This is an indication of how incredibly bloody-minded I can be.
This amp belonged to Paddy; he used it as a spare, and so was not in any rush to get it back. But this amp stretched the limits of credibility and patience. Most jobs I turn around in a week, this one took THREE YEARS.
Paddy would turn up from time to time, with various jobs (a Stingray bass, an Ampeg SVT Pro, an Ampeg SVT) and would ask, more or less in passing, how his Ampeg BA300 was coming along.
It was, I would shamefully say, work in progress. That was fairly obvious as it was in a hundred bits on the bench, and, as I mentioned, didn’t move for three years. I use three benches in the workshop, and the BA300 just sat on the middle bench, daring me to repair it. Every spare hour I would go back to it, work through my notes, order further bits. The fault is, it would seem, a common one. It should (don’t know why) switch off if the output power exceeds 300 watts. So if you give your bass a hefty slap, it will switch off, if you are already pumping some power through it.
I think this is quite a daft idea. But it’s double-daft if it switches off when pushing 12 watts out. Or so. This is what Paddy’s amp was happy to do. Why Paddy didn’t actually chuck it over a cliff I’ve no idea, and it is a great compliment to his incredible patience.
At the risk of repeating myself (there is another, earlier and desultory blog on this subject) this is a mosfet, class D amplifier, with bridgemode output. A class D amp has carrier signal (of around 400+ KHz if I remember right) and this is modulated by the audio signal. The modulation in a class D amp can be either frequency or amplitude modulation; this one is frequency modulated. Why anybody would want to modulate it in the first place is a mystery to me. Probably to make sure it switches off at embarrassing moments.
I did replace a hell of a lot of components in this amp (the sign of being lost in a swamp) and every time it made no difference at all. However…..
You need a schematic for this next bit, and I must say that it is not difficult to find them on this amp. SO FLAGS OUT FOR AMPEG. Not everybody is in the Secret Society just yet. To the left of the 2nd page is the dual preamp chip U1 (4558) in which the two op amps are used in cascade. The output from pin 7 sends the analogue signal via R1 to pin 6 of U2 and the other opamp in this package is used to mix the 400+ kHz carrier signal and frequency modulate it. This dual chip is a TL082. YOU CANNOT USE A TL072 (as would often be ok ). In this case, it won’t work.
You don’t want to see a dc voltage at pins 1 or 7 on this chip. I was seriously stuffed with this as I changed the chip I think three times, and each time I got a dc voltage at pin 7. Eventually, you start to think that there is something here that you don’t know about, and assume that this is how it should be. No it isn’t.
Eventually, it turned out that there was a crack in a track on the pcb UNDER THE CHIP. ARRRRGGGGGHHHHH!!!! Even when I had the chip out, you couldn’t see it. I resorted to testing for continuity on all the tracks associated with the U2 chip.
The eventual reason for the fault was that the pcb was too thin and flexed when the vibration hit it. That was my guess, anyway.
I would not wish this repair on anyone.
But I did get a celebratory pair of macaroons. And in the evening, a celebratory Bowmore malt scotch. Did I deserve that one. Oh yes!!!!!