TOA idle current setting process??

[bnwitt]

I am replacing the power board in a TOA A-912MK2 PA amplifier. I'm a little confused on the written procedure for setting the idling current on the amp. It seems I am to take millivolt readings across each of the 4 power transistors' emitter resistors with the power off first??

Then if the readings match those in a temperature table, I am turn on the power and set the control pot to achieve the readings in the table. That doesn't seem right. Should I get identical readings with the amp on and the amp off? Am I misunderstanding the instructions? Am I to check the idling current at the emitter resistors of all 4 power resistors both times and set for an average of the 4? I've attached the instructions and the schematic.

It seems to me that I should set the control pot full CC, power on the amp, check the 4 Q's emitter resistor voltage drops in mv, adjust to the match the table reading at ambient temp, then wait 3 to 5 minutes before readjusting. So for instance, at 77 degrees ambient I should be aiming for 4.4mv across the 4 emitter resistors. All of this with zero signal and no load right? Or should I have a 41ohm 200 watt load resistor on the 70 volt output and a 1kHz signal injected for the idle current set up like is done for the distortion adjustment.

Thanks for any help in advance

[52 Bill]

It seems to me that I should set the control pot full CC, power on the amp, check the 4 Q's emitter resistor voltage drops in mv, adjust to the match the table reading at ambient temp, then wait 3 to 5 minutes before readjusting. So for instance, at 77 degrees ambient I should be aiming for 4.4mv across the 4 emitter resistors. All of this with zero signal and no load right? Or should I have a 41ohm 200 watt load resistor on the 70 volt output and a 1kHz signal injected for the idle current set up like is done for the distortion adjustment.

Thanks for any help in advance

These instructions are written very poorly.

I agree with you, set up the amp with no signal and no load. First check the output for voltage (This is where they say set the output to direct).

Then read the voltage across all 4 emitter resistors, this way you can see how well matched the outputs are. If any one of them reads too far off value from the others, then replace the mis-matched transistor.

Let it run for a while to warm up and re-check the readings.

[bnwitt]

thanks Bill, I'll set the switch to direct and check pin 2 of connector 107 for DC first. All should be fine as this is a brand new board with all new components.