Crate TV60 Bad Output Transformer?

[misterc57]

Received this Crate TV60 with no tubes and told that it may have a bad OT.

With all OT wires disconnected, on primary I have continuity from center tap to outer legs, reading 70 ohms and 90 ohms. Across both outer legs I cannot get a reading. The meter readout keeps jumping. I thought I would see the sum of the 70 and 90 ohms.

Secondary looks okay on resistance readings.

I also noticed that F5 is blown. F6 is okay.

I have attached schematic. I do not think this amp is worth putting money into but I wanted to see if I can bring it back to operation as I have nothing else to work on right now.

Thoughts on issues? Thank you

[Mick Bailey]

I have a DMM that jumps when taking readings as you describe. In my case its noise being picked up by the windings. I also get it with some pickups with the same meter. Even if you get readings that add up you could still have a shorted turn and this is not detectable with a DMM. You either have to use the 'neon trick' or get hold of a ring tester that essentially does the same thing in a more fancy way. Transformer failures aren't at the top of my list when I'm looking for a fault, but when they are defective a shorted turn is quite common.

The OT has flyback protection diodes so that's reassuring. It could have just had a shorted output tube on the side where the fuse had blown.

[misterc57]

Thanks so far. It looks like I am missing some of the schematic. I do not see any output tubes on the one I have.

[Helmholtz]

The reason for the measuring artefacts is the high primary inductance (full primary has 4 times the inductance of a primary half) and the counter EMF generated when connecting the meter. (I could explain in more detail if someone cares.) It is actually a good sign showing that there are most likely are no shorted turns and that the winding is not open.

Just short the secondary while measuring primary DCR to facilitate a stable reading.

[Jazz P Bass]

Here is the TV-120 schematic.
TV-120 Schematic.pdf.zip
It may help.

[misterc57]

Interesting. Not sure I have seen a circuit where each output tube has its own fused protection. Help me understand what is the general concept here. Thank you

[g1]

The power tube cathodes are fused. It helps prevent a bad tube from damaging other components, like screen resistors, or in worst case, transformers.
The Marshall JCM900 series had them mounted on the rear panel, with indicator lights showing that a power tube fuse had blown.

[misterc57]

Just short the secondary while measuring primary DCR to facilitate a stable reading.

That worked!

[Enzo]

Just short the secondary while measuring primary DCR to facilitate a stable reading.

That is what I always did, facing this.

[Helmholtz]

That is what I always did, facing this.

Sure, the method is obvious to anybody who understands transformer (and meters).