This schematic diagram was reverse engineered around 2004 from the original modules my brother asked me to fix. I even posted it on some forums, which unfortunately were killed off by WhatsApp groups.

At the time one of the major challenges was to figure out the internal configuration of the resistor networks which are non-standard types.

The module is composed by an 8051ish microcontroller, an H-bridge, an ADC, a clipping circuit and a hardware watchdog circuit.