Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ToodW on April 13, 2020, 07:10:32 pm
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I have a 63 Fender Pro AA763 amp I am trying to bring it back to life. So I am repairing and testing 1 stage at a time. First, I isolated the power transformer completely except for the 6.3VAC heater feed. Standby switch is off. Removed all tubes. There was no GZ34 tube because it was replaced by a couple of diodes soldered into the tube base to get the full wave rectification from center tap. I unsoldered the bias feedback tap to the PT. I checked the PT secondard and got 350 - 0 - 350. Then I removed all doghouse caps. I only put back in the 2 caps for DC filtering. So basically at the point I have a PT going through a couple of diodes and then to the 2 20uF DC filter cap wired in parallel and double checked for correct cap polarity. So its basically nothing but a PT to DC with no load at all. Even though I did not unsolder the taps for the heater circuits, there are no tubes so it is an open circuit.
So first fire up attempt, 1A MDL fuse, immediate blown fuse, then same thing for a 2 and 2.5A fuse. I have the proper 2A slow blow fuses but wanted to start out with undersized/fast acting fuses for first tests.
My question is is it normal for the amp to just blow fast acting fuses because of something like maybe in-rush current? I would just go ahead and stick a 2A slo blow in there but am worried there is something wrong. Or maybe it is normal to blow fast acting fuses on start up and thats why you use a slow blo. Any advice on trying the slow blo or should I test the unit more? Thing is, there not much I can think to test, its literally just the PT secondary feeding 2 DC filter caps. Thanks.
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WITHOUT THE PLUG IN;
ohm the secondary to chassis ground, basically meter across the cap. It should be many thousands of ohms "charging".
you can disconnect everything on the 2nd side, then do a fuse check, slo blo is correct. a good tech has a handful, any more than 2 you might want to pass :laugh:
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OK will do those checks. Here is a pick of what I am testing.
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Turns out it was a bad diode. Once I took it out and powered on, the fuse did not blow.
Still going to get a variac/bulb tester. Can see where they would be useful, especially one with a current indicator.
Also, going to put the GZ34 back in after testing power circuits.
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Diodes are the first thing to check, they could be damaged if somebody solder them with no heat sink
No fuse blow with no diode = no filter caps. issue my be come from not only from bad diode .....
A bulb current limiter will help and it is a must for somebody who fix amps.
Variac is not useful for this issue.
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I just went ahead and removed both diodes and wired it up for GZ34 operation. Stuck the old GZ34 tube in (not sure if original, but pretty old). No problems with blowing fuses and I get 538 Vdc for B+ across the filter caps and 378 - 0 - 378 on PT secondary. I have a brand new GZ34 tube as well but as the old one seems to be working I will just use it. So 100Vdc higher on B+ than schematic calls out (440 Vdc)