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Hi, one and all. I am repairing a marshall AS50R acoustic combo. It was lent out, wired to a generator and Ipod for a party. When it was returned, my friend discovered the whole amp dead.
Initial investigation revealed the main output power transistor LM3886 had gone, the +v 7815 rectifier was also a goner, and the speakers were open circuited. These, I replaced, and tested the amp. It worked, sounding great with the two new speakers. However, the chorus and reverb circuits were dead.
Voltages from the LT+ and LT- on the main PCB were approx 32v to the two main 5w 100 ohm resistors, BUT ..on the circuit side of these - going to the two main rectifiers -the +ve voltage had almost halved to 16.9v, and the 5w resistor was very warm On the -ve rail, all appears well at -30.5v, and that resistor is cool.
On the other side of the rectifiers I have a reasonable voltage of +14.6 and - 14.9
5 IC's are on this main 15v circuit, and I considered that invariably the IC's in a circuit usually go first with a voltage spike (which it seems had occured to this amp). Perhaps, I thought, one of them has failed and is pulling this +v voltage down?
However, they all have over 14 volts, +ve to pin 8, -ve to pin 4, as they should. There is also one CA 3080 IC which instead has roughly the correct voltage at pin 7, again, as it should.
Additionally, none of these are in the reverb or chorus circuit. Somewhere, something is causing the voltage drop - 50% - on the Lt side of the rectifier. But where and what, is the question? Could it be the 2 chorus IC's MN3007 & MN3101? But then again, the reverb circuit also is dead. The switching IC M5201, possibly?
Anyone have experience of similar problems, and/or maybe an idea as to the possible solution?
Initial investigation revealed the main output power transistor LM3886 had gone, the +v 7815 rectifier was also a goner, and the speakers were open circuited. These, I replaced, and tested the amp. It worked, sounding great with the two new speakers. However, the chorus and reverb circuits were dead.
Voltages from the LT+ and LT- on the main PCB were approx 32v to the two main 5w 100 ohm resistors, BUT ..on the circuit side of these - going to the two main rectifiers -the +ve voltage had almost halved to 16.9v, and the 5w resistor was very warm On the -ve rail, all appears well at -30.5v, and that resistor is cool.
On the other side of the rectifiers I have a reasonable voltage of +14.6 and - 14.9
5 IC's are on this main 15v circuit, and I considered that invariably the IC's in a circuit usually go first with a voltage spike (which it seems had occured to this amp). Perhaps, I thought, one of them has failed and is pulling this +v voltage down?
However, they all have over 14 volts, +ve to pin 8, -ve to pin 4, as they should. There is also one CA 3080 IC which instead has roughly the correct voltage at pin 7, again, as it should.
Additionally, none of these are in the reverb or chorus circuit. Somewhere, something is causing the voltage drop - 50% - on the Lt side of the rectifier. But where and what, is the question? Could it be the 2 chorus IC's MN3007 & MN3101? But then again, the reverb circuit also is dead. The switching IC M5201, possibly?
Anyone have experience of similar problems, and/or maybe an idea as to the possible solution?