Quick and easy OT testing

[Gibsonman63]

I have a '73 Fender Champ on my bench. The guy said that it powers up, but no sound comes out and after a few minutes the fuse blows. I took it apart and noticed some yellow waxy stuff on the speaker. Apparantly, the yellow waxy stuff dripped out of the output transformer... not a good sign. Is there an easy way to test the OT using just a multi-meter?

[Chevy]

Possible one of the xformer windings are shorting to ground? Or to itself? That may account for the fuse blowing... and the heat. Anyways, just measre all the resistance values from all xformer leads to ground (or to the xformer metal) with the VOM if that's all you have. Also measure the primary and seconday winding R values. That info would help troubleshoot things. Check on the schematic, you may have to disconnect a xformer lead or two to get an accurate reading.

Also, I don't know which specific schematic that year is, but looking at the CBS champ schem, there appears to be a 470R cathode resistor. If you could measure the voltage drop across that while the amp is running, it'll tell how much primary current you've got in the xformer... which may also help point out the problem.
Chevy

[Gibsonman63]

I pulled the preamp and power tube and checked voltages and everything looked reasonable. Put the tubes back and powered up with the light bulb current limiter. The amp turned on and didn't seem to pull excessive current, but no sound. I put in some tubes from a working Champ and voila... sound. The power tube is no good and there is a burnt looking spot on the side, but I wasn't sure from just looking at it is that was wierd or not. With a working power tube, it sounds like it should even with some of the potting melted out of the transformer.

I guess I am getting to the point that it is time to invest in a function generator and a scope.

[R.G.]

Is there an easy way to test the OT using just a multi-meter?

No. But there is with a multimeter, plus an inductive kickback tester.

The multimeter tells you
(a) are the windings burned open?
and
(b) are the windings shorted to each other or to the shell
both of which are "transformer dead" conditions.

It does not tell you whether there is an internal short between wires, which is a common way for the transformer to die.

Fortunately, it's simple to make a transformer short tester. For reference, this is on my web site as Transformer Short Tester.

You might enjoy looking at these:
Tube Amp Tips

[Gibsonman63]

Thanks RG. I like tools I can build myself. Normally, I just work on my own stuff, but I have all of my amps working so I am starting to take on a few odd jobs to learn more about tube amps and make a couple of bucks on the side.

[vintagekiki]

The simplest way to examine the OT, is the measurement of transformation as the PT.
Bring external 6V AC voltage to the secondary OT. Put AC DVM to the primary OT. Primary AC voltage will be increased by the transformer ratio (n). Transformer ratio (n) is square root of Ra divided by Rspkr.
For the SE Ra is 5000 Ohm, and Rspkr is 4 Ohm transformer ratio n = square root 5000/4 = 35.35
At right OT, for 6 VAC on the secondary, primary voltage must be 200 - 220 VAC, or Uprim = Usec * transformer ratio.
When examining OT, amplifier (Champ) must be disconnected from AC line voltage.