Replacement MPU board for Bally / Stern (1977–1985): which one to choose?
Is your Bally or Stern pinball from 1977-1985 failing to start, or has the battery leaked on the MPU board? It's the classic failure of this generation. The lasting fix: replace the original MPU board (AS-2518-17/35/133, Stern MPU-100/200) with a modern, battery-free board. This guide explains how to choose.
What is a replacement MPU board?
The MPU (Micro Processor Unit) is the brain of the pinball. On Bally/Stern "-35" machines, it uses a battery that eventually leaks and eats away the board. A modern replacement MPU board takes over the same role without a battery, Plug & Play, and permanently removes the risk of corrosion.
The criteria to choose well
- Compatibility: does it properly replace the AS-2518-17/35/133 and Stern MPU-100/200?
- Game fidelity: does it interpret the original ROMs, or use reprogrammed games?
- Battery-free: no more corrosion.
- Freeplay.
- Repairability & support: can it be repaired easily, with help?
Our recommendation: the BallyFA
The BallyFA replaces the original MPU of Bally, Stern and Bell Games pinballs from 1977 to 1985. It interprets the original ROMs directly in its FPGA (gameplay faithful to the era), runs without a battery (no more corrosion) and installs Plug & Play.
And if something ever goes wrong? The failure-prone components (the ones that can fail) are mounted on sockets: replacement without soldering, in seconds. We also provide repair tutorials and free technical support. No throwaway board.
Configuration: the dip switch settings
The BallyFA is configured simply via two dip switch blocks — an asset for installation and troubleshooting.
S1 — Game selection
The S1 block selects the game the BallyFA should run, based on the ROMs present on the SD card (see the full game list in the manual).
S2 — Options (default: all OFF)
- S2-Dip1 — Zero Cross emulator: Bally MPUs need a "Zero Cross" signal (from the 12 V supply) to operate, checked at startup. On ON, the BallyFA emulates this signal — handy to test on the bench with a simple 5 V supply (timing differs with the emulated zero cross).
- S2-Dip2 — NVRAM save to EEPROM: the BallyFA saves the NVRAM contents (high scores, extended settings) to an EEPROM, each game having its own area. On the bench, momentarily switching Dip2 ON forces the save. In normal play, saving is automatic (test button, end-of-game relay, credit button).
- S2-Dip3 — Unused (reserved, not implemented yet).
- S2-Dip4 — Anti-flicker (LEDs): on ON, the BallyFA changes the Zero Cross timing for flicker-free LEDs, without having to add a parallel resistor on each LED (a known issue on Bally).
- S2-Dip5 — Coin validator: on ON, enables the coin validator. Do not enable if no coin validator is connected.
- S2-Dip6 — NVRAM initialization: on ON, the BallyFA initializes (resets) the NVRAM of the selected game at startup — useful to fully reset the RAM.
What about other solutions?
The Alltek Ultimate MPU is the American market leader: an excellent board that replaces Bally/Stern MPUs, packs 90+ games on one chip (dip switch selection), Freeplay, a self-test and a self-resetting fuse. Its logic relies on reprogrammed PROMs. The BallyFA takes a different approach: it interprets the original ROMs on FPGA (hardware fidelity), and focuses on repairability (socketed components) + Europe-based technical support. The right choice depends on your priorities.
Comparison
| Criterion | BallyFA | Alltek Ultimate MPU |
|---|---|---|
| Replaces | Bally / Stern MPU (1977-1985) | Bally / Stern MPU (1977-1985) |
| Battery-free | Yes | Yes |
| Game logic | Original ROMs interpreted (FPGA) | Reprogrammed PROMs (90+ games, dip switch) |
| Freeplay | Yes | Yes |
| Configuration | Dip switches: game selection, Zero Cross emulator, LED anti-flicker, coin validator, NVRAM save/init | Dip switches: game selection, Freeplay |
| Repairability & support | Failure-prone components socketed + tutorials + EU support | Self-test + self-resetting fuse |
FAQ: replacement MPU board for Bally / Stern
Which boards does the BallyFA replace?
The original MPU of Bally, Stern and Bell Games pinballs made between 1977 and 1985.
Is there a battery to change?
No. The BallyFA runs without a battery: no more corrosion risk.
Does it run on the original ROMs?
Yes, it interprets the original ROMs in its FPGA, for gameplay faithful to the original.
Can it be configured (game, LEDs, coin validator…)?
Yes, via dip switches: game selection (S1) and S2 options (Zero Cross emulator for bench testing, LED anti-flicker, coin validator, NVRAM save/initialization).
Is it repairable?
Yes. Its failure-prone components are socketed (replacement without soldering), with repair tutorials and free technical support.
Unsure about your model? Contact us — support is included.
