primaries are usually within tens or hundreds of ohms on output xfmrs, the output is almost a short; in power transformers input is tens of ohms, outputs - could be hundreds. But in any case, you need to see the resistance, not continuity. If it is within the range shown in the sch - this is what you need.

Meantime I would recommend you familiarize yourself with the basics of the components: what is what, how to measure it and what physical meaning it has.