"ODEP" crown macrotech

the good Channel (B) reads 34.5Mv

Thats too high, must be under 10mV and in reality is usually less than 5.

What's the cause and should it be under 10 at start-up?

I've adjusted odep to spec and the protect issue stopped.

there isn't an IOC circuit schematic as such, it's all incorporated across multiple boards.
The final IOC led drive circuit is on the front pcb behind the front panel, then it's driven off the main control board analogue integrator which is sensing error conditions via the ES signal (overall amp error signal ie clip detection) and the high side error flag which is itself driven from one half of the output section.

In short, anything that prevents the amp from reaching equilibrium will light the IOC light...which is, as the name suggests, an input-output comparator.



readme

its still got a fault then, either the odep is still throttling the drive or one of the amp stages is not conducting. Dont forget each channel has 4 parts to it, high side positive, high side negative, low side positive and low side negative.

maybe a heatsink tracking issue. The bias should stay fairly steady, with the bias voltage automatically reducing as the transistors warm up to keep the bias current steady. There are a string of small diodes on the output transistor heatsink that are in thermal contact with it, check those are ok. On some variants they are sealed under silicone and on others just buried in heatsink compound. If any are leaky the bias will be too low as it uses the forward voltage drop as a reference for the bias voltage.
Also check the associated fusible resistors and zero-ohm links. Both resistors and diodes can suffer if the thing's been damp.

If this is not addressed then the amp channel may go into thermal runaway when in heavy use and blow the output transistors again...this may have been the original cause of the fault!