Turntable Modification

[Goddard, Duncan]

>>>I have an old Philips 312 turntable.  I need to run at 78 rpm (this
> thing is 33/45 stock).  Since it has a servo control for the platter
> speed, I was thinking I could pervert this thing to run at 78.  Has any
> body ever attempted this, and if so, were you succesful.  I would not
> want to ruin a perfectly good turntable on a fools errand.<<<
> 
> 
if the intention is to attempt a good-quality transcription of these discs
onto (say) cd-r, you'd best stick with the turntable you have and steer
clear of the old bsr's and so forth that one might find at a car-boot
sale..... rumble's less of a problem with older vinyl and shellac because of
the sheer mass of it, but still.....

but rather than frig the servo and risk the motor overheating, have a look
at making a new pulley for it. I'm assuming it's belt-drive but even if it's
one of those multi-diameter bush things on the end of the motor, driving the
platter via an idler, then you need to find a mate with a lathe. if the
existing one doesn't already live too close to the platter, you'll be able
to fit a large diameter replacement. if it's direct drive then check what
the motor's maximum rating is- you might get away with it. usually the servo
is designed to free-run within limits, with some sort of feedback path to
stabilise it under varying load; the limits are arbitrary to some degree,
designed to allow accurate adjustment, but servos can go unstable outside of
their normal range- very often because the power transistor gets unhappy.

once you've got a larger diameter drive wheel roughly right, then the servo
can do the remaining adjustment.
how will you check the speed? if you need me to, I can scan my goldring
strobe-disc for you- you'll need to cut it out but it doesn't matter if it's
not exactly circular as long as the hole is in the right place.......

d.




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