How it took me 2 years to find out in my Mac 1900 trouble shooting....

My Mac 1900 was sounding good but after paying for few days, this was after replacing all the lytic coupling caps in the signal path, but not the power supply, however PIO and high quality lytics were used to bypassed the PS lytics. So I found after playing the R ch had very hot output transistors heat sink. So I tried to find out... after changing the driver transistor to TIP 31c/32c, and adding the by-pass small caps at the b-c junction all over the power amp, it does not remove the problem. The problem may come in all of the sudden and I could not detect any problem or any damage in any of the transistors. At the end I sent it to an experience technician to repair. When it was back, the transistors of that ch from the diff input pair to the driver had all been changed, except the driver right after the input pair. But the driver board is now plugged into the L ch instead by the tech. Of course by then I could not remember which board was driving the ch as this was after more the a year.
Well, I played it a long time later after I got it back, and it was playing all right, but the L ch may have some intermittent muting on and off. The sound would be muted for a few minutes to half an hour, and then it would come back to normal by it self.
Anyway, i could not find out the cause as everything measured normal, with all active devices working well, resistors within spec, all caps has been replaced.
One day, the other R ch, the one that originally have over heating problem, blew the fuse. In the course of repairing it, I blew both the output transistors. So I used MJ 21195/96 to replace the original Motorola NPN output devices, ad covert the circuit to complimentary Emitter output configuration, which I wanted to try all along anyway.
Now with a 22ohm i place of the fuse, I tried to find out the cause that blew the fuses and output devices.
It seemed that the OCL output could suddenly swing to -3.8V, from the normal -.9V if I switch it on and off a few times. this is without ay output load. When the amp was off, the output would swing around zero, and if I turn the amp on when the output was above +1.5V, it will trigger the output to show -3.8V, ad thus very high current to the -ve rail.
Finally, i have to conclude that some how the Triac at the output stage that supposed to protect the amp would some how behave like a diode, became conductive triggered by the swig of the output voltage.
When I remove the Triac of both ch, it cured both the intermittent muting and the intermittent over heating problem of R ch.
It was a very long work for such a problem... Otherwise, the circuit is stable, and the sound is fantastic now, after I removed the input caps ad resistors, as well as increase the bias to more the 100ma.

So conclusion, do suspect the triac in the amps!

Best rgds

William Lee

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