Pinball Pop Bumper Not Working: How to Find the Fault on Gottlieb, Bally and Williams

A pop bumper that has gone quiet is one of the cheapest faults on a solid-state machine, and one of the more satisfying to chase, because the order of checking barely changes from game to game.

The short version: if the bumper is dead, start at the switch under the skirt, not at a circuit board. A worn plastic skirt, a divot in the spoon, or a switch gap that has drifted open accounts for most dead bumpers on a forty-year-old playfield. If it fires but feels feeble, the coil sleeve and the plunger come next. Only when both are ruled out does the driver circuit matter — and on a Gottlieb System 80 that circuit is a small board of its own, one per bumper, which the CPU has nothing to do with.

One exception. If the bumper fires and stays energised, switch the machine off now and read pinball coil stuck on instead: a coil rated for milliseconds cooks itself in minutes.

Contents

What the symptom already tells you

Watch the bumper before you lift the playfield. A pop bumper does two separate things when the ball hits it — it kicks, and it scores — and which of the two has stopped narrows the fault enormously.

What you see Where to look first
Nothing at all: no kick, no score The switch stack under the skirt, or the wiring to it
It scores, but does not kick The coil side: coil, sleeve, plunger, driver circuit
It kicks, but does not score The scoring switch, or the switch matrix that reads it
It fires, but the ball barely moves Coil sleeve, plunger drag, worn ring or yoke, low supply voltage
Every bumper is dead at once Shared power: a fuse, a connector, or the relay that enables them
It works sometimes, worse when the game is warm A connector, or a switch gap sitting right on the edge
It energises the instant you switch on Shorted driver transistor — power off first

How a pop bumper actually fires

Above the playfield sit the cap, the lamp and the skirt — the round plastic wafer the ball nudges. Below it, a bracket carries the coil, a plunger pulled down through it, and metal and fibre yokes linking that plunger to the metal ring that does the actual kicking. Beside the plunger is a switch stack with a plastic spoon on top: the skirt tilts, presses the spoon, and the spoon closes the switch.

The part that trips people up is that Gottlieb bumpers usually carry two switches in that stack, doing different jobs. As one long-time repairer put it on Pinside, discussing a System 80 Spider-Man: "Your Spider-Man pop activation switches are not in the switch matrix. The scoring switch on the pop, which closes when the pop fires, is in the matrix." The upper switch fires the coil; the lower one tells the CPU it happened so the score can go up. That split explains most bumpers that kick without scoring or score without kicking — and the scoring switch is the one that fails more often.

Step 1: the switch stack

With the game off and the playfield up, work through this before touching anything electrical:

  • The skirt. Worn skirts lose the tip that reaches the spoon. Chewed plastic is a couple of euros to replace, and the single most common cause of a lazy or dead bumper.
  • The spoon. Forty years of the skirt landing in the same place wears a divot into the plastic spoon, so the skirt drops into the dent instead of pushing the switch. Replace it rather than bending round the problem.
  • The gap. Close enough that a light nudge of the skirt closes the switch, not so close that it rests closed. Adjust the blade, not the spoon.
  • The contacts and the wires. Dull grey deposits stop a switch conducting long before they look bad, and bumper wires flex every time the playfield is raised — check the solder lugs. Our maintenance guide covers what to clean contacts with.

If the bumper kicks but never scores, suspect the scoring switch — and since that one lives in the matrix, read pinball switch matrix not working if other switches are misbehaving too.

Step 2: the coil, sleeve and plunger

A bumper that fires weakly is nearly always mechanical. Pull the coil and look inside the sleeve: a worn, scored or dirty sleeve drags on the plunger and robs the stroke of its speed. Sleeves are consumable, cost almost nothing, and are the first thing to change on any tired bumper. Then check the plunger for wear and the metal and fibre yokes for cracks — a broken yoke gives you a coil that fires audibly while the ring barely moves. The plunger and coil stop can also become magnetised over the years, leaving the ring reluctant to return even though everything else is sound.

Then measure the coil. It should read in the same ballpark as its twin across the playfield and match the manual. An open winding is a dead bumper; a cooked, partially shorted winding reads low and will destroy driver transistors one after another. Our guide to testing a pinball coil with a multimeter has the method — and check the coil diode while you are there, because fitted backwards it takes the driver with it.

Step 3: what fires the coil on your machine

This is where the three platforms genuinely differ, and where people waste evenings testing the wrong board. For the wider map of which board does what, see pinball machine electronics explained.

Gottlieb System 80, 80A and 80B

Gottlieb gave each pop bumper its own little pop bumper driver board, mounted near the bumper and cross-referenced under several Gottlieb numbers depending on revision — A-19741, C-19740 and D-20923 all appear. One board per bumper, so a game with four bumpers has four of them.

The important point: the CPU does not fire these coils. The bumper switch triggers the board directly, and the board is simply enabled when power reaches the flipper circuit through the game's relay. As one Pinside contributor puts it, there are "no other drive transistors on the MPU or Driver board for the pop bumper boards." So pops that are dead while everything else works point at the enable relay or the bumper board rather than the MPU — and no CPU solenoid test will fire them, because the CPU has no way to.

The board's job is a one-shot timer: "The whole point of the pop bumper board was to only energize the coil for a set amount of time, even if the spoon switch gets stuck." Its failure modes are correspondingly specific:

  • Coil energises the moment you power up — a shorted transistor on the board.
  • Coil fires and locks on — the electrolytic capacitor in the one-shot has dried out. Owners chasing this fault report fitting a fresh 4.7 µF 25 V part.
  • Intermittent or dead — cold solder joints on the connector pins, a classic here.

The fastest test needs no meter: swap the suspect board with one from a bumper that works. If the fault follows the board, you have your answer in two minutes — easy on multi-bumper games like Black Hole, Haunted House or The Amazing Spider-Man. If the bumpers misbehave alongside other wandering faults, treat it as a grounding problem first: see our System 80 ground mods guide.

GoPOP80 pop bumper driver board for Gottlieb System 80, 80A and 80B, the small one-per-bumper board that fires the pop bumper coil independently of the CPU
On a Gottlieb System 80 each pop bumper has its own small driver board in this position. Shown here is the GoPOP80, the modern replacement for it. Photo: Pinballs Store.

Bally and Stern, 1977 to 1985

Bally did it the other way round. The bumper switch sits in the switch matrix, the MPU reads it, and the MPU turns on a transistor on the solenoid driver board — AS-2518-16 on early games, AS-2518-22 later — which grounds the coil. Driving is by TIP120 or TIP102 power transistors, behind a CA3081 transistor array as the pre-driver stage.

Because the CPU is in the loop, the coin-door self-test earns its keep: the machine cycles the solenoids one by one, and the switch test reports each closure by number on the displays. A bumper that fires in the solenoid test but never in a game is a switch or matrix problem; one that fails the test is a coil, a transistor or the wiring between them. And since the CA3081 is an array, one failed IC can take out more than one solenoid — worth remembering when two bumpers die together.

Williams System 3 to 7

Williams ran two kinds of coil circuit side by side. Most solenoids are computer-controlled: the switch goes through the matrix, the CPU decides, and a 6821 PIA drives logic that turns on a pre-driver and then a power transistor. But slingshots and pop bumpers are usually wired as special solenoids, where the playfield switch itself triggers the logic and the CPU is never asked — a 7402 logic gate, a 2N4401 pre-driver, a TIP122 or TIP102 power transistor. Which category your bumper falls into varies by title, so check the schematic rather than assume.

Direct firing has one weakness: nothing limits how long the coil stays on. While the switch is closed, the coil is energised — exactly the problem Gottlieb's one-shot board exists to avoid. On a Williams game, a bumper switch adjusted too tight is a burnt coil waiting to happen.

When it is power, not the bumper

If every bumper has gone at once, stop working on any one of them and look at what they share: the solenoid supply fuse, the relay that enables the flipper and bumper circuits when a game starts, and the connectors carrying that supply. A bumper that works cold and sulks after half an hour of play is describing a connector, not a coil — see burnt pinball connectors for the voltage-drop test that finds it. If a fuse keeps going the moment you fit a new one, you have a short: our guide to a machine that keeps blowing fuses takes it from there.

When a board is genuinely the answer

Most dead pop bumpers are a skirt, a spoon, a sleeve or a switch adjustment — and we would rather tell you that than sell you something. A board earns its place when it has been damaged rather than merely aged: scorched traces around the transistor, lifted pads from earlier rework, or a board that keeps failing after the coil and switch are sorted. Our honest comparison of repairing versus replacing a board sets out when each makes sense.

  • Gottlieb System 80 / 80A / 80B: the GoPOP80 replaces the original pop bumper driver board one for one, in the original mounting holes, with status LEDs and a test button that fires the bumper on demand. The Godri80 covers the A3 driver board if the fault is a general solenoid one.
  • Bally and Stern 1977-1985: BallyDri for the power and solenoid driver position.
  • Williams System 3 to 7: WillFA7 replaces the MPU and driver board together.
  • To diagnose first: a Lisy80 on a System 80 lets you watch switch states live, which settles a scoring-switch argument quickly. It is built on the LISY project, an independent developer's free, non-commercial DIY project — we supply and support it, we did not create it.

We test boards before they ship and support them afterwards. If you are not sure you need one, ask first: the honest answer is often a skirt and a coil sleeve.

FAQ

Why is my pinball pop bumper not working?

Most often the switch under the skirt. A worn skirt, a divot worn into the plastic spoon, or a switch gap that has drifted open will stop a bumper dead while everything around it works. Check those before you suspect a coil or a board.

My pop bumper scores but does not kick. What does that mean?

The switch that reports the hit to the CPU is working, so the fault is on the coil side: the coil itself, its sleeve or plunger, the wiring, or the driver circuit. On a Gottlieb System 80 that is the pop bumper's own little driver board.

My pop bumper kicks but does not score. Why?

Gottlieb bumpers usually carry two switches. The upper one fires the coil, the lower one is in the switch matrix and registers the points. A bumper that kicks without scoring points at that lower scoring switch, which is the one that fails more often.

Why is my pop bumper weak?

Nearly always mechanical drag. Change the coil sleeve first, then check the plunger, the yokes and the ring for wear. A magnetised plunger or coil stop can also leave the ring slow to return. If several coils are weak together, measure the solenoid supply voltage instead.

Does the solenoid test fire pop bumpers on a Gottlieb System 80?

No. On System 80, 80A and 80B the CPU does not control the pop bumper coils at all — the bumper switch triggers a dedicated driver board, one per bumper. So a self-test will not exercise them, and a dead bumper on those games rarely means a dead MPU.

All my pop bumpers stopped at once. Where do I start?

With what they share rather than any one bumper: the solenoid supply fuse, the relay that enables the flipper and bumper circuits when a game starts, and the connectors feeding them. Several coils failing together is a supply problem far more often than a coincidence.

Sources

Not sure which half of the bumper has failed? Tell us the machine, whether it scores, whether it kicks, and what changes as the game warms up. Our technical support is free, before and after any purchase — including when the answer is a two-euro skirt.

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