Fixing the WPC-95 GI Diodes

When Williams switched to WPC-95, they dropped from five controlled GI circuits down to three. They replaced two of them with always-on GI, eliminating two heatsinks and triacs from the board. A triac causes a voltage drop, so Willams inserted four diodes in each alwasy-on circuit in a series-parallel arrangement to reduce the GI voltage. Presumably, they did this to extend bulb life. But they didn't figure out that the diodes would make as much heat as the triacs - which have great big heatsinks. So the diodes get hot, and burn up the PCB. It blackens and delaminates. Williams eventually switched out the diodes for zero-ohm resistors. Here, we look at how you can easily remediate a bad board with just two wires.

The diode jumpers are equivalent to the following connections:

J103 pin 11 to J105 pin 5
J103 pin 12 to J105 pin 6

These connections can be reversed if you are soldering from header to header: it makes a tidier layout. You can jumper from header to header, across the diodes, or from diode in to header. The jumper will depend on how good the PCB is.

Here is a repair using one header-to-header jumper, and one diode in to header jumper. The header-to-header is always guaranteed, and completely bypasses the diodes. The best wire to use is solid core 22GA. Plenty thick enough for the current, and it will hold its form well. 22GA stranded is a good alternative. Thicker wire is unnecessary, and harder to solder neatly. For a full header to header repair, the other wire would go to the end pin on the input connector, adjacent to the other jumpered pin.

Here's the top side of the board, showing the cooked out diode area.

And a brand new board, showing the factory jumpers.