R80/RA81 diagnostic experience?

Breakthrough! (for no apparent reason) We revisited one of our sick RA81 drives again today, having aquired some proper schematics. Several of you had suggested the HDA spindle transducer, and it does indeed look like that's the problem, at least on one of our drives. In glorious ASCII-vision, the connection to the spindle sensor on the read/write board bolted to the top of the HDA is as follows: Spindle Read / write Sensor board conn. 82ohm red o----------XXX----------+------< +5V | black o-----<GND | | white o-----------------------+ | black o-----<GND X X 470 ohm o (KEY) X | green o----->TEMP SENS. OUT | | yellow o-----------------------+------> TACH. OUT Putting a scope on the tacho output from the sensor showed it at 5V (tied to +5V via the 470 ohm resistor) with the drive at rest. When we tried spinning the drive up, there was some fluctuation on the output, but it dropped to only 4.5V max or so and obviously didn't look healthy. Voltage at the pin where the red wire connects - i.e. supply to the sensor LED(s) was around 1.3V and so seemed healthy enough. We pulled the HDA and took the sensor off to try it on the bench in isolation by replicating the circuit above and repeatedly breaking the beam with a bit of card. Note that it showed no signs of being dirty. This is where things got strange - at first it behaved just as in the RA81, the output which was tied high by a 470ohm resistor dropping by maybe 0.5V. But then it quickly began to pick up, the voltage ducking further and further below 5V until it suddenly seemed to be operating perfectly, the output swinging between +5V and close to 0. We left it to cool down, thinking it might be a heat related thing. Came back - nope, still working just fine. We let it cool off for an hour or so again, then put the sensor back in the drive and put everything back together. Guess what - one fully operational RA81, spun up without any trouble! I'm totally confused by that - the sensor showed no signs of dirt. It hadn't worked in place in the drive, and nor did it work in isolation on the bench at first. Yet it showed signs of *some* life when originally in place within the drive, so there obviously wasn't a cable break. I'm at a loss to explain that one, and it also means we're just waiting for the drive to fail yet again, as we haven't actually fixed anything (at least we'll know where to look this time!). Next test will be an early morning start when the room that the drive's in is particularly cold I suppose, but today seemed to prove that it would at least work at normal room temperature and above.... cheers Jules