Terminator 2 Switch matrix column "GND Short"

[Parisno]

Noticed that the Jets, slings and kickback targets have stopped working so took a look at the switch matrix in diagnostics. Boot of the machine shows no errors enter diagnostics and test "T1 Switch Edges"

If I close any of the switches in column 4 that contains all the non-working items it shows the whole row as closed, see pictures. This happens for any switch in column 4 the whole row closes, the rest of the switches seem to work fine

Any ideas where to look next or what could be wrong?

 

[Jay Walker]

Hello,

It may be a limited help, but I think that Williams' procedure to isolate the problem between playfield and cpu board for System 11 should also apply to Wpc. Remove the switch matrix connectors from the cpu board, then (in a switch test) link a pair of the relevant pins, e.g. for Column 4, Row 4, with a piece of wire. The result should be simply Switch 4,4. If it's still a Row Short, then at least this shows that the problem's with the board rather than the playfield. AIR, the switch Columns are operated by a '2803' driver chip, U 20?

 

[Parisno]

OK, so I short out the pin corresponding to column 4 with any of the row pins and I still get a full row showing as closed, any other column pin shorted to a row pin one switch closed.

So that means its the board, yes?

 

[Ped]

Definitely look to isolate whether it's the playfield or the CPU first - once you know that you can narrow it down further.
If it's playfield, and all switches are closing in the column when you activate one then it's likely a diode failed on one of them. Do check for any ground shorts but in practice it's more likely you won't find any and the problem is elsewhere

If it's CPU then the U20 ULN2803 is a common failure point which can show as a ground short - if you're going to replace it then use a socket so if it goes again it's an easy replacement.

 

[Sam C]

If that's my old T2 (which it looks like) I'd suspect it's the board, it had some very not-pretty hacks on the back side done to it by a previous owner which may have at last bitten the dust. You may want to pull it off and look at the rear side of the CPU board to see what is going on there.

 

[Parisno]

Thanks for all the replies!

@Jay Walker your comments regarding 2083 and U20 rang a bell, replaced it and bingo all back to working order, when I say rang a bell when I first got this pinball it pretty much died on me and turned out it was this same chip and could only buy a pack of three (I see why now they are rubbish). Am I doing something wrong or do these blow a lot?

@Ped Its a replacement board by PinLED and already has a socket made things really easy

@Sam C How can I tell if this board is yours or if it has had some dodgy stuff done to it?

 

[Pick Holder]

If the ULN2803A constantly blows there is some voltage going down the switch line somewhere. Look for stray strands of wires touching lamps or coils near by! These faults can be sods to find.... but the 2803 chip is a.k.a the 'fuse' chip by Todd Tuckey at TNT amusements. On most CPU boards they are socketed.

 

[Parisno]

Its blown twice in six months, do I need to be looking for a problem somewhere. It is played pretty much every day.

 

[Sam C]

It looks like my old game because of the LED caps fitted and I also put in those light up ramp signs. The red rubbers were on it too.

I had the legend Andy round to sort some small issues out at the time and this was what greeted him on the back of the CPU board, he said it was okay at the time but it would want replacing eventually.

 

[Pick Holder]

Its blown twice in six months, do I need to be looking for a problem somewhere. It is played pretty much every day.

They usually go when someone messes with say a lamp holder next to a switch and wires cross from the switch matrix to the lamp matrix (or solenoids). They do not really fail for no reason.

It is also one of those faults I could find in 2 minutes or 2 days ! Intermediate faults are horrid to find but I would look first at all the cables near switches on row 1 and 2.

 

[Parisno]

It looks like my old game because of the LED caps fitted and I also put in those light up ramp signs. The red rubbers were on it too.

I had the legend Andy round to sort some small issues out at the time and this was what greeted him on the back of the CPU board, he said it was okay at the time but it would want replacing eventually.

View attachment 127626

It may well have been yours at some point in its life, but nothing like that on the back of the board when I took it off to replace the 2803, and it had black rubber on when I got it the red is all me.

 

[Jay Walker]

Re. frequent failure of U 20; a Getaway at work was actually pulled out of its location early due to repeated failure of U 20. Almost as soon as it was back at the workshop, I found a solenoid winding (the lock release pin) touching its coil bracket where the outermost part had rubbed through the wrapper. After that, I often added insulation to coil brackets if a large coil such as a 23-800 was fitted.

Maybe it was fitting, though. That location had also seen off an original High Speed for random lock-ups. It was in disgrace in the workshop when I started there. The problem was a broken diode on the upper flipper.