What you need to repair a pinball machine, what you should measure first, and how to read a schematic without being an electronics engineer. 80% of faults are found with a multimeter and a bit of method.
The tools
Essential
- Digital multimeter with DC and AC voltage, resistance, audible continuity and a diode range. It doesn't need to be expensive; it does need good leads with sharp probe tips and, ideally, a pair of alligator clips so you can measure with one hand.
- Temperature-controlled soldering iron, 40 to 60 W, with a fine tip, and 60/40 or 63/37 rosin-core solder (leaded solder flows far more easily than lead-free, and the old boards were made with it).
- Desoldering pump and desoldering braid for removing components from boards. A good-quality pump makes half the difference.
- Nut drivers in 1/4", 5/16" and 11/32" with a screwdriver handle. Pinball machines are American, their screws are in inches. The 1/4" is the one you'll be using all the time.
- Phillips and flat screwdrivers in various sizes, and a magnetic 5/16" nut driver for the coil screws.
- Pliers: needle-nose pliers, side cutters, and a wire stripper.
- Head torch. Inside the cabinet you can't see a thing.
Useful
- Switch adjustment tool: a blade with a notch for bending the blades of leaf switches without breaking them. A pair of thin pliers will do instead, but the proper tool costs 5 euros.
- Connector pin tool (crimper for Molex / Trifurcon). You will be replacing a lot of connectors.
- Test lamp: a socket with a #44 lamp and two wires. It tells you immediately whether power is reaching a lamp socket.
- Logic probe, if you want to go deeper into the boards. For most faults it isn't needed.
- Isopropyl alcohol, cotton swabs, a soft toothbrush for cleaning boards and contacts.
What not to buy
Sandpaper or a file for switch contacts: it destroys their plating and afterwards they oxidise faster. Contact spray on matrix switches: it leaves a residue. Oil on anything: coils and sleeves run dry.
The voltages you need to check
Before you look for anything else, measure the basic voltages with the machine switched on. Black lead on ground (a metal point of the frame or the spot marked GND on the board), red lead on the test point or on the connector pin. The values below are the nominal ones; a 5% deviation is normal, but the +5 V must be between 4.9 and 5.2 V.
| System | Logic | Coils | Displays | Other |
|---|---|---|---|---|
| Williams System 3–7 | +5 V | +28 V | +100 V and -100 V | +12 V, -12 V (sound), 6.3 V AC illumination |
| Bally -17 / -35 | +5 V | +43 V | +190 V | +12 V, 6.3 V AC illumination |
| Williams System 11 | +5 V | +50 V (and +25 V for flash lamps) | +100 V and -100 V | +12 V, -12 V, 6.3 V AC illumination |
| WPC | +5 V | +50 V (+20 V flash lamps) | DMD about +62 V and -100 V | +12 V, +18 V lamps, 6.3 V AC illumination |
The exact locations of the test points are in each machine's manual. What each deviation means:
- +5 V low or "wobbling" during play: connectors, bridge rectifier, filter capacitor, regulator. This is the cause of resets.
- Coil voltage low: bridge rectifier or the big capacitor. Weak flippers.
- Display voltage zero: fuse or transistor in the high-voltage circuit.
- -12 V missing: usually only the sound is lost.
Caution: the display voltage is dangerous and its capacitors hold a charge. Measure with one hand, and don't touch the back sides of the boards.
The three measurements that solve most faults
Transistors with the diode range
Multimeter on the diode range. On an NPN transistor (like most of the coil ones): red on the base, black on the collector, it should read 0.5 to 0.7 V (on darlingtons around 1.2). The same with red on the base, black on the emitter. Reversed on both: nothing (OL). Collector to emitter in both directions: nothing. If it reads close to zero anywhere, it's shorted. TO-3 and TO-220 transistors can also be measured on the board, as long as the machine is switched off.
Diodes
The diode on each coil protects the transistor. Diode range: 0.5 to 0.7 V in one direction, nothing in the other. If it conducts in both directions it's shorted and it will keep blowing transistors or fuses. Measure it with one leg desoldered, otherwise you're measuring the coil too.
Coils
Resistance across the coil's terminals: a flipper coil has a few ohms on the power winding and over 100 ohms on the hold winding. The other coils are between 4 and 30 ohms depending on the type (the part number is printed on them). Zero or "infinite" means a burnt coil. A coil that smells burnt is burnt, no matter how well it measures.
How to read a schematic
The schematic in the manual looks frightening, but you only need one piece of it at a time. What to know:
- Connectors have names like 1J1, 2J3, J114: the letter J means connector, the number in front (where there is one) says which board it's on, the number after says which connector. The pins are numbered, and the "key" position is a missing pin so it can't be plugged in backwards.
- Wire colours are written as abbreviations: BLK, WHT, RED, GRN, BLU, YEL, ORN, VIO, GRY, BRN. "WHT-RED" is white with a red stripe. Follow the colour from the schematic into the machine and you've found the wire.
- The switch and lamp matrices are rectangular grids: a row and a column define a lamp or a switch. When a whole row doesn't work, look at the row driver on the board.
- Every coil has a number (solenoid 1, 2, 3...) and the table in the manual says which transistor drives it and which connector it passes through. That table is the most useful part of the manual.
- The test points (TP1, TP2...) are the spots where you measure voltages, and they are also marked on the board.
If you don't have a manual, the IPDB is the first stop. And if you get stuck, send a message via the Contact page with a photo of the board and your measurements.