drinking my morning coffee. found the amp you are talking about.
EPAC10010B or maybe 10010D, as also in the Galaxy DX-94HP 4 IRF520's.
that bias procedure is for the main board. TP7 VR13 just like most
of them other IRF520 RF AMP radios.
for your external amp, since you worked on it.
I would key up, in LSB mode. mic audio turned off.
carfully, no shakey hands, measure the voltage at the gate of any of the 520's.
turn VR2 for 3 volts. then measure the 3 volts at all the gates to make
sure this bias circuit is working. Then turn up to 3.7 volts and try it out.
the MOSFETS should be conducting then.
But better is to measure the current into the amp, through the red wire.
turn up VR2 for 200 to 300 mA.