Servo as a spindle motor replacement - what to look out for?

[Voicecoil]

You may have noticed that some drives show a higher PPR for differential mode than for single ended and this can only be accounted for by the LED being flipped more quickly.

That's a good point, so if you are going to drive them single ended you want a driver that can both sink and source current, not an open collector type.

[Muzzer]

No, generally a single ended driver just sinks current. When you turn it off, the current into the driver stops. If there is a decent capacitance in / around the opto LED, it takes a few hundred nanoseconds to actually turn off completely.

The differential line driver (RS485 etc) both sinks and sources current by swapping the polarity over using a totem pole output stage. So generally you have a choice between a sink-only single-ended driver or a full sink-source differential driver.

It's a pity the current solution already uses differential drivers. Do both sides of the driver output still work? Or is the failure due to perhaps one of the lines no longer switching? It's possible that might explain why it bursts into life when the scope grounds one side.

[Voicecoil]

No, generally a single ended driver just sinks current. When you turn it off, the current into the driver stops. If there is a decent capacitance in / around the opto LED, it takes a few hundred nanoseconds to actually turn off completely.

Depends whether it's a bipolar/TTL or CMOS driver. The former tend to be MUCH better at sinking than sourcing, whilst the latter are pretty symmetrical due to the complementary output stage, e.g. the datasheet for the 74HC245 which is quite popular on cheapo BOBs shows 0.18V droop when sourcing 6mA and 0.15V when sinking 6mA: 30mV difference in the ~3V across the series resistor. Differential would still be a bit better as you potentially have 2x the voltage discharging aforementioned capacitance, but it's a darn sight better tah open collector drive.