Output Transformer Troubleshooting

[PepeSilvia]

Hey guys,

I'm trying to identify whether or not I have a blown output transformer, using the instructions here: Output Transformers

There are a couple points I'd like clarification on. First of all, when testing the primaries, it says to "measure the resistance from the B+ lead to each plate of the output tube(s)". I'm not sure if this varies from amp to amp, I'm currently working a JCM2000 DSL100, but isn't what it saying just to test the resistance across the windings of the transformer? That is, the B+ supply connects to the center tap, and the heater pins connect to the other two taps on the transformer. If that's the correct method, I was measuring in the sub 1Ohm range. Does this indicate that there is a short in the windings?

[R.G.]

Hey guys,
There are a couple points I'd like clarification on. First of all, when testing the primaries, it says to "measure the resistance from the B+ lead to each plate of the output tube(s)". I'm not sure if this varies from amp to amp,

Yes, it does. The DC resistance of a primary winding is not something that is standardized.

but isn't what it saying just to test the resistance across the windings of the transformer?

Yes.

That is, the B+ supply connects to the center tap, and the heater pins connect to the other two taps on the transformer.

Plate pins. The heaters connect to the power transformer.

If that's the correct method, I was measuring in the sub 1Ohm range. Does this indicate that there is a short in the windings?

No, it doesn't. What measuring a resistance tells you is that the winding is not open. The difference in resistance with a shorted turn will not necessarily be enough to show up on a resistance meter.

When measuring resistances on an output transformer, you're just trying to establish what's connected to what. The primaries may be a very small resistance, or may be a couple of hundred ohms. What matters is that neither half-primary is open. And that the secondaries are not open. And that the primaries are not shorted to the secondaries anywhere, nor to the core.

The way you test for a shorted turn is quite different. It's at GEOFEX too:
Tranformer Shorted Turns Tester
What this does is directly test whether the inductive nature of the transformer is being poisoned by a short. If (1) an output transformer ever worked correctly and (2) it passes the what's-connected-to-what test you're doing and (3) it passes the shorted turns test, then it's most likely good. The only possible way around that is an intermittent short or open that will not show up at room temperature or at low voltages. Thermal and voltage intermittents like that can happen, but they're quite rare.