How to test voltages on secondaries?

[riotubes]

Need help understanding how to test secondary voltages on 3 transformers for dac project with tube stage. Although I have read and re-read my Fluke DVM manual...it is not that thorough. Just want to pass this by an experienced builder before I test.

1. As far as all secondary leads, they are not hooked up to their respective dac and tube pcb terminals. Also, they are not touching anything (i.e. metal) nor are they touching any other lead.

2. For the 6.3V (B+) and 9V (filament) secondary windings, there are just two leads each. Take the 9V transformer for example. In AC mode, do I just clip the + lead of my DVM to one of the secondary leads, and the minus lead to the other lead, and then plug in the dac, turn it on, and observe the voltage reading? Then power off, unplug, and then unclip the leads?

3. The other transformers are 18v-0v-18v (Dac) and 230v-0v-230v (tube stage). Hence, there are three leads for each, and they are color coded. For instance, the 18v-0v-18 is blue-white-blue. Now for this one, do I clip the DVM's + Lead to ONE of the blues, and the minus lead clipped to the white wire (with nothing on the other blue? And then power up and read the voltage on the DVM? Do I repeat this same exercise for the other blue wire? Since only 2 of the wires would be hooked up, am I looking for an 18v reading, or half (9v)?

Sorry for the long message. I really do appreciate any guidance you can share. I won't be testing anything without first receiving feedback from an experienced builder. Thanks!

[SY]

Your fear and trepidation are wise. This is dangerous stuff we do.

You have the basic idea: get the secondary leads isolated and insulated, clip the meter across them, then plug it in and power on. I usually have the switch in the "on" position, then control power by plugging/unplugging at a safe distance. If there's any loud humming, smells, sparks, or smoke, unplug it ASAP. If all seems well, you can read the voltage. (You know all this, it just needs to be said)

Dumb stuff:

Make sure the meter is set to "AC Volts."

If your meter doesn't autorange, pick a higher setting than you expect, just to be sure, then start lowering it to get a precise reading.

The insulation on test leads and clips should always be thought of as flawed. Power off before you clip and unclip!

Unloaded, the voltages will be higher than you expect.

99% of the time, there's no problem in a measurement like this. You just want to make sure that the 1% doesn't kill you.


The blue-to-blue voltage will be 36V. You don't need to worry about the CT (usually!).

[pinkmouse]

When testing an unknown traffo, I usually fix all the secondaries into an insulated connection block, then there is absolutely no chance of any of them shorting during measurement.

[riotubes]

Thank you Sy and Pinkmouse for your quick and helpful feedback!

[mpmarino]

I know nothing about the hi volt stuff but I learned a tip here on the board that I use any time I test a trafo now.

power up a small 120v primary/12v secondary traffo. Apply the 12vac from your small trafo to the 120v primary of the trafo under test. Measure the secondaries of the trafo under test and multiply by 10 to get the 'real number'.

It's nice and safe, you get to deal with lower voltages and an isolated trafo.

All of the above safety suggestions still apply of course.

works fer me.

[aletheian]

Originally posted by SY
... I usually have the switch in the "on" position, then control power by plugging/unplugging at a safe distance. If there's any loud humming, smells, sparks, or smoke, unplug it ASAP...

I converted the power switch on my power strip to a push button... or more accurately a "stomp" button... that can be easily smashed off with my foot is sparks fly and smoke pours...