Kenwood KAC-X401M Amp need help

[zeppo4]

I have tried to attach a schematic.

This amp has a CPU which keeps powering off the unit. When I power the unit it starts and makes 65volts + & - about 5 seconds later it shouts down. In the diagram Q20 & Q21 are part of the Protect circuit and Q21 stays at 5V. Q20 has 65volts on the Base and Emitter and .001volt on the collector. I also replaced the 5.1 zener although it checked out.

There is .04 volts at the most on the outputs but I don't think that would shout it down. I did find diode D32 across the relay to be shorted and replaced it. I have also pulled all outputs and checked them ouyt of the board(they were fine).

Please help me with any ideas.
Thanks Jeff

[Perry Babin]

I don't have the manual for the 401. Using the manual for the 201, there is a transistor connected to the positive rail that drives the protect input of the processor. The 5.1v Zener is used to limit the voltage sent into the input of the processor. If you remove that transistor, the amp shouldn't be able to go into protection and you should be able to troubleshoot the problem.

When the protection circuit is disabled, you must be very careful/observant. You should have the power semiconductors clamped to the sink and you must monitor current draw. You should have a current limiter on the B+ supply line. If there is a real fault causing the amp to go into protection, it's possible that damage can occur very quickly if you're not careful.