Transformer Troubleshooting

[Nictrolis]

Hi all!

Just working on my Ampeg J12A the other day, which has a developed some sort of fuse-blowing short. I think I've found that the power transformer's bad, but I haven't worked much with them before so I wanted to do an idiot check with you guys before I drop $160 on a new one.

Here's the question: how does a power transformer behave when unloaded?

I'm worried that when I disconnected all the secondaries, the transformer will blow a fuse anyway because it's completely unloaded. As I understand it, when you connect your mains up to the transformer, the inductance of the primary should keep the winding from looking like a short between the line and neutral... but I have a nagging feeling that I'm forgetting something I learned 10 years ago.

Also, what's the best way to test that a transformer meets its specs?

Thanks for your help!!

[Bruce / Mission Amps]

+9 times out of 10, blowing fuses is caused by bad power tubes or a singular power tube.
But, fuses are cheap compared to R&R the trannys.
You will not blow up anything with no load on the PT.

But first, just pull all the tubes out and start over with a fresh fuse.... if it doesn't blow, I rest my case.

If it does blow, then remove HI-v secondary center tap wire from the OT to the rectifier to see if the OT has a short and causing the overload.
If it still blows, then remove the B+ rectifier lead to the main filter cap(s) circuit to see if there is a short in the caps or the circuit board somewhere.
If it still blows then you might have a short on the filament wiring somewhere.
Seriously, removing and replacing the PT is the very last thing I would do .... that doesn't mean it won't be bad but I do all the easy stuff first.

[52 Bill]

Here's the question: how does a power transformer behave when unloaded?

When you disconnect all of the secondaries of the transformer from the amp circuits, the fuse should hold unless there is a problem in the transformer or in the primary wiring. For me, that is usually the last test before I order a new one. Well actually, I pull the transformer completely out of the amp and hook it up through a current limiter as the final test.

Check each winding with an ohmmeter. Look for any continuity between windings and from the windings to the core.

Check the primary wiring for shorts either between hot and neutral, or hot to ground. Be sure to check the ground cap if it's still in circuit.

Go to the geofx site and find the transformer shorted turns tester schematic. It only uses a few comonents and will test the transformer for any shorted windings.