On Nov 7, 2022, at 6:51 PM, Robert Armstrong via
cctalk <cctalk(a)classiccmp.org> wrote:
I have three DEC RA8x drives that have failed (all of them fault with
"spin error") because of bad photo-interrupter tachometer sensors. After
talking to a few friends, it sounds like this is a pretty common fault.
Photo sensors like this are fairly common, even today, but the specific
parts DEC used are weird and unobtainable.
Does the tachometer have to be accurate, or does it just have to indicate "spinning
fast enough" to satisfy the spin-up logic? In other words, does the actual laying
out of bits on the media depend on the tachometer?
If not, you could just use a 555 to generate a pulse train at the nominal frequency,
perhaps with a second 555 to delay that signal after power up to correspond to the typical
time it takes the drive to get up to speed.