Epson PF10 problems

I am currently working on an Epson PX8 system. I bought this on E-bay, so the history is essentially unknown, but I was told it was 'untested' which I took to mean non-working. The PX8 had suffered a minor burn-up in the PSU area, which I have cured (one new transsitor, resistor and zener diode. The computer now seems to work find (and it's a very nice toy). With it, I got a PF10 floppy drive. This is a single 3.5" unit, with a 38400 baud serial interdace back to the PX8. I had to make up my own cable, but I have checked and double-checked it, and anyway it works fine to link a PX8 to a PX4 via the RS232 interfaces (essentially the same pinout as the 'high speed' serial interface to the disk drive). In fact what I did was make up a cable from the 8 pin nini-DIN plug to fit the PX8 to a DB25 plug,, wired as a DTE and a second cable with a DB25 socket to a 8 pin mini-DIN, wired to do the right swaps and jumpers. OK, a bit of background on the PF10 internals. There's a 6303X microcontroller (in an 80 pin package...) linked to a 27C64 EPROM (socketed) and a 6116 SRAM (SMD again). Address decoding is mainly done by a '138, with a few other TTL chips. Linked to that is a 765 disk controller, drivers for the head stepper motor, a data seperator chip, read/write chain, etc (note, there is no seperate logi board on the drive mechanism, the heads, stepper, etc plug straight into the controller board). There are several power supply lines, most of them under the control of the microcontroller. In particular, there's an always-on 5V line, a switcehd 5V Line (the disk controller, etc, is powered down when the unit is idle), +/- supplies for the RS232 drivers, a +12V line for the analogue circuitry. OK, at power-on, the 'power' LED comes on (I have connected 5V from my bench supply in place of the NiCd battery pack). The always-on 5V line is at 4.95V. None of the othher supples are present (this could be right at this point) I connected it to the PX8, DIR D: gives 'BDOS ERROR ON D: SELECT'. A breakout box connected between the computer and the drive (RS232 levels, rememeber) shows that when I run that command, the computer's serial port is turned on, something comes on on the TxD line, but the disk drive never turns on its port or sends anything back. More worrying, reducing the voltage from by bench supply never turns on the low battery LED on the drive unit. This is controlled by the microcontroller, BTW. I pulled it apart, of course. The serial data from the PX8 does get to the appropriate pin on the microcontroller. The microcontroller is clocking (1.23MHz on the Eclk pin), it's accessing the ROM, but it also appears to be wandering around the memory map (outputs on the '138 decoder that are not used for anything are being asserted at times). The address lines look odd to me, with pulses narrower than the Eclk on some of them. Since the ROM is socketed, I pulled it and read it out. It looks sane. Certainly no data bit is stuck high or stuck low, and all the address lines do something. Assuming it's like a 68xx processor, with reset and interrupt vectors at the top of memory, that looks sane too. I suspect the microocontroller or the RAM. Alas both are SMD (and therefore a pain to remove), I have spares for neither and no way to get them. Any comments, suggestions, or things to check? (Yes, I know about PC-based emulators for these drives, but I'd like to get the real unit working too). -tony