Analogue-sequencer

> I've powered up only the mainboard
> but
> the lcd doesn't write at all

Can you see attempted writes to the LCD ?
The CPU should be strobing pin 6 of the LCD connector.
In one case I have seen, the transistor used for Q1 had too little gain to
drive the LCD enable line.
You could try a different transistor in there, and/or reduce R20 to 10k.

The bootloader will attempt to write to the LCD display, and if it doesn't
see a clear 'busy' flag, it wont continue.
You can fool the CPU into thinking the LCD is responding by removing the
display module, then wiring pin 14 to pin 1 of the LCD connector.
That will ensure the CPU thinks 'busy' is always clear.
If doing that makes it continue booting (you see LED activity), then the
problem is with the LCD interface.

> what is the current consumption of the main board?

About 200mA.

Best regards,
Colin Fraser
Sequentix Music Systems Ltd
http://www.sequentix.com