Bally Black Jack instantly blowing the under-playfield fuse

[GaryD]

I have a 1978ish Black Jack.

It's been working, then starting blowing the under-the-playfield 1amp fuse when the game starts up.

I have checked all the other fuses are intact, and all the voltages are correct on the test points on the rectifier and on the test points on the solenoid board.

Some troubleshooting guides hint toward a short somewhere, but I don't see anything obvious from visual inspection. How would I go about finding a short otherwise?

 

[Pop Bumper Pete]

OK, that fuse protects all the coils on the playfield.
Are any of them locked on (engaged)?
Do any of the wrappers look brown?

 

[GaryD]

Hey Pete, thanks for the suggestion of what to look for. Sure enough, one of the coils was locked, it didn't move freely like the others did. Once I disconnected it, the machine starts up correctly and the rest of the coils work just fine. Now I just need to get a replacement ordered, lol.

Thanks again!

 

[Pop Bumper Pete]

OK, good so far.
The problem is not the coil, but the transistor powering it. (but you were probably going to have to replace the coil)
In the backbox there are 3 circuit boards. The one on the upper right is the power/driver board, There is a transistor on that board that needs replacing.
This is beyond what I can do online

 

[Pop Bumper Pete]

Which coil?

 

[GaryD]

It's number 12, the right slingshot.

It's darker than the others, and even after disconnecting it, it won't move, so I think it's toast. Do you think there is a transistor problem as well?

confirming- I took the board off and checked with the multimeter, and it does appear that transistor is junked. Time to risk mess up my board with my horrible soldering skills, lol. Worst case, I'll order an ultimate replacement solenoid board I guess.

 

[Pop Bumper Pete]

Something caused the coil to lock on, usually it is a bad transistor. Q14 (for the right pop bumper) on the power/driver board controls that coil (I think, I could be wrong), it will need to be replaced.

The bigger issue is, the game is almost 50 years old, it has done remarkably well to have lasted this long. Bally built these things like tanks.
Parts are going to fail
The battery on the MPU board should have been replaced 20 years ago.
The electrolytic capacitors (the big blue parts on the power/driver board) are currently failing and need to be replaced
You can either need to find someone to work on your boards, or buy an aftermarket replacement board, and then you will have years more fun

 

[Pop Bumper Pete]

confirming- I took the board off and checked with the multimeter, and it does appear that transistor is junked. Time to risk mess up my board with my horrible soldering skills, lol. Worst case, I'll order an ultimate replacement solenoid board I guess.

OK, right slingshot is controlled by Q12

It is a single-sided board, traces are big enough , you will be fine