Vintage Ampeg blowing fuses, indicator lights on dim when amp is off...

[dcline]

I'm working on an a 1968 Ampeg Portaflex SB-12.
When I plug it in, the indicator lights come on dim indicating a short somewhere which blows the fuse when I turn the amp on.
Pulled all tubes... problem remains.
Checked all filter caps... all have been replaced and even after disconnecting them... problem remains.
Checked the only solid state diode in the amp and it's good. All resistors good.
Disconnected all secondaries on PT. Does not blow fuse. Only when I replace the 370 volt center tap wire the indicator lights come back on dim and fuse blows.
When I check the ohms from the two red 370 volt secondaries to one of the primaries, one reads 3.40 ohms and the other reads 95 ohms. I'm starting to think the PT is bad but just want to be sure before I inform the owner. Funny that it don't blow any fuses when the secondary center tap is disconnected.
Any suggestions?
thanks in advance

[g-one]

What resistance do you read from secondary center tap to primary? I would think the center tap is ground and without it your secondary is floating. When you connect the center tap you are basically shorting the primary to ground (due to the short from primary to secondary).

[dcline]

43 ohms to one side of primary and 45 to the other

[Enzo]

SOunds like the transformer is shorted. You have the two secondary wires plus their center tap. With all three of those wires disconnected from things, if there is ANY resistive continuity to the frame of the transformer or to the primary windings, the transformer is shorted.

[R.G.]

When I check the ohms from the two red 370 volt secondaries to one of the primaries, one reads 3.40 ohms and the other reads 95 ohms.

Lessee here. 3.4 ohms to AC VOLTAGE PRIMARY(!!) and 95 ohms to AC voltage primary on the other one, with two half-secondaries that measure about 90 ohms total?

It sure seems to me that one side of the HV secondary is shorted to the AC power primary. To test that, open the AC power primary(ies). Unsolder them, cap them off so they don't touch anything. Now look for continuity from the 3.4ohm secondary to any of the primary wires. If this is less than a solid open circuit, the power trans is unsafe, and must be replaced. One of the major jobs of a PT is to isolate the secondaries from the AC power lines in a DC sense - no DC continuity, just an open circuit.

I'm starting to think the PT is bad but just want to be sure before I inform the owner. Funny that it don't blow any fuses when the secondary center tap is disconnected.

That makes sense. The CT is what connects the HV to the chassis, and connecting the CT to the chassis open it up enough to not blow the fuses. Do the DC continuity test. If you get DC ohms less than an indicated open circuit from any primary wiring to the HV secondaries, the transformer is a goner in almost all cases. There may be some clever circuitry that makes for some leakage - that's what unsoldering the primaries for testing is intended to isolate.

This thing is dangerous if there is a primary-secondary short.

[dcline]

The HV wires have been disconnected all along. And I am getting continuity between one of the secondaries and the primaries. Thank you for helping me verify that the PT is bad.
I've only found one exact replacement for this transformer and it's $125.00
The owner is not gonna be happy. Anyone know where a guy might find one cheaper?
Thanks again for all the assistance.

[R.G.]

The HV wires have been disconnected all along. And I am getting continuity between one of the secondaries and the primaries. Thank you for helping me verify that the PT is bad.
I've only found one exact replacement for this transformer and it's $125.00
The owner is not gonna be happy.

I feel for the guy, but you can't let him use a PT with a primary-secondary short, if only on grounds of not wanting to explain it to your lawyer.

I'd offer him choices, not suggestions. An exact replacement may be $125.00, but perhaps there is some non-exact replacement that he'd be $50 or $60 happier with than perfection. The exact specs on a power transformer (at this point, I look guardedly to either side...) are not an enormous factor in the sound. Some people have an almost religious faith that they are, but - well, the owner might take less than perfection, dependent on his wallet.

See if you can find a close replacement with a reasonable amount of searching, and offer him the choice. He has three choices - perfect replacement for a price, "good enough" replacement for cheaper, or taking a chance on dying for his music. Don't help him with the last one.

[Steve Conner]

Well, the amp doesn't even work, so he can't die for his music. He'd just die.

I'm sure a generic power transformer from Weber, Hammond, or whoever would work. The hard part is finding one that fits in the original mounting hole. If you have to start cutting bits out of the chassis, your labour costs can easily eat the difference between the cheap PT and the "right" one.

You also ought to factor in your hourly rate, and the time spent searching for the PT.

Also beware, Weber have been known to sell random transformers on special offer because they have weird design flaws, like the CT is about 25 volts off the centre of the winding, etc.

[dcline]

Once again I'd like to thank everyone for all their help and suggestions.
Hammond has a Fender replacement that is so close. Just not enough voltage on the HV secondaries. Gonna do some more searching when I get in this afternoon.

[Steve Conner]

How much is the voltage difference? Could you make it up by replacing the tube rectifier with diodes? (Cheap and nasty I know)