it sounds like a bee? i have heard that too when they don't work and i think it is an oscillation in the feedback loop on the TL494 controller. i never understood how to isolate the source of it.
you can measure the correct voltage on the output but it does not push current. or current cannot be measured because the pulses of current are so short.
the fan runs off the 12V that comes from the voltage regulator through that transistor next to the 8 pin op amp and 100 ohm current limiting resistor for fan speed.
there is a set of 4 small diodes right behind the transformer that come off of the second set of windings in the secondary of the transformer. those 4 diodes make a rectifier diode bridge to create the current used to power the electronics in the 'back end' of the charger.
put the black probe on the negative output or on the big wire shunt which is also at ground, and probe the ends of the four diodes there to see if you can see 20V DC while the charger is powered up.
edit, there should be voltage there since the TL494 is running the oscillator so maybe there is a short somewhere between there and the 12V regulator over on the side of the case. the trace that carries the current goes under the choke so sometimes the choke will melt through the coating over the trace and short it out so there is no 12V coming from the regulator to power the fan. so look for a short between the place where the choke is soldered to the board and the trace underneath it that goes over to the voltage regulator. you should be able to check continuity with your meter.
