I have a Roller Coaster Tycoon with a lamp matrix issue. In the single lamp test, when testing any lamp in Row 7, the lamp lights-up as it should, but the same column lamp in Row 6 also lights-up (just not quite as bright). If I turn on the entire Row 7, the entire Row 6 lights-up too (just not quite as bright).

I fiddled around checking all the diodes in Row 6 and 7 with a multimeter, but no luck. What the heck is going on? It's like there is some kind of semi-short circuit between Row 6 and 7.

Only rows 6 and 7. I completely disconnected the playfield (J12 and J13, rows and columns) and tested Row 6 with a lamp and alligator clips. Same thing. When Row 7 is active, Row 6 lights-up too, just not as bright. I replaced the Mosfet at Row 6, and even replaced the Mosfet at Row 7 for laughs and giggles. No luck. I plan to replace the next component upstream, the 74HCT273 next... however, I've never seen one go bad before.

One other small clue, I noticed that the Mosfet at Row 6 had been replaced before, prior to me getting the game.

I carefully replaced the 74HCT273N. Used flush cutters to cut out the old IC, and a Hakko FR300 to clean it up. That went smoothly, but the result... no change, same problem as before. I guess that means the problem could be anywhere on the data bus. Dang. Kinda wish I owned another Whitestar game so I could at least swap boards to isolate it to the board level :-/

Disconnecting J12 and J13 confirmed that the problem is not caused by something connected to J12 and J13. So one would think the problem must be on the I/O Power Driver Board. Problem is, I've replaced the MOSFETs driving those rows, and the IC driving those MOSFETs, tested the resistors in that circuit, and looked for shorted traces in that circuit. None of that helped. It appears that something further upstream is effecting the signal that drives those rows. That's where I'm not 100% sure the problem is on the I/O Power Driver Board. I don't think I've totally ruled out the CPU board. Just for the heck of it, I reversed both ends of the ribbon cable connecting the CPU board to the I/O Power Driver Board, but that didn't help. I guess I can confidently say it's not the ribbon cable

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Resistance between J12-2 and J12-3 is the same as the resistance between J12-6 and J12-8. It's infinity (even when I reverse the polarity of the test leads). I haven't totally ruled out the CPU board as I was thinking something might be affecting timing on one particular data line (where I only notice it in the lamp matrix because the lamp matrix is visible, and sensitive to timing). But I might be overthinking it .

Well, this is interesting. The manual says the MOSFETs driving each row are STP19N06L. On my game, STP20NE06L's were used. OK, that's fine. On the bad row (Row 6, which lights-up dimly when Row 7 is activated), somebody before me had installed an STP40NF10L. I replaced that with an IRL540N and there is no improvement.

Here's the breakthrough. If I move that IRL540N over to Row 5, the dimly lit row follows the IRL540N! And Row 6 works fine with an STP20NE06L. Apparently, in my game, I can't use IRL540N's (or STP40NF10L's) in the lamp matrix! Very strange.

I just ordered some STP19N06L's from Marco. I hope they don't send me IRL540N's

Replaced the IRL540N FET with an STP19N06L FET from Marco Specialties. Problem solved! Based on various posts, I had thought that IRL540N was a perfectly good substitute for that MOSFET. In my game, it's not, at least not on the lamp returns. I don't know if it's due to the timing of the lamp matrix or what, but I do know that the lamp matrix is a life sucking b@&^h from which there is no escape

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