Hi all.

Trying to fix some issues with a System 3 driver board out of a SMB. I am suddenly getting switch shorts reported on startup for returns 3, 4, 5 and sometimes 6. This issue is on the board - replacing the board fixes the problem. Unplugging the switch matrix connectors allow the game to boot so it is definitely related to the switch/lamp matrix circuitry. All of the transistor tabs are reading 20v. I tested each transistor but they appear to be OK. What is the next place to go from here? I suspect U6 or U8 but I've never had to replace these before and it seems like an odd place for the circuit to fail.

Any ideas?

The issue is on the board rather than the playfield as swapping boards resolves it.

OK, I've done some more troubleshooting on this. The switch return shorts seem to change around for no reason. Today there are shorts on returns 0, 4, 5. I thought it might be an issue with the MOSFETs on the lamp returns. I replaced the FET for lamp return 5 but there is still a short being reported for that return.

Most of the shorted returns were associated with the LM339 at U11, so I replaced that with no change.

Now I'm thinking U5, the 74HC273, might be the issue. Is there anything else I should be considering in the strobe side of the board or am I going in the right direction with the return side? I appreciate everyone's input.

Found it.

A header pin at P2 was bent. I presume that someone jammed the connector in too roughly. When I tried to straighten it, it broke free from the board completely. I jumpered the connection normally made by this pin and the ribbon cable (from the driver board to the CPU board) and the switch return shorts disappeared.

So, took me a while to realise a relatively simple fault, but this is the first issue I've ever had with a Gottlieb header, especially this one on the driver board. I was on the right track with the troubleshooting. The next step in the chain after U5 would have been P2. Good thing I saw it first.
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