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Somewhat unknown toroidal transformer specs, how to find out more?
Somewhat unknown toroidal transformer specs, how to find out more?
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Old 16th April 2012, 01:27 PM   #1
Coconuts 500 is offline Coconuts 500
diyAudio Member
Join Date: Apr 2010
Question Somewhat unknown toroidal transformer specs, how to find out more?


I opened up an old generic branded broken home theater receiver (no info about it online, I've looked hard) and noticed that it had a pretty nice toroidal transformer in it.

I took it out.

It's 6,5 cm high, and 12 cm in diameter. It weighs about 4 kg.

Primary is 230V, and on the back of the amp, it said maximum power consumption: 200W.

The primary fuse was T5AL (what's L?).

The secondary fuses (for 2x 26VAC) are T10A.

When connected to the rest of the power supply, I measured +/- 29VDC on the smoothing caps, so I think this could pretty suitable for a DIY amp?

The transformer has ten wires coming out of it, with five connected to a plug each.

The first, going to the power amp section:

Two green ones, which are the ones marked 26VAC on the board (again, fused with T10A each).
Two yellow wires, marked FF.
One black wire, GND
All of these five are 18 AWG.

The second connector, going to another misc board, with its own smaller power supply.

Two blue wires (fused with T4AL each)
Two white ones (marked 20 AWG)
And a black one (GND).
Blue and black ones are 18 AWG.

How can I figure out what these other wires output? I only know the +26VAC ones because it was marked on the board, and because it was connected to the rectifier and smoothing capacitors.

Any help greatly appreciated. Please bear with me - I'm learning!

Last edited by Coconuts 500; 16th April 2012 at 01:46 PM.
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Old 16th April 2012, 01:49 PM   #2
ric-paul is offline ric-paul  Netherlands
diyAudio Member
Join Date: May 2005
Location: Rotterdam Netherlands
Well, get a voltmeter/multimeter would be a simple answer...

So the toroid is still in the amplifier, right? You don't have the leads separated from the board yet?

If the wires are loose and not soldered onto the board, you could measure resistance between each set of wires (with no primary AC voltage present!!!!!). Any two wires showing less than ~10 ohms will be an output. Anything OL, or over 100kohm will NOT be connected internally and therefor not be worthwhile measuring whilst hooked up to 230V (or 115V). (If they are crimped into a plug and you can disconnect these plugs: then do this. Write down all wire-colours, resistances between each combination and then fire it up and measure the AC voltage between each combination with a low resistance between them)

If they are still soldered to the board and you can switch it ON, then simply try every combination of wires and note the AC voltage between them. BEWARE of not shorting anything out with your measuring leads and not touching the primary windings!!!

As far as I can see now you can simply find out the primary winding, right? See if there are extra wires for e.g. switching between 220 and 240V~ or 115/230V~.

And as for 26VAC leading to 29VDC, sounds strange to me. 26AC should result in some 35VDC in regular simple powersupplies.

Last edited by ric-paul; 16th April 2012 at 01:51 PM.
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Old 16th April 2012, 02:06 PM   #3
Coconuts 500 is offline Coconuts 500
diyAudio Member
Join Date: Apr 2010
Hi, it just says 26VAC on the board. The transformer may output something else, but the end result is 29VDC. The amplifier (STK chip) can take 25-36VDC, so I guess the board markings are just a "recommendation", but the actual used transformer was different.

I already know what wire belongs to which, from examining the pcb of the amp. But I am unsure of their voltages. The primary winding is connected to a power cord.

I have a couple of multimeters, but was unsure of how to measure the wires. Do I measure one wire against ground, or the two matching colored wires together? I did that, first, and got 42,2V between the two green ones (the ones that were PCB marked 2x26VAC became +/- 29VDC)

I only have the amplifier/power supply board still. It can be attached to the transformer with a connector of five (of the ten) wires.
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Old 16th April 2012, 02:19 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
Connect each wire end into a separate terminal of an insulated terminal strip.
The screw down will ensure you break through corrosion and insulation and thus avoid false readings.

Identify which winding are electrically separate from all the others. Some windings may have only two ends. Other windings can have intermediate taps. Sometimes as many as 5 ends on a single winding. Label all these different windings (maybe a, b, c, etc.)

Third: Thoroughly check that what you have done so far is accurate.

Measure the resistance from end to end of every winding. Write them down. Where you have more than two ends to a winding you are going to have to find the highest resistance and assume those are the two furthest apart tappings. Then by looking at the intermediate resistances you can work out from one end which order the tappings make contact with the winding in which order.

Check thoroughly. You don't want inadvertent shorting when you come to power up.

Build a bulb tester, test it is working correctly.

Power up the transformer via the bulb tester.
Measure the voltage from end to end of every winding and write them down.
Measure the voltage of any intermediate taps always from the same "end". Write them down.

Power OFF and take a well earned rest.

At this stage you have no idea what current you can draw from any winding.
That requires quite a bit of measuring and guesstimating.
regards Andrew T.

Last edited by AndrewT; 16th April 2012 at 02:22 PM.
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