Medusa (Bally, 1981) : pannes et cartes de remplacement (MPU Bally)
Is your Medusa by Bally no longer starting, are its displays dead or the coils running wild? These symptoms are typical of Bally microprocessor pinballs, whose original MPU board is now over 40 years old and almost always suffers from a leaking battery. Good news: the BallyFA replacement MPU board, Plug & Play and battery-free, gives your machine a second life.
Medusa (Bally, 1981): overview
Released in 1981 on a Greek mythology theme (Medusa), this Bally pinball favours continuous action (design by Wally Welch, art by Kevin O'Connor). It uses the Bally MPU of the AS-2518 series.
- Manufacturer: Bally
- Year: 1981
- Electronic system: Bally MPU (AS-2518)
- Type: electronic pinball (solid state)
- Theme: Greek mythology / Medusa
Common faults (Bally / Stern)
On Bally and Stern microprocessor pinballs (1977-1985), recurring faults come from ageing: battery leaking on the MPU (corrosion of traces and sockets around U10/U11, the number-one cause of dead boards), tired PIA 6821 (U10/U11) (oxidised pins, no start-up), RAM 5101 and sockets to replace, MPU/power connectors oxidised or burnt (look for heat marks), displays with missing segments, and power/driver board (rectifier, fuses, coil transistors) to service.
Issues specific to Medusa (forum feedback)
- Battery leaked on the MPU: corrosion around U10/U11, to be dealt with first.
- Temperamental drop targets: switches and reset coils to check.
- Dead or partial displays: display and MPU power supply to check.
- Runaway or dead coils: driver board (transistors, fuses) to service.
- Oxidised or burnt connectors: to replace (re-pin) to make the machine reliable.
- Blowing fuses: look for a shorted rectifier or coil.
Replacement boards compatible with Medusa
- BallyFA — replacement MPU board (battery-free).
- BallyDri — power + driver.
- BallyLa_60 — lamp driver.
📚 To go further: Guide: which MPU board for a Bally / Stern (1977-1985)?
The BallyFA replaces the original MPU board (start-up, logic, battery-free memory) and removes the battery, the number-one cause of corrosion. For the coils and power, the BallyDri takes over; the lamps via the BallyLa_60. Plug & Play installation, battery-free, free support. Contact us.
FAQ — Medusa Bally
My Medusa no longer starts.
Check the battery and corrosion around U10/U11. The BallyFA replaces the MPU and works without a battery.
Some coils run wild or don't respond.
On the power side, the driver board is to blame: the BallyDri (power + driver) replaces it.
Is the BallyFA compatible with my Bally?
Yes, for Bally/Stern microprocessor pinballs (1977-1985). If in doubt, contact us.
Should I keep the original battery?
No. The BallyFA works without a battery and removes the number-one cause of corrosion.
How long does it take to install a replacement board?
Installation is Plug & Play: a few minutes, no soldering, with tutorials and free support.
Does a battery-free board keep the settings and high scores?
Yes. Modern replacement boards use non-volatile memory: no more battery, no more corrosion, and the settings are kept when powered off.
Step-by-step diagnosis
- Powered off, visual inspection: look for any trace of battery or corrosion on your Medusa's MPU, especially around U10/U11; remove the original battery if it is still present.
- Power supply: check the rectifier and the voltages (5 V logic, coil and lamp voltages); replace tired fuses.
- Grounds and connectors: redo the grounds and re-pin the oxidised or burnt MPU/power connectors.
- MPU: check the PIA 6821 chips (U10/U11) and the RAM; the BallyFA replaces the MPU and works without a battery.
- Displays: never plug or unplug a display while powered on; test with a known-good display.
- Final test: check start-up, credits, coils and displays; if needed, contact free support.
