RD54 Stopped Spinning

Robert Jarratt robert.jarratt at ntlworld.com
Sat Apr 4 16:51:03 CDT 2015

> -----Original Message-----
> From: cctech [mailto:cctech-bounces at classiccmp.org] On Behalf Of Jon Elson
> Sent: 04 April 2015 22:18
> To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic Posts
> Subject: Re: RD54 Stopped Spinning
> On 04/04/2015 04:06 PM, Robert Jarratt wrote:
> > So when I fired it up again with the scope, the disk started to spin.
> > This was a stroke of luck as I was able to probe what I believe were
> > the outputs of the hall sensors. All three oscillated, but one of them
> > showed significant sideways wobble on the scope, which suggests to me
> > that the signal is not absolutely regular. It does seem to suggest
> > that one of the sensors is not working well. However, would this be
> > enough to explain why it stopped dead in its tracks when it was
> > working before?
> Yes, the logic is generally that you decode 6 legal states of the Hall
sensors to
> decide which of the 3 motor terminals to drive high and low.  if the Hall
> give a signal that has all 3 high, or all 3 low, the decode logic will
fail, generally
> leaving all transistors off.  If it gets in that state again, a vigorous
twist of the
> drive around the spindle axis might shift the rotor to a position where it
> start up again.
> > Changing one of these sensors is probably going to be beyond me :-(
> > Thanks Rob
> I'd get the data off it quickly and retire the drive.  You may only have a
> minutes run time before the sensors go more flaky and the drive shuts off.
> now that you know the secret, you ought to be able to recover data, if
that is
> the plan.
> Anyway, it seems you have completely diagnosed the problem.

Thanks. There is no critical data on the drive. I just wanted to use it to
show my MicroVAX II at DEC Legacy (in the UK next week) working without
booting off a network :-(



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