About the amp: This is from an old solid state audio amplifier with an AM/FM tuner. No digital parts or ICs were inside. It was all point to point. Unfortunately I threw out the amp and didn't write down the number and now I am trying to use the transformer to build an audio amplifier.

About the transformer:

On 'one side' the wires go as follows: White, Yellow, Brown, Orange, Red, Grey, Black

On the 'other side' the wire go as follows: Red, Red, Yellow, Grey, Yellow, Blue, Blue

My experiments to find out what the primary wires are: After reading about how to find out by measuring resistance, I found that the primary wires should be about 4-8ohms I measured all the combinations of wires on each side and applied mains to the transformer. My first thought was that the 'other side' had pairs of wires so it must be the side with the primary. To test I (very quickly) hooked up 120V to the pairs and measured the other wires with my scope. This stepped the voltage WAY up (200Vp2p was my first measurement) and buzzed a lot. Once again.. it was on for less than a second and never was remotely hot. I tried a few more wires on that side and had the same result. After doing a similar thing to the 'other side' I found that the white and grey produce approx 80Vp2p, 40Vp2p and 10Vp2p. This is the best combination I have found so far. Does this make sense? I'd like to know a little better before designing a circuit around it.

Any advice or thoughts will be very much appreciated!!!

edit: Really important detail I missed. The outside of the transformer says t52-131 on the first line and C-AS-QD7 on the second. It is a big hefty transformer weighing two or three pounds? (I'm bad at estimating weight)


5 Answers 5


Since transformers by their nature are bi-directional, the selection of the primary side totally depends on your input voltage and desired output voltage.

The transformer you describe likely has multiple taps on the "primary" side, may have multiple windings on the "primary" side and likely has multiple windings on the secondary side. Start with a low range DMM, and check for continuity between different leads on each side of the transformer. Once you have mapped continuity, check resistance between the same leads. You should be prepared for the transformer to be as complex as this:

Just an example - this is not your transformer

The "secondary" side may be a single coil with multiple taps, or it may have multiple outputs more like the above example.

Once you've reverse-engineered the coil arrangement, you'll need to determine the turns ratio between each set of coils. I would NOT recommend your 120VAC test for this. Start with a much lower (and safer) voltage. Find a small "wall-wart" type power supply that you can sacrifice. The lower the output voltage the better. You want it just for its transformer, not the rectification and regulation components, so if you can find an AC-output wall-wart, you can use it's output as-is. What you want is a low voltage AC source that you can use to test individual windings. Note that applying a low voltage AC source to the "secondary" may result in lethal voltages on the "primary", so be careful!

Find one set of windings to apply your AC input to, and measure the resulting output on each set of coils and on each tap. Transformers are ratiometric, so the relative voltages will be the same using your low voltage AC test vs. when you identify the intended primary winding and apply 115VAC to it.

Doing this, you should have a good sense as to what windings are present that the relative turns ratio between each. Good luck!

  • \$\begingroup\$ Thanks so much! I originally used my function generator, but its output impedance was too high to drive the transformer. I hadn't thought of using another transformer to test! I have a few known valued transformers I can test with it. Hopefully the lower voltage does not affect the way the transformer works. Once I get my lab set up today I will do this and check! Thanks again. \$\endgroup\$ Jul 28, 2012 at 19:11
  • \$\begingroup\$ +1 for "no real primary side". I recently threw together a little hacked-up high-voltage DC supply by driving the secondary of a 12V wall-wart transformer with a H-bridge. A little clever voltage-doubling of the output, and I have isolated ~650V DC at a few milliamperes at the output, from a 12V source. \$\endgroup\$ Apr 8, 2014 at 5:53

Electrician, butting in.

In the field, we frequently test transformer ratios as a service to industrial customers. Normally you'd have an automatic or semi-automatic test set that would give you the complete characteristic of, say, a current transformer. But since you don't have that, you have to get creative.

Using a voltage source can be very unpredictable and quite dangerous, as HikeOnPast and Martin James pointed out. A quick multimeter reading from terminal to terminal on each coil will tell you which one is longer - that is, the high-voltage side. Short it from end to end using an ammeter. Then pass a controlled current source through the low-voltage side. This can actually give you incredibly accurate results. If you find that you have a multiple winding transformer, all of them have to be shorted out.

This obviously has the disadvantage that now, you need a source of ac that produce a controlled amount of current. These aren't exactly just laying around, but they could be built fairly simply. So, yeah, what's the point?

No more dangerous voltages, and no need for high voltage probes. The relatively low-impedance coils will not generate any significant amount of voltage in this arrangement - as long as you short all of the other coils out. Do NOT forget that part, or you will be very sad.

If you do this sort of thing a lot, I strongly recommend keeping around an AC current source anyway. There is no better way of getting highly accurate resistance readings on low-resistance stuff, and no safer way of testing unknown transformers and coils.


'To test I (very quickly) hooked up 120V to the pairs and measured the other wires with my scope.'

Don't do this, even for a second. Apart from the danger of death by stepped-up voltages, an inadvertent gross step-up resulting from such experimentation is quite likely to result in internal arc-over, straining the insulation and resulting in either immediate, or subsequent, failure of windings.

Find a pair and shove on a much. much lower voltage. A variac would be a good start - notch up carefully and see what is happening on all the other windings.


I use this method of identifying unknown transformers for tube circuits. I use a known small filament transformer that has a 6.3VAC output. A lot of the transformers I test have a pair of green wires or a pair of yellow wires that generally signify the filament supply voltage but other times I'll find a pair of wires that have a reading of a couple of ohms. I connect my 6.3 volt transformer to these wires then test the other wires using high voltage probe in series with my multimeter. The probe has a 100x attenuation so millivolts on the meters indicates voltage. Once I find the winding that has approx 115 volts output I've found the primary 120vac winding. Then I'll hook up 120vac to the winding and ID the other windings with using the high voltage probe. Use clip wires to connect everything up so your not handy to it with the power it applied. My probe consists of a 10 Megaohm 1/2 watt resister in series with the positive lead on the meter then a 10K 1/2 watt resistor between the meters positive and negative leads to form a voltage divider with 100x attenuation. You can find lots of diy HV probes on the internet. be careful and good luck.

  • \$\begingroup\$ Please revise your answer, as there is a Typo (10M instead of 1M). For a 100:1 attenuation your resistors should be 1M:”10K”, as the correct “10K” should be 10K1: (10K+100R). For a 10M impedance of a DMM, this would be accurate to ~0.1%. \$\endgroup\$
    – EJE
    Sep 1, 2022 at 16:01

Try "googling" the model number of that AM/FM reciever. If you have the case the transformer came from; look for a model number. The schematic might be available on line. Be safe, an isolation transformer should be used; rather than using mains directly.


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