The Korg Poly-61 has the same type of battery as the Korg Polysix, the type that creates the infamous battery-leak disasters. Here’s how to modify the Poly-61 for a standard litium coin-cell battery. The coin-cell is seated in a battery holder, so can easily be changed by the owner, without soldering.
The original batteries in Korg Poly-61 (and Polysix) are charged at any time the synth is on. The coin-cell litium battery is not rechargeable. If you would just insert a litium battery in the circuit, the battery would break and possibly catch fire or explode. So you need to stop the current flow from the PSU to the battery.
Also don’t replace the NiCd battery with a new rechargeable battery such as Lithium-Ion or NiMh. The Poly-61 charging method is not suited for those, and they will break.
Old Crow (RIP) has info on battery modification and battery leak repair for the Polysix. I Adapted it for Poly-61.
- Old battery
- Resistor R70 (replace with diode)
- Capacitor C29
- Diode, eg 1N4148 (Please watch the direction, see schematic)
- Coin cell battery holder