Amstrad 3in disc drive troubleshooting

[Philip Pemberton]

OK, this is seriously weird.

I have two Amstrad 3-inch disc drives: an EME-156, and an EME-231.

Both drives will spin up, select and generally "work". For varying 
values of "work". On both, the "activity" (selected, whatever) LED is 
stuck on at half-brightness, but switches to full brightness when the 
drive is selected.

For reference: both drives are showing the exact same symptoms.

I can select the drive, spin the motor, and seek around the disc. 
Writing seems to work (more or less) -- when WR GATE goes low, the coils 
of the head are driven to ~10V with smaller positive and negative pulses 
(sort of like an exponential curve, synchronised with the falling edge 
of WRDATA). When WR GATE goes high again, the head voltage falls back to 
about 2V.

When I try and read anything, the head shows no response whatsoever. No 
pulses on either side of the head at any amplitude (measured with a 1:10 
probe on a Tek TDS2024B). Similarly, RD DATA shows no pulses either, 
except for a brief glitch (2.8us or so) when the drive is selected.

The drive controller ASIC also gets rather hot.. like, too hot to touch. 
I've confirmed that the power is correct -- 12V and 5V have been applied 
to the correct pins.

I'm using "new old stock" Amsoft discs which I bought from an ebay 
seller. No idea if they're good or bad, but they arrived in 
shrink-wrapped side-opening plastic boxes and were apparently "Made in 
Japan."

My current suspects are:
   - Discs. The discs aren't actually magnetically coated in any way, or 
the coating has failed in some way. Catch is, I don't have a known-good 
disc to test with.
   - Heads. Dirty, out of alignment or otherwise completely pooched.

Can anyone suggest some things I could check, or are these drives likely 
to be toast?

Thanks,
-- 
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/