Stern SAM board switch drive transistor failure (SOLVED)

[lilshawn]

any reason in particular on a S.A.M. cpu/sound board, that the transistor used to drive the switch rows (Q1) would fail?

it failed open... entire row of switches acted weird (ghost switches on same columns and even different columns) or it didn't work at all. Replaced the transistor and it tested okay in another machine. but machine is out of town. so i don't want it to not work.

I'm all for random failure, but I'm just wondering if I should look for a wiring issue. Maybe a grounded line some place?

[lilshawn]

well last week I replaced the board in the machine. (Family Guy)  Booted it up and started testing switches on the row. I found an electronic optical switch for the single dropdown target that wasn't working on that row. Pulled it out and disabled the switch in the test mode.

it has an emitter and a receiver on it (slot + shutter type switch) and then a surface mounted MOSFET to send the switch signal back to the MPU. perhaps this MOSFET shorted and killed the switch row transistor. not entirely sure, but I'll be ordering a new switch anyhow.

[lilshawn]

UPDATE:

received the replacement opto switch. received a call the next day that there was an issue with the machine. no big deal, i'll fix it when I take this opto out there and fix the works.

damn drive transistor for the switch row is toast again. 3 weeks of working and now toast again. 

back to square one.

[lilshawn]

The weird problem is no longer weird. It's possible to blow Q1 on Family Guy if you have 70V from a bad coil on the upper left flipper base plate.

seems there is some extra turns on the coil causing it to rub through and touch on the mounting plate.

The lower wood screw for mounting Flat Rail FR14 can touch one of the mounting screws for the base plate of the upper
left flipper from below. If the ball hits the G-10 target switch (Meg Standup target...The standup switch has a metal point rivet for the white plastic face which has a direct connection to switch drive #1. ) and also touches the rail at the same time... 70V goes into the switch matrix. Bzzt!

[lilshawn]

seems you need to have the stars aligned just right.

the combo of the close (touching) proximity of the screw for the flat rail... the coil rubbing through the paper insulation...the ball hitting the target while touching the rail AND the metal rivet in the centre of the target.... gotta be worth a few million points and a free game for that combo shot. 

the toasted opto switch was the start of the give away. the opto switch and the switch matrix both operate on 5 volts... why is it fried??

there is only a small handful of switches on the first row so eliminating them was pretty easy.  (2 of the switches aren't part of the playfield and 3 aren't used and the one opto I removed leaving 10)..... I started out looking for rubbed wires or touching parts...looking for anything that looks wrong. switches all seemed good. diodes tested good. decided the microswitches aren't the issue (eliminated all but 4) and focused on the standup switches (as they had the most exposed metal.)

started looking around and decided the only thing powerful enough to kill something under the playfield is the flippers.

3 of these switches have nothing around them. only one left... of course it's Meg... Shut up Meg!

damn it!  looked under the playfeild didn't see anything touching the wires or the lugs. pulled off the coil and see it's rubbed through. well that will do it.

for shits and giggles I continuity buzzered the switch and the rail with my VOM... it's actually fairly hard, but it does connect. which is why it was fine for a couple of weeks then crapped out.

so ya, coil windings to mounting plate to screw holding the flat rail to pinball to rivet to switch drive transitor... BLAMO!

kinda like a sick electronic version of how people had a dozen things happen before they die in the Final Desination movies

[lilshawn]

i used some thicker "splicing tape" it's like electical tape, but about 5x wider and about 3x thicker.

ya i painted some clear nailpolish over the rubbed through portion of the coil wire insulation... i also wrapped splicing tape around once on the coil and placed a layer on the plate under the coil for good measure. i also ground down the tips of the screws for both the mounting plate and the flat rail. i also used a plastic washer on the flatrail screw for extra insurance.

this scenario is unlikely to happen again.

[BobA]

Nice troubleshooting.