I'm helping a friend w/ his Grand Lizard and I'm very much a hobbiest newbie. Recently all of the solenoids (except flippers) quit working. The 2.5A fuse was blown. A replacement fuse blew again immediately. Prior to this, the game was running fine after replacing all lights to LEDs, new rubbers, etc... Tonight I checked every transistor for each solenoid and the smaller transistor that drives the bigger one. All looked normal in that the readings were mostly similar to each other. I also checked (and replaced) all diodes on the 7 solenoids. Old diodes checked out fine but I put in new ones while there.

What next? I'm not sure what I'm missing and not sure what to look at next. Any tips? I did find a leg on a diode disconnected on a EOS switch on the right flipper but can't see how that's related.

Ok thanks for the tip. So I had previously checked all the driver (power) transistors and the smaller transistor connected to them by removing the board and using my multimeter in diode mode to check between the three tabs of each transistor. Of the transistors like each other, their readings were very similar. Since this game is at a friend's house, I'm only over there every so often so it will be a while before I visit again. Following your suggestion (just to confirm), you're saying to mount the board back, disconnect the three connectors at P11, P12, P19, replace the diode, then while it is powered up, test the transistor leg resistance relative to ground?? Just curious, would that show me anything different than the method I did earlier with the board unpowered and out of the machine? Any risk in further blowing a transistor with it powered up doing the resistance test (such as slipping the probe shorting adjacent legs)?

One other thought would be to disconnect all three connectors, turn on the game then plug in one at a time until the 2.5A fuse blows again. Just thinking that would narrow it somewhat to at least which group of solenoids or flashers (or associated circuit) is causing the failure.

Again, thanks for the help. I was a little surprised I didn't find a blown transistor the way I was previously testing. Hope to find the culprit soon.

Solenoids and flashers (resized).png

Ahhh... this makes good sense. Thanks again for the explanation. I’ll give this method a try soon and will report back with what I find.

So I began testing like described above, checking resistance from ground to collectors on the power transistors with game powered up. Prior to this, I just reflowed all header pins, and remounted the board. I also found a disconnect diode on the right flipper switch ( lane change switch) so I replaced and reattached that. A new 2.5A SB fuse also replaced. After checking a few transistors appearing normal, I get to one that is 0 ohms. Ah, “that’s it”, I think. Then I found that one went to the coin door lockout, which doesn’t that normally stay on during game play? Something with the coin mechanism looked a bit bent up and mangled, and there were a couple quarters someone had put in around the time the game quit working. Not sure if there was a temporary short that blew the fuse or what, but the game has apparently played flawlessly now for at least 75 plays. I’ll probably disconnect that lockout solenoid the next time I visit my friend and his game.

I agree. I’ll just disconnect it. One less thing to cause a problem.