Quoted from Tuukka:There is a preventive measure which can be taken for this. Install a blocking diode on the driver board ceramic 10 watt resistor R224 (or R9 on WPC-95). To do this, first remove the lower leg of resistor resistor R224 (the leg just above TP7). Connect the anode (non-banded end) of a 1N4004 (or 1N4007) diode to the resistor's leg. Then solder the cathode (banded side) of the diode back into the driver board (where one leg of R224 was removed). This will prevent the problem.
Excellent advice!
This safety measure is implemented in my reproduction WPC-89 power board. One reason that new reproduction circuitry can be better than new re-manufacturing of the original Williams circuitry.
blocking_diode_20v_flasher_power.jpg
Hagerstown, MD
