Testing Output Transformers

[fossilshark]

Hi all! Working on a Fender Deville and I suspect the OT is fried. I made up this test fixture to easily test OTs (by applying AC to the speaker output) however I am unsure if it works correctly.

When I connect the transformer to the output (connected through the speaker jack) the voltage drops to 0, wich I have ran this test before (on a much smaller, functional OT) and it worked fine. The only change I have made since last time I tested it was a pot and resistor so I can vary the voltage the 1VAC, as the current OT im testing would turn 6VAC into like 1600VAC (or about that) and my meter only goes to 750VAC.

I test 2ohms DCR on the 8ohm secondary and 104ohms DCR (52ohms between the CT on each side) on the primary, does this seem normal?

Heres some pics of the deathtrap.

[shooter]

When I connect the transformer to the output (connected through the speaker jack) the voltage drops to 0,

put the VAC directly to the OT wires, nothing else
an OT is  Step-Down, so when you put 6vac on the output it WILL step-UP at the input, IIRC typical is something ~~~~~~~~~ 30:1
so i would expect 6 X 30 VAC at the input

[fossilshark]

Yes thats what I am expecting, the OT is soldered right to the speaker jack so thats why I made that little adapter.

My problem is the voltage drops to 0 even tho the DCR of he 8ohm line is 2 ohms so with the test fixture set to 1VAC (no load) it should only be drawing 500ma wich is perfectly reasonable for the transformer im using in the test fixture.

My question is if that is normal or if that is indicating a short in the transformer/blown transformer. Could me adding the resistor and pot to make the ac voltage of my test fixture variable be adding some sort of impedance or something that would cause the voltage to drop to 0?

[tubeswell]

Could me adding the resistor and pot to make the ac voltage of my test fixture variable be adding some sort of impedance or something that would cause the voltage to drop to 0?

You won't be able to get much useful information out of that IMO..

[Merlin]

Why do you need it to be variable? Easiest way to test an OT is to hook the primary up to the wall voltage, just like a PT. If it blows a fuse, it's shorted.

[fossilshark]

Well I wanted to test it by sending 1VAC to the secondary of the transformer and reading the output on the primary, why would that not give me any useful information?

Would the fuse still blow if it was the secondary of the transformer that had a short?

[Merlin]

Well I wanted to test it by sending 1VAC to the secondary of the transformer

The OT secondary looks like a very low impedance, so you would need R1 to be just a few ohms to knock the voltage down to ~1V, and no pot needed at all.
But even 6V into the secondary should NOT be giving you 1000V+ on the primary

Would the fuse still blow if it was the secondary of the transformer that had a short?

It should do yes, or it should at least buzz loudly and give a nonsensical voltage reading at the secondary (you can work out what the voltage should be from the impedance ratio, if you know it)

[fossilshark]

OK I will do the test using mains.

The math I ran to get the higher then 1000v number was the input impedance (4200) / 8 = 525 * 6VAC = 3150VAC, did I bungle that up?

[Merlin]

OK I will do the test using mains.

The math I ran to get the higher then 1000v number was the input impedance (4200) / 8 = 525 * 6VAC = 3150VAC, did I bungle that up?

Impedance ratio is the square of the turns ratio, so:
Voltage ratio = square root (4200 / 8) = square root (525) = 22.9
22.9 * 6VAC = 137VAC

Stick with 6V into the secondary, since you have built it into a nice box. Get rid of your pot system, you don't need it.

[fossilshark]

Awesome, running 6VAC right into the speaker jack give me about 10VAC on the output on the primary, I am going to assume this transformer is blown. Thanks for the help!