next is for a technician to check the fan circuit. Maybe a bad connection, maybe C17, the capacitor across the fan is shorted, maybe something else.
The fan should run all the time slowly, and speed up as the amp gets hot. The "base level" of speed is set just by resistors from the positive power supply "rail", so if it doesn't run, and doesn't start when the amp is hot, then either there is a bad fan, a bad connection, fan power shorted, no power to the circuit, or the circuit is not working.
You checked the fan, apparently, and the amp works, so we assume that power is available to the fan. It is unlikely for both sections of the fan circuit to be bad together. So probably a bad connection, or a shorted C17.
Yes I USED TO work for Ampeg...but I haven't forgotten everything.