Burnt Pinball Connectors: The Cause of Most Intermittent Faults on Bally, Williams and Gottlieb

If your machine works, then doesn't, then works again — displays flickering, a solenoid firing on its own, a reset halfway through ball two, lamps that come and go when you nudge the cabinet — the cause is usually not a board. It is a connector.

The boards in a 1977–1989 Bally, Stern, Williams or Gottlieb machine are joined to each other and to the playfield by plastic plugs full of small metal terminals — the oldest unmaintained part of the machine. They carry real current and they fail gradually, which is exactly what an intermittent fault is. As Clay Harrell's connector guide puts it, “if a pinball machine is over five years old, chances are really good it will need at least some connector replacement.” Yours is over forty. Terminals cost a few euros per connector, and you can fit them yourself.

Contents

The symptoms that point at a connector

A board fault is usually stubborn: the same wrong behaviour every time. A degraded contact still conducts, just not well — so the machine boots, then falls over when current demand rises, the backbox warms up, or a flipper shakes the cabinet. That inconsistency is the tell.

  • The fault changes when you touch the wiring. The strongest single clue: if flexing a loom or pressing a plug makes a display flicker or a lamp return, you have found the area.
  • Random resets or freezes with no repeatable pattern.
  • Displays flickering, dimming or losing segments, especially on Gottlieb. If yours is simply dead, start with pinball score display not working.
  • Solenoids firing by themselves, or several dead at once.
  • A group of lamps coming and going with no fuse blowing, or the machine refusing to boot some days.
  • Browning or a melted smell at a plug — the visible record of a voltage drop that has been heating that pin for years.

Rule out three non-connector faults first: a coil locked on (an emergency), a fuse that keeps blowing (a short), and battery corrosion.

How a forty-year-old connector fails

A crimped terminal is meant to make a gas-tight connection: metal pressed onto the wire hard enough that no air reaches the join, so it cannot oxidise. The female terminal's grip on the male header pin soldered to the board relies on spring tension to do the same.

Tension goes, then oxidation gets in. Every unplug and replug spreads the terminal slightly, and the tin-plated terminals in your machine are rated for roughly 25 mating cycles. Once the seal is broken the surfaces corrode, and corrosion is resistance.

Then heat runs away with it. Resistance in a contact carrying current makes heat there. Heat raises the resistance and softens the plastic holding the terminal, loosening the contact, raising the resistance again. The housing browns, then chars — which is why a plug that was “a bit warm” last season is melted this one.

Gottlieb gets an extra dose, having used IDC connectors — insulation-displacement terminals, where a blade cuts through the insulation instead of the wire being stripped and crimped. Harrell gives the numbers: an IDC contact rated 7–8 amps at 20 °C is down to about 4.5 amps at 60 °C and below 4 amps at 75 °C, and a backbox in summer reaches that easily. The connector isn't broken; it has run out of capacity and works again once cool — precisely the fault you cannot reproduce on the bench.

Where to look: Bally, Williams and Gottlieb

Designations vary between games, so treat these as starting points; our guide to pinball machine electronics explains how these boards relate.

Bally and Stern, 1977–1985

The rectifier board — the AS-2518-18, or Stern's equivalent TA-100 — is the known trouble spot. Home Pinball Repair rates burnt rectifier-board connectors as “Common”; common enough that Marco Specialties sells a 49-piece rebuild kit for that board's J1, J2 and J3 connectors alone, built around 40 high-current Trifurcon terminals. Next likeliest is the voltage regulator and solenoid driver board, “fairly common” for plugs and header pins alike, with J3 and J4 worth checking. On the MPU, connectors and header pins sit at the top of the failure list, ahead of the chips — so read the boot LED first: Bally MPU LED flashes tells you whether the CPU runs at all.

Williams System 3 to 7

One connection dominates: the interconnect between the MPU and the solenoid driver board. Home Pinball Repair is direct — “that large line of 40 pins and plugs between the MPU and the solenoid board has a high failure rate” — and singles out 3J1 and 3J2, where “burn is common.” These use a pin general suppliers don't stock; Great Plains Electronics carries the correct ones. The CPU LED shows how far the machine gets: see Williams System 7 diagnostic codes.

Gottlieb System 1 and System 80

Gottlieb is the worst case, for a structural reason. Bally and Williams bolted their boards to a metal plate that served as a common ground; Gottlieb passed ground from board to board through the connectors, in series — so one tired contact lifts the logic ground of everything downstream.

Wiring harness and connectors running from the A1 CPU board to the A3 driver board in a Gottlieb System 80 backbox, carrying the single logic ground link between the two boards
The harness between the A1 CPU board and the A3 driver board on a Gottlieb System 80. Logic ground reaches the CPU through a single pin in this link. Photo: Pinballs Store.

This is documented, not folklore. In February 1987 John Robertson wrote to Premier Technology naming the culprits: logic ground reaches the CPU board only via the bridge between A1-J4 pin 1 and A3-J1 pin 1, and upstream sits a single ground pin at A2-J1 pin 2, which he identified as the main cause of the problem. A few ohms there lifts logic ground by a volt or two, driver transistors go into partial conduction instead of switching off cleanly, and they cook slowly without blowing a fuse. Being a design flaw as much as a connector problem, it has its own remedy: our System 80 ground mods guide, an evening well spent on a Black Hole, a Mars God of War or any System 80 you mean to keep.

Gottlieb's IDC display connectors need their own note: flicker, garbage characters and missing segments that worsen when the wires move. These are often feed-through contacts, where one terminal serves wires carrying on to the next display — awkward to repair with a plain crimp. There is a way round it below.

Finding it: the voltage-drop test

The mistake is testing with the machine off. A contact measuring a fraction of an ohm cold can still collapse once current flows, because a resistance check reproduces neither the load nor the heat. Test it working.

  1. Pick the rail you suspect — usually +5 V logic, or the solenoid supply.
  2. Measure at the source, on the pin where that voltage leaves its board, with the game on.
  3. Measure at the destination, on the matching pin at the board receiving it.
  4. Compare. The two should sit within a few hundredths of a volt. Anything more is dropped in the wire and connector between them — nearly always the connector.

One owner chasing flickering bulbs, random solenoids and flashing displays on a System 80B measured 5.3 V leaving the power supply at A2-J2 and roughly 1.2 V arriving at the CPU at A1-J1. That thread never posted a confirmed fix, but the measurement is the lesson: four volts lost where almost none should be.

Measure warm, after half an hour of play, when a marginal contact is at its worst — and read while gently flexing the loom, since a figure that jumps tells you which plug to work on. If the machine is dead rather than erratic, start with our won't-turn-on diagnostic.

The fix: repinning, properly

Reseating is a diagnosis, not a repair. Unplug the connector, plug it back in, the machine works — which confirms the connector is your fault and nothing more. Harrell is emphatic that reseating fixes nothing and spends what little contact tension remains; Home Pinball Repair agrees that if replugging gets the game to boot, “the fix is likely temporary and the connectors should be replaced.” Contact cleaner removes oxide but cannot restore tension or un-char a housing.

Replace the pins and the header. If a plug has burnt, the header pin on the board sat in the same heat, so a new plug on an oxidised header puts you back within the year: “it is important to replace both the plug and the pins.” That means desoldering — if it is past your comfort level, ask for help rather than press on.

Use the right terminals. These machines use .100 inch and .156 inch pitch connectors plus .093 inch and .062 inch round pins, the .156 inch ones carrying power. Where you have the choice, fit Trifurcon terminals: three contact points instead of one, far more tolerant of vibration. Don't mix platings — a gold terminal on a tin header raises contact resistance rather than lowering it.

Crimp it correctly. A proper crimp is two crimps: an inner one gripping bare wire, an outer one gripping insulation. A ratcheting crimper is worth buying — the Hanlong HT-225D is the usual pick for occasional use — because a consistent crimp is what makes the joint gas-tight. Don't solder the terminal: solder wicks into the locking tangs that retain the pin and stiffens them, and can melt the insulation.

Two details that catch people out. Where a Gottlieb IDC contact has wires passing through it, cut the wire at the connector, make a 75 mm jumper, solder that junction away from the terminal, insulate with heat-shrink, then crimp each end into its own position — two wires forced into one barrel is not a reliable join. And always fit the blanking plug that stops a connector going in one position out: “we have seen games literally burn because someone left out this plug.”

When the board really is the problem

Most intermittent faults here are connectors, grounds and mechanical wear, so repin first: a new board in the same burnt connector inherits the same fault. A board earns its place for other reasons — the same driver transistor failing repeatedly, battery electrolyte under the chips, rework that has left lifted pads and cracked traces, or wanting the fragile inter-board links gone. On System 80 and 80A, replacing CPU, driver and power supply with one board removes the A1-J4 to A3-J1 bridge and the A2-J1 ground pin altogether, since no cable runs between those boards any more.

  • Bally / Stern 1977–1985: BallyDri for the power and solenoid driver boards; BallyFA for the MPU.
  • Williams System 3–7: WillFA7 replaces MPU and driver board together, retiring that 40-pin interconnect.
  • Gottlieb System 80 / 80A / 80B: Godri80 for the driver board, or GottFA80_Plus for CPU, driver and power supply as one.
  • To diagnose first: the Lisy80 drives lamps, solenoids and switches one at a time, separating a bad contact from a bad mechanism quickly. It is built on the LISY project, an independent developer's free, non-commercial open-source work.

We test boards before they ship and support them afterwards. If you are not sure you need one, ask before you buy — the honest answer is often crimp terminals.

FAQ

Why does my pinball machine work intermittently?

Usually a connector whose contacts have lost tension or oxidised. A degraded contact still conducts, so the machine boots, but it fails under load, heat or vibration. Measure the voltage at both ends of the suspect connection with the game running and warm.

How do I know if a pinball connector is bad?

Look for browning or melted plastic around a terminal; even slight discolouration means that pin has been dropping voltage. Then measure: with the game on, compare the voltage leaving one board with the voltage arriving at the next. More than a few hundredths of a volt points at the connector.

Can I just clean pinball connectors instead of replacing them?

Cleaning removes surface oxidation and sometimes helps briefly. It cannot restore the spring tension of a terminal unplugged too many times, nor undo heat damage to the housing. If a plug is browned, replace the terminals and the header pins.

Why does unplugging and replugging the connector fix it temporarily?

The movement scrapes oxide off the contact surfaces. It is a good diagnostic and a poor repair, and every cycle spends some of the tension left in the terminal — tin-plated terminals are rated for only about 25 mating cycles.

Should I solder pinball crimp terminals?

No. A correct crimp is already gas-tight and needs no solder. Solder wicks into the locking tangs that retain the pin and makes them rigid, it can melt the insulation, and flux residue can interfere with the contact. Buy a ratcheting crimper instead.

Sources

Not sure whether you are chasing a connector or a board? Tell us the machine, the symptom, and whether it changes as the machine warms up or when you move the wiring. Our technical support is free, before and after any purchase.

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