A Power Control circuit question

[jlovda]

Well, after three weeks of listening to the MC7100 amp I purchased, I discovered that the Power Control circuit does not work.

Looking at the schematic I see that the input power goes through a relay controlled by a Triac with an opto-isolator gate. In the normal ON position the Triac is shorted to turn on the relay. When a Power Control signal comes in, it turns on the LED which turns on the Triac. I asked a question earlier about paralleling the Power Control output of my preamp. I see from the schematic that when the Power Control signal comes into the amp it goes out through a buffer made up of four Dual NAND gates. It's not a straight pass-thru.

Has anyone else here had to replace a Power Control Triac on a modern amplifier? How robust are they? The Power Supply board on this amp is not easy to remove. One quirky problem is that half the wires going into or out of the board are socketed yet several others are soldered directly to pins. The MA6200 and C33/C34 units I have worked on are beautifully modular to disassemble. Not so with this unit.

Thanks,
John

 

[damacman]

John - there was a thread a while ago here from another 7100 owner with the same problem. He was never able to make it work IIRC, but I don't think he attempted any board level repairs either.

 

[jlovda]

John - there was a thread a while ago here from another 7100 owner with the same problem. He was never able to make it work IIRC, but I don't think he attempted any board level repairs either.

I think I found the problem but I am mystified at the situation. I took a 6v lantern battery and applied it to the open end of a power control cable. Nothing. I looked very closely at the service manual and saw that the PC "in" and "out" sockets are on a little board attached to the back panel. The service manual shows three wires coming out of the board and going back to the power supply board and the main relay. On my unit the third, ground wire is missing. The is both a little pad on the circuit board for a third wire and a third position on the socket at the other end. If I take some wires and directly apply the hot lead of the 6v to the "in" trace and the negative to the main chassis, the PC works. The "in" and "out" sockets do not have a ground return??? I find it hard to believe the unit could leave the factory that way. I am looking very carefully to see if the wiring was changed to save money by trying to use the chassis for the ground return. The little circuit board is stuffed into a very tight spot so it's hard to see all the traces. I may call Mcintosh tomorrow to see if they have seen this situation before.