Tip: When adjusting the 5 volt supply on a Gottlieb Systems 80B pinball machine, adjust it to 5.00 volts or lower, but not below 4.85 volts.
The reason for this is the poor design of the Memory Protect circuit, located on the CPU board. There is a 3V zener diode (VR1) located on the CPU board that will start getting hot and fail if the supply voltage goes above 5.0 volts.
Zener diode (VR1) with a bulge and crack along right side.
Although the failed zener diode shown above was still basically working, I suspect it was acting intermittently, causing the CPU board to freeze up. Regardless, a bulging and cracked diode shouldn’t be trusted. This was from a machine where the 5 volt supply was adjusted over 5.00 volts (5.12 volts).
Also, the 5 volt adjustment pot on the power supply should be replaced with a fixed resistor. The pot will get dirty and become sensitive to vibration, causing voltage fluctuations. The best thing to do is adjust it for 5 volts, de-solder the pot from the circuit board, measure the resistance, and replace it with a fixed resistor or a combination of fixed resistors to obtain an equivalent resistance.
Symptom: Cheap Squeak sound board blowing fuses.
I had another person send me their Cheap Squeak board from their Spy Hunter pinball machine after it was blowing fuses. Compared to last time, it was much easier to identify where the short was located. Both C10 and C22 capacitors were shorted.
Tantalum capacitors were used for both C10 and C22, and a variety of other locations on the sound board. Although tantalum capacitors don’t age like aluminum electrolytic capacitors, they have a weakness: they don’t tolerate voltage spikes very well (nor reverse polarity, where they will likely explode).
Both C10 and C22 are located on the unregulated 12 Volt supply. This supply normally runs a little higher, and since it’s unregulated can have voltage spikes on it. So if your Cheap Squeak is blowing fuses, replace both of these capacitors. The original caps were rated at 25 volts, but I use either a 35 volt or a 50 volt for a replacement to make them more resistant to voltage spikes. The value is 4.7uF.
The other tantalum capacitors on the board should be fine since they are downstream from the 5 volt regulator and it’s very unlikely a voltage spike would get that far. They should last forever.
Also note that aluminum electrolytic capacitors have the minus “-” side identified on them and tantalum capacitors have the positive or “+” identified on them. The circuit board has only “+” polarity identified for all capacitors regardless of type. So double check the polarity of the capacitors before soldering them.