> On Nov 7, 2022, at 6:51 PM, Robert Armstrong via cctalk
> <cctalk@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.
paul
