FIX - Gottlieb System1 MPU - Controlled Solenoids (resistor fault)

[JRK1971]

This is feedback on a repair I just did to my Gottlieb Countdown - system1 MPU.

Hopefully you guys with System1 boards find it useful.

I went to play the Countdown the other day and found that partway through the game, the Outhole or kicker at the top of the playfield would energise and stay on. Initially it started to "stick" on after it was activated by a ball, after a while it would energise after the machine was turned on for about 10 seconds.

What I thought was going to be a simple driver transistor on the "Master Driver" board ended up being a resistor - R38 6.8K on the MPU board.

The problem was quite strange as initially I replaced Q31 on the "Master Driver board" - originally a Se9300, replaced with a TIP102. This actually made the fault worse and the sound board changed its pitch for the sounds, plus produced a worrying squeal. I then tried replacing the 1N4004 on the solenoid, the 1N4148 on the "Master Driver" board to no result.

I ended up getting the CRO out and probed the circuit and found that the output from pin 12 of U6 on the MPU had a squarewave on it alternating between about 0.5V to 5V. Checking other controlled solenoid outputs from the same chip, I found this should actually be -ve 0.5V approx - seems the squarewave was slowly turning on the transistor energising the solenoid. Getting worried now, I replaced the U4 (one of the Rockewell chips) and the fault still didn't go away. I was starting to get ready to jump out the window when I had a look at the resistors in the cct. It ends up that the resistors are in the acid damage area from the rechargeable battery and R38 (pull down resistor) was measuring high.

I replaced all the resistors in that area (still have more to do for preventative maintenance) - see photo, and the machine is working again. Unreal - I never thought a passive resistor would be giving such a weird fault. It goes to show that as those guides say - you should replace EVERYTHING in the acid damaged section.


Just in case you got to here and I wasn't clear with the repair, replacing R38 6.8Kohms on the MPU was the solution to a Controlled Solenoid sticking on (and also energising by itself about 10 seconds after power up)

[goodolddays]

Well done on tracking that one down . 
I sure know how frustrating fault finding can be .

[JRK1971]

Thanks. If you asked my wife yesterday, she would have said I was ready to throw the machine(s) out the window

I've wasted days on that MPU over the last few years, but I'd rather have an orig board than an aftermarket replacement - I don't make my life easy, but IMHO original parts are what makes these machines nice - I've got an after market board, and the bootup sounds different and a couple of other things aren't the same still a great solution if you can't get the orig going.