If a playfield feature light gets stuck ON after a few minutes, and then after a few minutes more it finally goes out and acts normally, is that bad ???

Solid state Bally ( surprise surprise ) had a lamp board with issues.  i replaced about 10 transistors on it. Only problem remaining seems to be one insert that will stay lit after one or two minutes of play.  It is essentially a bonus light, so after the ball drains, the bonus is counted and each light is supposed to go out, but this one light does not.  Once it gets stuck, it may take a game or two for it to go off ( when it's supposed to ).  Or, if you reset the MPU, the light goes back to normal.

I've changed the transistor, checked the traces, checked connectors, checked the socket, checked voltages, etc.  The light is activated by a switch, so could it be a switch matrix issue or a switch diode issue ?  The switch does not have a cap.

This thing is driving me nuts.

Does the issue exist with another lamp driver board?
Was the playfield ever swapped in this game?
What game is it?

What part number did you use to replace the bad SCRs
(these are not transistors) on that lamp driver board ?

Perhaps time to use a logic probe, to identify if the issue
is with the output of the decoder chip feeding that specific SCR
on the lamp driver board, or if with the buffer, if indeed a new
SCR does not help.

The signal path to drive the controlled lamps is as-follows in early Bally SS:
MPU U10 (J1) - LampDriver (J4) - 4514 decoder - 4050 buffer - SCR - lamp

Good luck !
- Sylvain.

Quoted from sylvain

Does the issue exist with another lamp driver board?
Just put in another lamp board and this time a different insert stuck on and never went back to normal.  That probably is just a bad SCR, but I'm not sure.  I am out of SCRs after replacing so many !

Was the playfield ever swapped in this game?
No

What game is it?
KISS

What part number did you use to replace the bad SCRs
(these are not transistors) on that lamp driver board ?
2N5060 and a single 106

Perhaps time to use a logic probe, to identify if the issue
is with the output of the decoder chip feeding that specific SCR
on the lamp driver board, or if with the buffer, if indeed a new
SCR does not help.

The signal path to drive the controlled lamps is as-follows in early Bally SS:
MPU U10 (J1) - LampDriver (J4) - 4514 decoder - 4050 buffer - SCR - lamp
Hmmm....time to get a logic probe perhaps.  The MPU was refurbed for me, so I was leaving that possibility as a last resort.  I could probably just get the other lamp board working, but I hate to leave this lamp board with an issue.  If I don't fix it now, I will probably just forget all about it until I really need that board.

If this other lamp board works fine, then what would that mean for the problematic board ?  What type of problem would I be looking for if it was the board itself that has the issue ?

Well a fast way to to eliminate the MPU as the problem is to swap U10 and U11 and see if the problem still occurs. That *should* eliminate the MPU as the source of the problem. My money is on the 4514 or the 4050(if equipped).