OK, so power amp B is blown, but powr amp A by itself will work, right?
By channel board do you mean the little driver board with the row of pins along one edge, roughly the size of a pack of guitar strings? Are you saying that if you plug the B driver board into the A power module then A starts blowing fuses?
Or does module B blow fuses with either driver card?
First, go to the output terminal posts. Measure resistance between them. Are the B posts shorted together? When a channel blows, it often shorts the protective triac which is on the output terminals. If the red and black posts are shorted together, then you probably have a shorted SAC187 triac on the circuit board they are soldered to.
There are two rows of power semis on the module heat sink. Most are outputs, but the end one of each row is a driver transistor. Check each one for shorts. There are also a bunch of 10w ceramic resistors. Mostly low resistance like 0.33 or something. Check ALL of them for opens.
There is a diode on the heat sink with two wires running from it up to the driver board. Make sure it is not open. It is a dual diode, so the voltage drop across it will be twice what would be "normal."