Fender Champ AA764 I think '73

[nosaj]

[SAG Electronics]

I said "warped" because it was no longer square, and yes, I kept the old one since the only reason I replaced it was because my first DMM gave me bad readings. I like to advocate pawn shop supplies, except when they fail. As soon as I installed the new transformer, (after going through the whole scmeal of the supplier sending the wrong one first) I found it to be o.k. even though it was no longer square.

I am building the light-bulb limiter and, hopefully getting some new info from that. Funny how hard it is to find a simple incandescent bulb these days. At least, at my local hard ware store.

[SAG Electronics]

So, I built the light bulb limiter. Great little device, but my local hardware store was limited in the options for bulbs. I got a 75w work light. Anyway, Built the limiter, plugged it in (off position,) plugged in the amp (also off position.) Then I turned on the limiter, turned on the amp (no tubes, as had been the issue.) Pilot light (?) lit and bulb did not. Started with the preamp tube. started glowing! Next added the 6V6 and it too started glowing! I have to admit, This is my first time seeing this. Still no glow on the limiter bulb. Finally started to plug in the 5Y3 tube and ... sparks are a-flyin the limiter bulb is lit, electricity is a-crackling and I am fearing for my life. I think I have successfully isolated the problem part of the circuit.

Thank you guys for all your help so far. I am still overwhelmed. However, seeing those tubes light up thrills my heart and gives me the encouragement to carry on!

[Helmholtz]

I think I have successfully isolated the problem part of the circuit.

Maybe, maybe not.
Without the rectifier tube there is no HT/B+, so it could be the 5Y3 but a number of other things as well. The PT seems to be good, though.

[52 Bill]

You should start by powering the amp up through the limiter with no tubes installed. If there is little or no glow from the limiter lamp, then the amps' primary wiring and power transformer can be assumed to be okay. There may still be a power transformer problem, but no light from the limiter is a good sign. At this point you can test for the ac voltages coming from the transformer. There are two filament windings and the high voltage winding. Don't worry so much about the actual voltage readings, just see that they are close to what should be there.

Then install the rectifier 5Y3 tube. This will be the first test of the high voltage power supply circuit. If the light doesn't light up brightly, then start testing the high voltage dc voltages. If the high voltages are all present, then add the 6V6 power tube, again checking voltages as you proceed. Finally add the 12AX7 preamp tube.

If at some point your limiter light, lights up bright then you know that the tube that you installed either is bad or has connected the problem circuit to the power supply. At this point, you need to investigate to find the reason for the increased current draw, like a bad tube or bad tube socket or a shorted wire, etc.

Good luck

[SAG Electronics]

As soon as I add the 5Y3, the bulb lights up. The voltages to the socket all are good before that.

[Helmholtz]

As soon as I add the 5Y3, the bulb lights up. The voltages to the socket all are good before that.

Without the 5Y3 there is only the 6.3VAC heater supply but no operating HT voltage for the tubes.

[nosaj]

As soon as I add the 5Y3, the bulb lights up. The voltages to the socket all are good before that.

please specify lights up bright and stays or lights up bright then goes dim with a little glow?

You should probably power off before plugging tubes in, referring to earlier with sparks a flying.
nosaj

[SAG Electronics]

Lights up bright and stays bright. I am powering down between changes now.

[g1]

If you saw sparks in the 5Y3, it's probably bad. But like Helmholtz said, the 5Y3 provides power to the rest of the circuit. So if there is a fault right after it, the bulb will light up as soon as you install the 5Y3. And if it's a bad enough fault, like a shorted filter cap, it could probably make the 5Y3 spark internally as well.

The next step may be to disconnect the power to the output transformer and see if the bulb still lights (with 5Y3 in). From the layout it looks like that would be the OT red wire.
Schematic and layout for AA764 Champ attached.

[SAG Electronics]

Thanks g1! I have those diagrams in a book my professor gave me at graduation (I unsuccessfully made a pickup and a preamp for my final projects.) I had just replaced the O.T. because things a month or so ago had me looking to a problem there. I am planning to look into that section of the circuit tomorrow when I get a chance to work. Hopeful again. I will keep plugging away at it. More test leads, less soldering iron.

[SAG Electronics]

Whoa! I just noticed in the schematic that it shows 3 20MF caps. On the amp, though, it has a 20, 20, 40 can. I replaced with 2 22MF and a 47MF. Could that be a problem or is that a misprint in the schematic!?!

[Enzo]

It is just a guitar amp, part values can vary a wide range and it will still work. If I had to replace a three section can with 20/20/20 and all I had was a 20/20/40, I would use it in a heartbeat.

So, no, it is not a problem, and no the schematic is not likely wrong. It is just a difference. Look on the schematic notes, upper right. It says right on it "voltages...values shown + or - 20%".

[Mick Bailey]

And it wasn't uncommon for electrolytic caps to have a wide tolerance. Many used to be marked -20%/+80%.

[eschertron]

...and many champs were upgraded at the factory with 20/20/40 cans, schematics often followed production practices, not the other way around.