You may remember I’ve written a few times before about using your multimeter in diode test to find faulty 74xx series logic chips. You can also use this same technique to troubleshoot problems further down the line.

After repairing this dead Williams High Speed system 11 logic board, I found the “f” segments on the displays were locked on. Back to the bench I put my meter in diode test and probed all of the display output pins. I found the problem on 1J22-24 (connector 1J22, pin 24), the ribbon cable connector.

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An important thing to note is you’ll need a schematic so you know which pins are grounds. Most pins gave me a ~.530-.730 ish reading. One pin gave me a .092. Thinking it may have been bad, I saw on the schematic that it’s tied to the RESET line. Checking other places on the RESET circuit verified the reading is correct.

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One pin (pin 24) was reading 1.921. I traced this pin back through SRC2 (resistor/capacitor network) to PIA U41. Since there’s a decent amount of battery damage I removed SRC2 to make sure it wasn’t causing the problem. It wasn’t so the next logical step is to check continuity between connector pin 24 and pin 15 on the PIA. I had continuity between the SRC and the pin, but not between the PIA and the SRC.

If you know me you know I hate jumper wires on the board, but sometimes it’s a necessary evil. So I did what I had to do and got the display circuit working correctly again. During testing I found the “g” segments weren’t working. I really didn’t suspect the board but I pulled it anyway and took it back to the bench.

I double checked all the display output pins again. For good measure I put my test ROM in and checked that all the relevant PIA output pins were pulsing. Everything checked out.

Someone had replaced all the headers and pins on the harness for 1J1, 1J2 and 1J3. The pins weren’t crimped properly and the wire for pin 2 on 1J3 pulled out. I crimped a new pin and all is well with the displays.