Trsnformer Lead Determination?

[croat47]

I have a multi-tap transformer from an old piece of medical equipment. It was a tube-based instrument (6SL7, 6SN7, and others). What is the best and safest way to determine what the voltage and currents are? I have only looked at and used new transformers.

My inclination is to hook up mains via a Variac to the "1" and "2" labelled leads where the mains were attached previosly. Then run each of the other taps to ground. I would then measure each lead after slowly bringing up the supply.

Please advise the best method. I would rather know that I am an idiot than find out I am a goner.

I can not find the original schematic on line. It is an old UTC transformer that was probably made specifically for the instrument.

Also, are there any possible noise problems with using the transformer for audio equipment power supply?

[anatech]

Hi croat47,
Your primary should have black coloured leads is it's using a standard wiring colour code. Power the equipment up to it's nominal supply voltage and you can then measure the AC voltages on the secondaries.

The highest voltage should be between the red leads. A center tap for this would be coloured red / yellow. Be aware that these voltages may be very high, the rectifier heater may be biased at the B+ DC. This means it could be running from 300 VDC to 600 VDC or higher.

Do not assume anything. Have someone near that can render assistance if you get a shock. Read this entire thread before doing anything. If you are not comfortable, then get an expert to help you. Nothing is worth being dead.

-Chris

[croat47]

As this was from medical equipment, the transformer is potted and has numbered solder tabs on the bottom of a big round can. There are two larger ones labeled "1" and "2" which I believe to be the primaries. Then there are 8 or 10 (not looking at it just now) others with smaller tabs which I believe to be the secondaries.

I will read the thread, thanks!

Aaron

[anatech]

Hi Aaron,
No leads. What a drag. My nice colour chart is useless!

Make sure you have the primary leads and follow my previous advice. How was the AC disconnected?

-Chris

[croat47]

The wires were clipped and the soldered remnants were desoldered with solder wick. The leads are intact with a little solder left on the pads.

No trace of what the circuit looked like to tell me what was what. Wish I at least knew the filament leads!

I am currently reading the thread. Two pages in, it is things I was aware of. But, I wanted to make sure my plan was sound. Looking for the experienced guys for a plan. Does my method look okay?

[anatech]

Hi Aaron,
Okay, so you have a raw transformer there I guess. If this is the case, use an ohmmeter and draw a picture of all your windings. You may as well list the DC resistance as it may give some clues.

-Chris

[AndrewT]

Hi,
the maximum voltage from some of the windings could be well over 500V since this is from medical equipment.
These windings are likely to have very high resistance, low voltage secondaries will have lowish resistance depending on current capacity.

You could power the transformer to only 10% of mains voltage to do an initial check on output volts to get an idea of what you might have to handle when fully powered. Use the highest (750Vac) scale on the DVM.

Do not connect any windings to earth. It will be safer for you if the secondary windings are floating with respect to ground.

It would be wise to clip one voltmeter probe to a terminal, then you only have to concentrate on accurately guiding one probe tip at time. That is why you must not earth any winding otherwise the testing probe tip could be live with respect to ground.