Quoted from vbobrusev:
Hi! So, i trying to find solution on forum but didnt find any topics which can helps me. I have Grand Prix/NASCAR pinball which have issue with coil Q5. Before this it have issue with coil Q1, but it was issue with U1 Chip, i changed U1 it and Q1 works fine. But, Coil Q5 still keep blowing fuse and burn transistor Q5. I change capacitor near transistor Q5 and put new transistor on Q5, it works fine few times. i turn on and turn off machine. It works fine few days, but when i come back next day, fuse blows again and transistor burn also. I think maybe issue was with coil. i swap the coil and it works little bit and again same result, fuse is blowing, transistor burning. Any advices what it can be?
First, calling the coil you're having problems with by name (ie "Garage Release coil") is much more helpful than only referring to it by the transistor it's hooked to (ie Q5). I had to pull out a manual just to see what coil you were even talking about.
Now, that said. Check the area around the garage assembly under the PF. This is for the garage release, and if I remember right, there's a paper (plastic?) shield there that can get old and come off (but that might have been on the Q6 upkick coil, I can't remember). If it does, the motion of the garage causes the coil lugs to come in contact with metal and it blows the transistor.
If you can't make a shield to replace the broken/missing one, you can also unsolder the coil wires and put plastic tubing over the connection points on the coil there to help keep them from shorting to metal.
If this doesn't work, post a clear hi-res (change the dropdown menu to high-res before dragging and dropping) picture of your garage assembly from a number of sides so we can see if something jumps out as wrong.
Also, make sure you have the final ROM release because there's a problem with the trough up-kicker on older roms where it can burn out the coil and make Q1 blow. Since it was blown before, it's likely you have an older ROM that needs to be updated. If you do have older code and do not update it, the trough coil and Q1 will eventually fail prematurely again. I've attached the service bulletin that addresses this.