Electronic components

Components in solid state pinballs

Although there are quite complicated electronics in a modern pinball machine, microprocessors and stuff, most faults are easy to fix. The microprocessor itself or its related circuits seldom fail. Usually it is a coil or lamp driver transistor or a coil protection diode, and they are easily replaceable and inexpensive.


A diode is a device, that conducts electricity only in one direction. They are used in SS pinball connected in parallel to every coil to prevent a voltage surge appearing at coil turnoff, that could damage its driver transistor. Many machines also use diodes in series connection with lamps and switches. They make it possible to drive many lamps or switches with a few wires, or a so called matrix connection.

Diode is a simple component. It only has two wires. Diodes can be tested simply with a diode test function found in most multimeters. Connected one way the meter shows some value, the other way it behaves as if not connected at all. If your meter has no diode test range, you can probably use the lowest or second lowest resistance range.

A diode can fail in two ways. It either breaks open or goes shorted. If a diode connected parallel with a coil breaks open, there can be strange behavior: extra scoring, game lockup or reset. After some time the coil driver transistor usually fails. Flipper coils are strong and operate frequently. It is possible for the diodes at flipper coils to vibrate and break their wires. Although the diode itself is intact, it is of no use in one of its wires is loose. Usually this causes game resetting when pushing the flipper button. If a coils diode gets shorted, which is unusual, the coils fuse will blow every time the coil gets power. If you find a shorted diode, do not try operating the coil with diode removed. It may cause new problems. Replace diode and only try the coil after that.

If a lamp or switch series diode breaks open, the corresponding lamp or switch becomes inoperative. Finding the fault is then quite easy. If the diode shorts, the lamp or switch matrix gets confused. Some lamps may light even when they shouldn't. Some switches may seem to close by itself when another switch closes. Diagnosing these faults is easy with the games own test program, by checking one lamp or switch at a time. Consult your games manual.

When you check diodes with a multimeter, it is usually best to disconnect either end of diode. That way you can be sure that the other circuitry does not disturb your measurements. When checking switch series diodes, it is enough to make sure that the switch is open, there is no need to remove anything. Same goes for lamps, it is enough to remove the bulb from its socket.

Coils, lamps and switches can use the same kind of diode. Common and cheap type is 1N4007 (abt. 0.15 e). Some machines use smaller and cheaper 1N4148 diode for switches, but it is not big enough for coils and lamps. It is easier to keep only 1N4007's at hand.

1N4148 and 1N4007 diodes, and diode drawing symbol

Diode drawing symbols straight line corresponds to the band-marked end of a diode component. Current in diode can only flow in one direction: towards the marked end. Diodes are used in SS pinballs also in power supply, rectifying AC to DC, and at logic boards to steer logic levels. These applications will not be handled here because fixing of those needs a little bit more deeper understanding.


A transistor is not much more complicated thing than a diode. It has one extra lead, so totally three of them. Transistor's leads are marked E,B and C for Emitter, Base and Collector. In pinball machines, the transistor can be thought as a relay-like switch, where a small current (relay coil) can control much higher current (relay switch). Namely, when there is current flowing from B to E, the transistor lets higher current flow from C to E. When there is no current from B to E, the transistor is blocked and no current flows from C to E either.

TIP36C, TIP122C and 2N5401 transistors
and NPN-transistors drawing symbol

The most usual part to fail is a coils driver transistor. Coil driving is done in principle like in the following picture. Usually the coil drivers are TIP122C NPN Darlington transistors. Darlington transistor is sort of two transistors connected in series in one case. This makes it possible to switch large amount of current with very small control current.

Solenoid control in pinball

When the transistor's B-lead gets voltage from CPU board, the transistor begins to conduct and the coil between C and +50V gets full power. When the B is turned off by the CPU, the transistor blocks and there is no current through the coil. In parallel of the coil is a protection diode, which prevents turnoff spike damaging the transistor.

The picture shows an NPN type transistor, that are usually used in coil controlling. There are also PNP transistors, that have their polarity reversed. Sometimes PNP transistors are used in driving lamps and displays, when there is a need to switch negative voltages. The type can be seen from drawing symbol. In NPN transistor, the arrow at E-lead points out of the transistor, and in PNP the arrow points in. When replacing transistors, they must be of similar type than the replaced transistor.

A transistor can fail, like a diode, either by breaking open or shorting. Often high current transistors, like coil drivers get shorted. The failure can be seen from coil being constantly on. After replacing transistor, the coil usually works but be sure to check the coil diode. High voltage transistors, such as plasma display drivers, usually fail open. That shows in missing digits or segments on the display.

From the coil driving picture we see that the coil pulls when the transistors C-lead is grounded. On the other hand, TIP122C transistor's C is connected to its case. So if a coil does not work it is easy to troubleshoot. Just connect the transistors metal tab to ground point with a wire. If the coil is OK, it pulls now. Do not keep the coil on more than a few seconds. The transistor is probably open if the coil does not work when playing. But if the coil does not pull when grounding the transistor, the problem is either at the coil, fuse or wiring. But the transistor is probably OK. If the fuse blows but coil does not pull in, the coil or its diode may be shorted. See if the coil looks burned or brownish. If your multimeter has sufficiently low ohms range, you can check the coils resistance. Normal coils are 10-200 ohms, flipper pull-in coil about 5 ohms.

A repairman should keep some TIP122C transistors at hand. They have been used as coil drivers almost from the beginning of solid state era. Older Bally machines sometimes burn a 2N5401 on display modules. Get a few of those also if you work with Bally games. Newer WPC Fliptronic machines drive flipper coils with TIP36C. They do not fail very often though. TIP122C can be replaced with BDW93C.


There are mainly two types of capacitors used in pinballs: electrolytics and ceramics. Electrolytic capacitors are used in power supplies to filter voltages after rectifier. The power supply electrolytics are quite large cylindrical things that have two leads at one end or at opposite ends. Electrolytics lifetime depends on its quality and temperature and is usually between 5 and 15 years. In older solid state games the electrolytics are beginning to need replacing. They are often found at the power supply board, but sometimes especially the larger ones are off board and attached to backbox wall (Bally, Williams) or at the bottom of cabinet (Gottlieb).

A capacitor has two main parameters: voltage and capacitance. For example an electrolytic capacitor may have marking on its side saying 6800 uF / 40 V. It can be replaced with a new capacitor that has those values equal to or bigger than those. It is OK to replace that capacitor with a higher voltage one, for example 6800 uF / 100 V. Replacament part may have larger capacitance, but not too much. 8200 uF is probably OK to use, but 10000 uF maybe a bit too large. Too big capacitance value in power supply will take much current at startup and may blow the fuse. When an electrolytic ages, its capacitance gets smaller and its internal resistance gets larger. At some point it no longer filters the voltage enough. This shows in flickering displays, erratic game operation or game resetting unexcpectedly. Sometimes the game starts up very slowly.

You can quickly check electrolytics in power supplies by measuring the ripple voltage. This is the AC voltage present at the capacitor. Most digital multimeters are suitable for this measurement. The meter should be able to separate AC and DC voltages. You can check your meter by setting the meter to ACV range and measuring a battery voltage. If the meter shows 0 on ACV, it can be used to check ripple voltages. First, measure the capacitor DC voltage at game on. Then set the meter to ACV range and measure the capacitors AC voltage. The value shown in ACV should be less than 20% of the DC value. For example, if you measured 20VDC, you should get less than 4V of ACV. If the ripple is larger, the capacitor is dried up and should be replaced. This procedure works on filter capacitors only, it can't be used in other capacitors, for example sound boards speaker series capacitor.

Ceramic capacitors are used in logic boards to reduce noise in power lines. Also older Bally games had a 0.1 uF capacitor connected in parallel of many playfield switches. When you see an old Bally game which tilts constantly, or scores erratically, suspect these capacitors. They can be replaced with new caps but they are usually not needed so they can safely be removed.

Older solid state pinballs have a 1-3 uF ceramic capacitor in parallel of flipper EOS switches, to reduce sparking at switch. They are subject to vibration and wire breaking. If you see large blue sparks under playfield, suspect these capacitors. It might be wise to put some silicone rubber compound over the capacitor leads to reduce vibration.