I ordered some IRF9Z34N MOSFETs and put them into my Taxi. I turned the machine on, with only a few lamps connected to the board. I heard a pop, and immedately turned them off. I found that the two MOSFETs connected to lamps had blown, as well as their pre-driver transistors. So, I ordered some replacement ones.

Those replacements arrived, and before putting them in, I compared them to the non-fried ones I had in there from my first order. What I found was weird.

IMG_1936.JPG

With the red lead on the middle pin, and the black lead on the right pin on the top ones, I get .55 volts. I get nothing when the same is done on the bottom ones. When I reverse the leads, though, I get .55 volts. It seems to me that if these are both IRFZ34N's, then one pair of them is acting funny. Is it?

Wait. I figured it out. One pair of these is IRFZ34N, and the other is IRF9Z34N, which appears to move current in a different direction.

http://pdf.datasheetcatalog.com/datasheet/irf/irf9z34n.pdf
http://pdf.datasheetcatalog.com/datasheet/irf/irfz34n.pdf

That could explain why the two that were connected to lamps proceeded to explode their pre-driver transistors.

I'll have to take it up with the people I ordered these parts from and be a bit more aware of the parts I get in the future.

Would anyone happen to know if a 74F08 would be a good replacement for the 7408 ICs that sit in between the PIA and the pre-driver transistors? All signs point to yes, but I'd like to get someone else's confirmation.

I've seen the 74LS08 recommended as well. Would that be a better choice?

Here are the three datasheets for these ICs. Can anyone help me understand how I can determine this stuff for myself? What should I be looking for beyond simple input and output voltages and currents?

7408: http://pdf.datasheetcatalog.com/datasheets/166/500074_DS.pdf
74F08: http://pdf.datasheetcatalog.com/datasheet/fairchild/74F08.pdf
74LS08: http://pdf.datasheetcatalog.com/datasheets/70/375337_DS.pdf

Man. That is one hard-to-read page. I feel like it's a page taken out of some kind of CS class curriculum.

These aren't for solenoid drivers, though. These MOSFETs are to replace the lamp matrix transistors on the lower-right side of the board. Q52, 54, 56, 58, 60, 62 and 64, as per this guide:

https://pinside.com/pinball/forum/topic/williams-system-11-bullet-proofing-thread

So I'm trying to figure out if the existing 7408's were damaged when I powered the machine on. I don't have anything in the now-empty spots where the TIP42/MOSFET replacements go. I thought I could put the system in lamp test mode, put my logic probe against the different outputs of the 7408's and see the steady on/off of the lamp test, but no dice. I see a steady high on the pin that takes in the blanking signal, a pulse on the pin that goes to the PIA, and a low on the output. So either I'm not performing my test correctly (since I don't have actual transistors or lamps ultimately hooked up to the output pins of these 7408s) or they're both smoked.

I won't have access to another System 11 until Monday at the earliest, but since GPE has them for 35 cents a pop, I may as well get them and some sockets to go with them.