Peter wrote:
The “valid” output from these servos is a fake signal. It is derived from the amplifier output, only, and is thus valid even if the motor is burnt out. All it checks is that the amplifier output is the right polarity
According to the block diagram on page 4-5 of the KS 271C servo MM, the valid signal does make use of the tach output — or rather it compares the amplifier output to the speed error (the difference between the servo command input and the tach output). I can’t quite make out how that is supposed to work and I can’t find any explanation in the text of what exactly is required to get a valid output.
You need the servo MMs with the circuit diagrams. KS270C KS271C TB20 TB21 STC IM. The last one is for the installation overview. I have all the schematics of course, including the KC225 
Peter wrote:
You need the servo MMs with the circuit diagrams. […] KS271CThat’s the one I was referring to. But I haven’t look at the actual circuit diagram — just the block diagram. More interesting bedtime reading, then…
