[Info-vax] OT: the Daily WTF for today is a VAX/VMS story

[Chris]

On 02/15/16 23:00, JF Mezei wrote:
> RA82 had optical sensor over a small metal disk with notches the actual
> disk casing that would get the elctronics to calculate disk speed. (this
> was outside the casing, near where the belt from motor spun the platters
> inside).
>
> I have to assume one of the boards in an RA82 controlled motor speed.
> Not sure how it achieved disk speed control however. Did it have
> frequent on/off circuit (forget terminology) to calculate how much power
> went to motor, or would it change voltage ? or did it play with the
> induction coils ?

The tach sensor usually senses speed for the motor servo loop. How the
motor is speed controlled varies and don't have the schematic for that 
drive. For example, related, the Revox A77 reel to reel tape machine
used a small papst motor, which was wired in series with the ac legs of 
a bridge rectifier, to the ac line. At the dc side of the bridge, a
power transistor formed an effective variable resistance, which was
driven from the output of the servo loop. Pretty crude really, but it
was quite hard to control what is effectively a constanmt speed ac
motor at the time for that application. It worked quite well. Now,
they would just use an inverter drive to a 3 phase motor, which can
be done with a handful of parts these days.

At a guess, drives such as the RA80 would have a very tight servo loop,
with simple on / off proportional power control to the motor, to vary
the speed. Then, just design the electronics to put up with slight 
hunting around the set point speed. They were good engineers in those
days, with clever, low cost design that delivered results...

Regards,

Chris