Time for a battery upgrade!
First of all, the PSU board is very simple, with a bridge rectifier and a
So I rearranged wiring in a way that the on-off switch acts between the battery positive pole and the boost converter. The AC PSU was already always on when plugged in, only audio electronics were switched off. This fact gives me the chance to repurpose the PSU as a constant voltage Li-ion 1-cell charger. It is now voltage conditioned and fed through a diode (protection against reverse discharge) right to the battery.
The first circuit uses two LEDs (red and white in series) to step down from 9 V to 4.2 V max, but the current flow is too small to recharge in a decent amount of time. It's about 20 mA, as much as the stand-by device current consumption. We've got plenty of non-use time, but I don't want to have the keyboard around, keeping an AC outlet busy just for charging.
Second thought was to fit a Zener diode as voltage conditioner (Resistor - Zener to ground - Diode to battery). Still not optimal.
|The messy area inside the keyboard.|