Power Transformer - Crackle and Hot!

[soma_hero]

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?

[Phil_S]

Unfortunately, once you let the smoke out, you can't put it back in without a rewind. It will be cheaper to buy a new one.

Just curious...is there a center tap on the PT and did you ground it? With solid state rectifier, this is a no-no. Also, if you built a full wave bridge, you will get (would have gotten) ~1.4x the VAC output.

[soma_hero]

Well there was no smoke which is good, but definitely some crackling.

No, there's no center tap, so I did a full wave bridge rectifier. I don't remember the exact voltages now, but i'm going to disconnect the secondaries and see if the voltages are back to where I thought they were before. If not then I know i've goofed the thing up.

[Phil_S]

IMO crackle = smoke. Just the kind you don't see. The real question is, where did it come from? Sorry for your misfortune. You are right to disconnect. Then you will know if it is the amp or the tranny. It could be tubes. Good luck.

[soma_hero]

Well everything disconnected, I get 420VAC across the secondary.

The diodes i was using are 1N4002 which are only rated for 100V. That must be the issue. Hope the tranny will still be good to go, long-term, with new diodes.

[soma_hero]

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 and pre amp grounds from the chassis , 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?

[soma_hero]

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?

[Ears]

420V (no load) across a non-centre tapped secondary through a bridge will give voltages well over 500V into a capacitive input filter.
The problem here is that your descriptions just aren't accurate enough for any real assistance. Suggest you offer a schematic.

[Andy Le Blanc]

the hardest issues to overcome when scrounging are your own assumptions
without some refference or understanding of what it came from .......
you might be S.O.L. ...... if I were a manufacturer and could save a dollar
on 10,000 units by useing a halfwave rectifier and sourceing a tranny that
was internally wired to save another two bucks on assembly I'd have
$ 30,000.00......... a lot of consumer product is crap even if it is tube and
older than you are.... try and find a scheme of what it came from or try and
reconstruct the power side from whats left of the carcass if its still in your bone pile...

we've all had projects that went up in smoke....... flame or fame!?!?

[Phil_S]

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.

Are you certain that only the heaters are grounded this way? Maybe the HT is also grounded this way.

Unsolder the HT leads again and see if you get half voltage between each lead and the frame of the PT. You might check first by measuring ohms (with the power off).

With SS recto, you need to break such a ground if it exists, otherwise it WILL let out the smoke.