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Old 8th December 2012, 11:41 AM   #11
JMFahey is offline JMFahey  Argentina
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Well, it's a very straightforward circuit.
Problems are not just "bad parts", that's only half of the picture.
You must check *connections* too.
Getting 120 or 240V "at the input" is just the beginning.
1) with the amp unplugged, switch it on and measure continuity across hot and neutral plug pins.
Switch it off and on to check power switch.
2) now plug it into the wall socket, turn it on.
**CAREFULLY** measure voltage across transformer primary (those wires are soldered somewhere, either switch or fuse), does wall voltage reach the transformer?
3) then **CAREFULLY** measure voltage across secondaries, both 6.3 V filaments and HV secondaries. What do you get there?
4) follow 6.3V wiring towards tube filaments and pilot light, does 6.3V reach them?
And so on and on.
Good luck.
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Old 10th December 2012, 11:03 PM   #12
Enzo is offline Enzo  United States
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If it used to power up, and now after a cap swap it no longer does, then the first suspect must be your work. I assume you pulled the board out, or at least freed it, so I'd wager some connection is mising or loose, probably a wire that leaves the board. Or you have pushed one of the wires of the mains circuitry onto the wrong push post.
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Old 11th December 2012, 01:12 AM   #13
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Hrm, all I did is remove the aluminum cover over the caps.. then desolder each cap and soldered a new one in its place with correct polarity. I just soldered to the existing joints.. was very simple task and all the wires are connected still. I also did continuity checks on each solder joint and components connected to those joints.
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Old 11th December 2012, 02:03 AM   #14
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Ok, so I determined it was a bad fuse holder that kept me from getting voltage to the circuit. (Did a continuity check and was infinite resistance--even when I wiggled the leads to it and pressed them in.)

I tried to temporarily solder the leads to the fuse directly but the fuse didn't have the right metal to take on solder. (The fuse is good btw, I did a continuity test.)

So I risked it and connected the wires directly together. Then I plugged it in and the power light came on then after a couple seconds it went out--I figured the cord came loose from the wall. I then plugged it back in and the power light came on for another second then a big spark came from the wall with black powder smoke and the circuit breaker broke connection.

Go figures, it would short .. I'll never risk not having a fuse again.

So I have no idea what's wrong now. When it was initially smoking with the bad caps it did not blow the fuse.

I guess I can buy a NOS fuse folder for around $9 off ebay.. I'll start there.. and buy a few fuses.

Last edited by audiogeekess; 11th December 2012 at 02:05 AM.
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Old 11th December 2012, 02:13 AM   #15
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Does anyone have the schematic for AB763? I've tried to find it to no avail.
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Old 11th December 2012, 02:20 AM   #16
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My BF checked all the diodes in the rectifier and said they were working properly (current flowing only in one direction). He thinks it's probably a bad transformer. If this is the case, I don't know why it didn't blow the fuse initially, when the old caps started smoking.
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Old 11th December 2012, 07:56 AM   #17
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Quote:
Originally Posted by audiogeekess View Post
Does anyone have the schematic for AB763? I've tried to find it to no avail.
A simple google finds multiple sites with it.
Attached Images
File Type: gif twin_reverb_ab763_schem.gif (80.9 KB, 23 views)
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Old 11th December 2012, 12:09 PM   #18
JMFahey is offline JMFahey  Argentina
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Quote:
So I risked it and connected the wires directly together. Then I plugged it in and the power light came on then after a couple seconds it went out--I figured the cord came loose from the wall. I then plugged it back in and the power light came on for another second then a big spark came from the wall with black powder smoke and the circuit breaker broke connection.
PLEASE do yourself a favor and send that amp to a Tech.
Besides the obvious safety problems, it will end up being *cheaper*.
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Old 11th December 2012, 02:04 PM   #19
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Obviously you should have followed your gut and not jumped the fuse, I would chalk that up as a learning expierience, a line you don't cross ever again. You need to correct the safety feature of your amp first and then I think befor I went and ordered a transformer I would disconnect the secondaries (Heater and HT) and see if the fuse blows, if it doesn't may not be your transformer. If you have any reservations you should follow JMFahey's recommendation.

Last edited by jeeptechfred; 11th December 2012 at 02:09 PM.
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Old 11th December 2012, 02:18 PM   #20
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Transformers usually don't just go bad. There has got to be a cause.

I would step back for a moment, leave the power disconnected, and run a few tests with a multimeter. Are any resistors blown? Etc. You don't want to risk making another mistake and completely blowing up the wall.

Buy and oscilloscope and learn how to use it if you plan to do much more diagnostic work. These are extremely useful.
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