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It's pretty frustrating when stuff doesn't work as you expect and my own experience is that the "now what!?" emotion is a constant for me.

Follow the schematic with me. The amplifier's power for the regulator and assorted switching diodes is switched on and off by the PMOS transistor Q4 that R10 is connected to. R10 has one side connected to the source of the transistor and the V+ line and the other side is to the gate of the transistor. When The amp is not in receipt of a 5v signal from the QCX, the gate of Q4 is at source potential, held high by R10 which keeps the transistor in the off condition. You should see a green light from the leds which tells you there is main power.

Applying +5v to the PTT ring contact relative to ground will light the red led via its dropping resistor R16 (bottom right on the schematic) and will also turn on Q6 via R15. When Q6 is turned on, this pulls the drain low which pulls the bottom end of R11 almost?to ground. R11 and R10 form a divider which means that the gate voltage on Q4 gets pulled down to half the main supply voltage (more or less) which turns Q4 on, and its?drain at the top of R13 is pulled up to its source (remember this is a PMOS device, so the source is positive and the drain is usually a little more negative in this condition - upside down from what you get with the NMOS devices like the BS170 parts). Follow the line above R13 and you will see that it is in fact the line that connects power to the regulator. So if the red led isn't on, Q6 is off, so Q4 is off and there is no power to the regulator, hence zero voltage on the pot that you use to set the bias. If you want to make it independent of the QCX, take an audio cable and connect a pair of AA batteries in series so that the negative is to the ground of the 3.5mm plug and the positive is to the ring (NOT the tip, that isn't connected in this case) and you should see the red led come on. Now you should be able to set the bias current. Try this before you start tearing bits apart, you'll feel a lot better about it if you don't have a broken pot.

Having just spent a couple of nights fiddling with this very issue trying to persuade the amp to run with my FT-817 and without making it not work with my QCX, I was forced to work on this. What I ended up doing in the time honored?way of the experimenter, was to wire the input to the regulator to the V+ line directly?(chopping its pcb trace at its input) which means that you need to be careful not to set the pot wrongly or this will simply draw a lot of DC current through the two IRF510s and get warm or waste battery. I then routed an extra wire to the tip of the PTT socket so that I could turn on the amp remotely without needing external power; you simply short tip and ring together. Arranging for the FT-817 to control it was a bit more of an effort but it all seems to work quite well.

I hope this gets to you before you run out of patience, good luck and let us know how it all went.

Best,

Roland AE6VL