Help diagnosing Star Trek Pro left orbit gate

[Sleepybat]

Everything works great on my Star Trek Pro except for the left gate. I read a few threads online about the left gate coil failing, so I replaced it. It didn't help, and then I saw that the board was missing the Q51 transistor (had been snipped off, and it looks like there was some work done previously to q52 also). Fired up the game, and the gate worked for 2 min until it tried to eject ball 2... blown fuse (f8). Replaced the fuse to help diagnose, and now the left gate will not open anymore.

I'm new to all of this, so any help on what might be the cause or how to best diagnose would be super helpful to me. I assume the transistor is bad again? Could this be a problem with the board?

I've visually inspected wires and coils below the game to see if there's any obvious issues, but can't find anything. All other coils, switches, etc. work great.

Thanks for your time and suggestions.

[pinballcorpse]

The Q51 and Q52 transistors operate the left and right orbit gates respectively. 

If one of the transistors has been snipped off, I would address that first.  If a fuse is blowing, a locked (shorted) transistor can cause that. 

The only way to fix faulty transistors is to do board work.  A few people on here can probably offer you that service if that is not in your skillset.

Also, for the gates to open, (or basically for any coil to work) they have to be triggered by a switch.  Both orbits have a rollover switch.  The left orbit switch operates the right gate, the right orbit switch operates the left gate. Make sure these work also. Switches can also be checked in the test menu.

Good luck.

[Richie107]

I know that bad diodes on coils can blow transistors but what about a bad diode on a switch? would that back feed power and possibly burn up something on there board or will it just make the switch stick always open or closed?

[Mal7887]

The switch line is separate from coils and lamps. The switch is connected to the to the board, and when the the computer senses it is triggered, it fires the coil. A failed or missing diode on a switch usually causes weird issues with other switches that share the same wire, in addition to issues with the switch in question. To answer your question, generally, No, a bad diode on a switch does not burn anything up on the board. 

[Richie107]

The switch line is separate from coils and lamps. The switch is connected to the to the board, and when the the computer senses it is triggered, it fires the coil. A failed or missing diode on a switch usually causes weird issues with other switches that share the same wire, in addition to issues with the switch in question. To answer your question, generally, No, a bad diode on a switch does not burn anything up on the board. 

Thanks for clearing that up sir..

[Sleepybat]

Thank you all for the replies--I learned a lot from this thread. After replacing the transistor caused more problems than it fixed, I ordered a replacement six transistor board. Once that was installed, everything worked perfectly.

[pinballcorpse]

Glad it's working!

Off to Double Klingon Super Jackpots!!