Did you verify your new transistors are still good?

Did you do the test around U50 above and look further into the transistor circuit as suggested above; e.g. R102?

2 threads, same issue.

http://pinside.com/pinball/forum/topic/can-you-replace-a-23-800-coil-with-a-23-900-coil

It could be the PIA 6821 chip up line. Without a logic probe it is hard to say where the problem is.

This is the hazard of "shotgunning" repairs. It's a roll of the dice that more often than not is losing and costly. You might need to get someones experienced eyes on it.

For some folks, boards are cheap too - that would save you time.

You've got options. You could order the next two or three parts upstream that you think might be bad, but what if the problem isn't a bad part at all? What if there is an open or short you cannot see visually? What if the problem is downstream? Might as well order those parts there too.

Best option: Use the money you would be ordering needless parts to get a logic probe (or a oscilloscope) Learn how to use electronics tools of the trade.

Maybe this will help:

SSx: 6821 PIA 7407 7402 2N4401 TIP122
---------------------------------------------------
SSa: U38 (pin 39) to U49 to U45 to Q74 to Q75
SSb: U41 (pin 39) to U49 to U45 to Q70 to Q71
SSc: U41 (pin 19) to U49 to U45 to Q72 to Q73
SSd: U38 (pin 19) to U49 to U45 to Q68 to Q69
SSe: U54 (pin 19) to U49 to U50 to Q76 to Q77 <---
SSf: U54 (pin 39) to U49 to U50 to Q78 to Q79

U54:

SSe: U54 (pin 19) to U49 to U50 to Q76 to Q77 <---

Check your TIP36 transistors on the aux power board. I had a very similar issue happen with my earthshaker. I replaced the TIP102 and pre-driver on the CPU board with no change. Went upstream with the 7402 and 6821 chips..no luck. It end up being a TIP36 transistor on my aux power board. While you're at it, you might as well check the diodes on the aux power board at the same time.

I don't think there is a TIP36 to control special solenoid SSe on the aux power driver. If SSe is considered special solenoid number 5, I don't see a transistor for it on the aux power driver schematics. Not at my game now to check for certain.

I just read through this thread for the learning experience. I don't understand what you mean when you say that you get errors during the switch test. Switches either work properly, are stuck off, or are stuck on, right? Which one?

That error has nothing to do with what coil is in the game, FYI. The game cannot tell the difference between a 23-900, 23-800, etc...

...and you're not going to notice a difference in a coil with 100 turns difference (900 vs 800).

Exactly what error are you getting? What does it say exactly?

There is a distinct difference between getting this error in switch LEVEL or switch EDGE test. If the error happens in switch level, the switch is shorted/stuck closed or being reported as shorted. If the error is reported in switch edge test (switch name and number constantly on the display) you can be fairly certain that the problem is likely not the switch. (this is per the manual)

The switch plays no part in the coil test, so I don't understand how you are saying "it" works. The coils are instructed by the CPU to fire during coil test, totally bypassing playfield switch circuits.

What switch is it that is getting the error? Is it the switch that activates the coil, or the scoring switch?

The arrangement you are talking about (early sys 11 prior to big guns) only the scoring switch can possibly report back to the CPU. The coil firing switch activates the coils transistor.

Taxi has the special solenoids under CPU control, only one switch is used which tells the CPU to fire the coil and score the points.

Ahh. Did not know that! I was still assuming it had 2 like my HS.