Stern Star Trek Premium switch row issue

Hey guys, I have a Stern Star Trek Premium that had a CPU failure. Stern replaced it, but initially sent the wrong CPU which took a few days to figure out. Now, with the correct CPU all Row 3 switches are not working, even though no switch failures show in Diagnostics. Stern techs are really trying to help, so I appreciate that. They are asking me to take the CPU out and do some Diode tests on transistors Q2 and Q3. Anyone experience similar? I can do simple multi meter tests but I am not at all an Electrical Engineer. Any ideas or what to look for? The other switch Rows in the switch matrix seem fine during hand testing.

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#2 3 years ago

TPR

frunch

Pinside member

Your meter should hopefully have a diode test function on it already. It's easy enough to do, post a pic of your meter (and a good pic of the cpu board) and I'll (or another helpful pinsider may) point out which setting to put the meter on and where to place the probes.

Depending where the transistors are located on the board, you may even be able to test them without removing the board from the backbox. I would assume you still need to disconnect the connectors going to the board to ensure they're not allowing anything else to interfere with your meters readings. Ultimately it might make the most sense to pull the board out though. Just take pics of where all the connectors go and take your time. You can do this!

#3 3 years ago

REX

tomw1234

Inactive member

Thanks Frunch! I have a mini Radio Shack Multi Meter that has the Arrow with Plus symbol above the Ohms setting, which I am pretty sure is the Diode test. I tried to get readings with the CPU still installed but I was using the CPU ground strap and not an emitter or collector on the diode for the negative. I think I'll take it out and try again. Stern says I should be seeing between .515 and .650. and I should turn the board over and put the positive lead on the center and the negative lead on the outer transistor legs for the results. Does that sound right?

#4 3 years ago

REX

tomw1234

Inactive member

Transistor check.pdf

#5 3 years ago

TPR

frunch

Pinside member

Yep, you're following correctly! They seem to have pretty helpful instructions. You can always compare the readings you're getting on the suspect transistors to other nearby transistors of the same type. That can sometimes help when doing these tests. Definitely follow up with your results and any further questions!

#6 3 years ago

REX

tomw1234

Inactive member

So Frunch, does it seem likely that the CPU has an issue if an entire Row of switches are out? I guess that's my main concern. I don't want to go down a rabbit hole again. Has Stern twice sent me incorrect/faulty CPU's or is it something else.....under play-field wiring?... I've seen online that ST Premiums have had problems with switches. Just want to know if I'm going down the right path here.. It's definitely confusing.

#7 3 years ago

TPR

frunch

Pinside member

I don't have experience on modern sterns, but the switch matrix sounds similar to any others I've worked on. If your results from testing the transistor are telling you the transistor is ok, then indeed the next step would be to look for a broken wire or diode at one of the switches under the playfield on that particular row. Do you have the switch matrix chart handy?

#8 3 years ago

REX

tomw1234

Inactive member

So, I took the board out and had mostly .655 and .633 readings from the base to the collectors and emitters. But Q3 had .868 and 0. It seems Q3 is bad. So, warning to everyone: Even RMA CPU boards are not guaranteed to work. I learned this the hard way, but sure learned a lot about troubleshooting Stern SAM machines.....

#9 3 years ago

TPR

frunch

Pinside member