Sun 4/330 with faulty serial interface...

We've got a Sun 4/330 which has bit 6 stuck high in every character output on the console. I don't suppose anyone has schematics for these boards or know how the serial hardware works, do they? The machine looks to be booting OK, so it appears address & data buses are fine - but at the point where characters must be loaded into a parallel register and converted to serial for output it's falling over... I suspect the machine's hard-wired to use only serial port A as console, but either way I couldn't get hold of the right cable to see if the other three serial ports are OK (the latter three being 9-pin male, whilst port A is 25 pin male) I can't think of any line settings, faulty voltage rails etc. that'd cause bit 6 to be stuck high, so I'm assuming it's a hardware fault... cheers Jules