This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
The circuit on the mini-board is a low-gain 1-transistor amplifier which seems intended mainly as a buffer/voltage-level shifter between the 'old Apple' sound output of the Mega Chip and Op Amps which drive the Speaker and Sound jack. On the underside of the motherboard, beneath the boardlette, a surface- mounted resistor (SR1) has been scratched out. Leads from the mini-board run to SR1's connection points as well as Ground and a +12V supply point near Op Amp UM12. The circuit does not appear in the GS 'Hardware Reference ROM-01 schematics; but, it is shown in ROM-03 schematics. Some ROM-01 GS's do not have the mini board; so, it seems likely that the circuit was included on- motherboard in later ROM-01's and all ROM-03's. By the way, the circuit was certainly installed at the time of manufacture and was not part of the standard ROM-00 to ROM-01 upgrade. ----------------------------