Fender HRD with fried OT

[xtian]

I think I've identified a fried OT, but let me know if I missed anything.

First thing to note was a LOUD hum/buzz IMMEDIATELY on power up, even with power tubes removed. So, some AC getting on the OT, with no tubes installed.

I pulled the three OT primary spade connectors off the board to isolate the OT primary from the circuit--even so, I measure 70 ohms between OT primary red (center tap) and chassis.

Finally, powered up with OT disconnected, voltages look good.

[Helmholtz]

So we suspect a short between OT primary and core or secondary.
I would completely remove the OT from the amp and check with Ohmmeter.

[xtian]

Confirmed--even with all six OT wires completely disconnected, I still read 70 ohms from primary CT to chassis. Also, 180R from blue to chassis, 310K from brown to chassis. Now, that's weird, isn't it? And on the other side, I read no connection between any secondary wire and chassis.

Replacement OT ordered.

[Helmholtz]

Confirmed--even with all six OT wires completely disconnected, I still read 70 ohms from primary CT to chassis. Also, 180R from blue to chassis, 310K from brown to chassis. Now, that's weird, isn't it? And on the other side, I read no connection between any secondary wire and chassis.

Well, the measured R values look consistent. I calculate that yout OT has 240R between brown and CT and 250R between blue and CT. The 250R is divided by the short to core into 70R + 180R. Between brown and blue you should measure 490R.

[xtian]

My measurement of 310K was not a typo. That's what's weird. Anyway, we're on track. Will follow up next week.

[Helmholtz]

My measurement of 310K was not a typo. That's what's weird. Anyway, we're on track. Will follow up next week.

That's weird indeed. I expected 310R. What do you measure between brown and blue? Some meters have problems measuring the DCR of a large inductor.

[xtian]

Will have to wait--I packed the amp away to clear space.

[Leo_Gnardo]

Replacement OT ordered.

Good, that's what you'll be needing. You have shorts inside the OT between the windings and core or case. And that brown lead, looks like the winding burned open but there's probably some conduction between the brown wire and core/case due to leftover carbon from an arc. Anyway, it's toast.

[Mick Bailey]

I'd be wanting to take the end bells off to take a look at the construction and potential failure mode. Its a poor situation that a relatively modern Fender transformer would be able to short to the core. The case I can understand, as I've had insulation on old transformers go brittle and break.

[Jazz P Bass]

I had a HRD come in that was blowing fuses.
Until I lifted the OT off of the chassis.

[Mick Bailey]

Do you know if they're wound on a paper wrap over the core?

[xtian]

I went ahead and removed the OT, and opened it up.







Can't see any fault without digging further, but leads meter out the same.

[Helmholtz]

So "vintage style": no coilformer that could prevent a short to core.

but leads meter out the same.

Means what? Do you measure low DCR to core? To prevent meter error, short 2 secondary wires while measurung primary DCRs. What is DCR between brown and blue?

[xtian]

With end bells removed, DCR from red to code (secondaries shorted or not, makes no difference) is 1K5. DCR of red to brown reads 13M5--not completely open, but bad.

My meter is Meterman 38XR true RMS

[Mick Bailey]

Thanks for posting the pics. Quite educational and I wasn't expecting it to be so traditionally constructed. I guess there's a burn right through the first wrap of paper to the core.