Quoted from longtemps1:
I found a section in pinwiki.com on Williams WPC tech. Are there other specific documents you recommend? I can also go back to my experience as a kid in the pre-digital electronics age, as these seem to be good old fashioned discrete driver transistors, nowadays controlled by a digital circuit. If the trouble is at that level (and from what you describe this seems likely) I have a fighting chance.
When you mention “compare measurements at points of the column circuit that has failed to one that operates correctly“. If the transistor or associated passive components is not the cause, do you mean downstream, i.e. toward the lamps or upstream toward the digital controller?
Thanks again for letting me reach out to you.
Sorry I took so long to get back to you:
I would start with the lamp testing process that confirms the problem is on the board, to rule out the playfield as playing a part. The driver transistor is the typical weak link, although other things can contribute. If you find the signals going to the drive transistor are whacked, just move upstream from there. Only so much can be wrong with the "channel" if other columns are working correctly, and usually won't extend further upstream than the driving PIA.
This is an excerpt from the WPC lamp testing in the repair docs:
Testing the Lamp Columns.
If a TIP107 transistor that drives a lamp column is suspected as bad, test it:
Remove the backglass and fold down the display, to gain access to the Driver board.
Turn the game on.
After the game boots, press the "Begin Test" button in the front door. Go to the Test menu's "All Lamp Test" test.
Unplug the row connectors at J133 (or J124 on WPC-95) and column connector at J137 (or J121 on WPC-95). These are on the lower right portion of the Driver board.
Connect an alligator test lead to row connector pin 1 of J133 (or J121 on WPC-95). Pin 1 is the right most pin, as facing the board.
Connect the other end of this test lead to one lead of a 555 light socket. One can be temporarily borrowed from a playfield lamp (make sure the lamp works first!).
Connect another test lead to the second lead of the 555 light socket.
On the other end of the test lead, clip on a 1N4004 diode, with the banded end away from the alligator lead.
Touch the banded end of the diode to column connector J137 (or J121 on WPC-95) pin 1. Again, pin 1 is the right most pin, as facing the board.
The lamp should flash.
Move the diode/alligator lead on column connector J137 (or J121 on WPC-95) to the next pin. Again, the lamp should flash.
Repeat the previous step, until the last pin of column connector J137 (or J121 on WPC-95) is reached.
If a lamp column tested doesn't give a flashing test lamp, that column is bad (or the test diode is reversed!). No light or a non-flashing, bright lamp are signs that the respective column TIP107 transistor is bad. Test the transistor as described in Testing Transistors and Coils.