Power Transformer - Crackle and Hot!

[Michael Allen]

I cannibalized an old phonograph, with a 12AT7, 12AX7, 6V6, to make a Fender Champ.

Wired the power tranny up for testing with my multimeter and after solid state rectifying, had nearly 400V if i remember correctly.

Now I wired the whole amp up, turned her on and I hear crackling sound coming from the PT and it gets real hot! The voltage on the filter caps is now only 210V.

Did something perhaps fry/short in the power transformer?

[Enzo]

More likely something is wrong in your wiring and the poor transformer is being strangled.

How about the tubes? Pull them, especially the 6V6. Now what? If it is still loaded down, then disconnect the wires a winding at a time to see which is being loaded. For example ther might be a solder bridge at one of the tube sockets.

And lastly, power up that transformer with ALL the secondary wires disconnected. Now does it spit out full AC or doers it still get hot and have low voltage. If it gets all hot and stuff with nothing connceted to its secondaries, then it is bad.

[Steve Conner]

Irrespective of what caused it, I'd put any transformer that made crackling noises in the trash.

The only things that could cause a crackling sound from a transformer are internal arcing, or bubbling wax due to extreme overheating.

Even if it was the fault of the external circuit, and it stops doing it when disconnected, it's probably damaged enough to be a fire hazard if you continue to use it.

[Michael Allen]

Well, I was using 1N4002 diodes which are only rated for 100V. I think this was the issue. Unconnected, the transformer measures 420VAC across the secondaries so I think its still good. Need new diodes..

[Robert M. Martinelli]

You should use 1N4007s which are rated at 1 Amp 1000 V, but, just like Enzo and Steve said, I would not trust a transformer which has suffered from overloading/overheating, there is a chance the internal insulation is not that good anymore and, even if it looks ok, you could have problems in the future, especially if the insulation' s been damaged ( think about the possibility of shorting coils ). If you' re determined to use it anyway, I would advise you to run an insulation test with a "megger" ( an instrument used to check winding insulation on motors/ inductances/impedances/transformers ) to state if it is the case to use it or not....

Hope this helps

Regards

Bob

[Michael Allen]

I replaced the diodes with 1N4005 and was still getting bad voltages, 130V after rectifying.

I built this champ on a PCB i threw together, with the ability to have isolated star grounds connected off-board at a chassis star ground. Right now the pre amp and power amp have different grounds that are connected on the chassis with the output transformer secondary. If I disconnect the power section ground from the chassis and pre amp ground, then the bridge works fine, 373V after the bridge. When I take the amp off standby the voltage shoots up to over 550V. I hear a little fizzing which I'm sure is my power filter caps which are only rated at 450V so I kill the power.

How can I lower the voltage before the caps? With a big enough resistor? Or do I need a lower voltage PT?

Also, any ideas why connecting grounds to chassis would mess with my voltages on the PT? Possibly a short somewhere?

[Michael Allen]

I put a 220K resistor after the standby switch and the voltages are about 440V after bridge rectifier, voltages look good.

I took apart the power transformer and found that the 6.3vac heater has a center tap connected to the frame so when the PT is bolted to the chassis the center tap is grounded. When the pre amp and power supply grounds are connected to chassis the transformer gets loaded down and voltage drops to 130v, and gets hot. Why would this be? The center tap of the heaters should be grounded so why would this jack up the power transformer?