hey johnboypitt. I don't know if you're still looking for answers, but here's my thoughts on your problem if you're interested:
I'm an electrical engineering student, and I'm pretty much in love with this amp. I actually just built my own modified version of it for a school project, so I know a little bit about it to say the least. If you have no electronics background, you could perhaps present some of this advice to a repairman.
Here's my logic flow from the problem which you described:
(Before all of this excessive technical explaination I'm assuming that the problem is not because of some dirty pots or because of a backwards capacitor.)
I'm assuming that your clean channel works because you said that the problem only occurs when you're on the gain/distorted channel. This implies that your power amp is working just fine, which is good because that's the hardest part to fix due to the high voltages/currents and the much greater complexity compared to the pre-amp. Also, the fact that one of your pre-amp channels works implies that the problem must lie in circuitry specific to channel A of the pre-amp. This is good because you could replace the entire channel A for about 15 bucks (so it's definitely worth it for you to keep this gem). If I am correct in assuming that the sound coming from the amp is the same, but the volume is the only change, then that means the signal is being passed through the EQ and distortion stages (because these stages contribute the most to how the signal will actually sound). This leads me to say that the problem must exist at the last gain stage of channel A. There are two parts that would be of concern here: an op-amp and a JFET. The problem is most likely in active parts because these can be damaged non-visibly unlike resistors for example. Also, depending on what damage has been incurred either of these parts could cause the problem you're describing.
If I had to guess, I would say to replace that last op-amp in channel A's signal path. The part number is AC4558P. There are 3 of these on the pre-amp board (if you decide to open up the amp, the pre-amp is the board with the control knobs); the middle one is the one that you want to replace. You prrobably won't be able to find that specific part anymore, but any op-amp with the 4558 number in it would do the trick. The JFET part number is J112.
As for the capacitor being the problem, I would say that if it failed multiple times, the capacitor blowing is because of one of the active components failing, not because of the capacitor itself.
If getting it repaired causes too many issues, I have seen these on ebay several times periodically. You just have to check it occationally and within a month or two one should come up. It's tough to say how much I'd pay for one of these. I got mine for 150, but that was because it was broken. How much did you pay for it 20 years ago? I can't find an original price for this thing, and I'd be interested to know what the original design went for.
Hoped this helps,
Rob
I'm an electrical engineering student, and I'm pretty much in love with this amp. I actually just built my own modified version of it for a school project, so I know a little bit about it to say the least. If you have no electronics background, you could perhaps present some of this advice to a repairman.
Here's my logic flow from the problem which you described:
(Before all of this excessive technical explaination I'm assuming that the problem is not because of some dirty pots or because of a backwards capacitor.)
I'm assuming that your clean channel works because you said that the problem only occurs when you're on the gain/distorted channel. This implies that your power amp is working just fine, which is good because that's the hardest part to fix due to the high voltages/currents and the much greater complexity compared to the pre-amp. Also, the fact that one of your pre-amp channels works implies that the problem must lie in circuitry specific to channel A of the pre-amp. This is good because you could replace the entire channel A for about 15 bucks (so it's definitely worth it for you to keep this gem). If I am correct in assuming that the sound coming from the amp is the same, but the volume is the only change, then that means the signal is being passed through the EQ and distortion stages (because these stages contribute the most to how the signal will actually sound). This leads me to say that the problem must exist at the last gain stage of channel A. There are two parts that would be of concern here: an op-amp and a JFET. The problem is most likely in active parts because these can be damaged non-visibly unlike resistors for example. Also, depending on what damage has been incurred either of these parts could cause the problem you're describing.
If I had to guess, I would say to replace that last op-amp in channel A's signal path. The part number is AC4558P. There are 3 of these on the pre-amp board (if you decide to open up the amp, the pre-amp is the board with the control knobs); the middle one is the one that you want to replace. You prrobably won't be able to find that specific part anymore, but any op-amp with the 4558 number in it would do the trick. The JFET part number is J112.
As for the capacitor being the problem, I would say that if it failed multiple times, the capacitor blowing is because of one of the active components failing, not because of the capacitor itself.
If getting it repaired causes too many issues, I have seen these on ebay several times periodically. You just have to check it occationally and within a month or two one should come up. It's tough to say how much I'd pay for one of these. I got mine for 150, but that was because it was broken. How much did you pay for it 20 years ago? I can't find an original price for this thing, and I'd be interested to know what the original design went for.
Hoped this helps,
Rob