RD54 Stopped Spinning

Robert Jarratt robert.jarratt at ntlworld.com
Sun Apr 5 16:40:44 CDT 2015

> -----Original Message-----
> From: cctech [mailto:cctech-bounces at classiccmp.org] On Behalf Of Jon Elson
> Sent: 05 April 2015 17:55
> To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic Posts
> Subject: Re: RD54 Stopped Spinning
> On 04/05/2015 02:55 AM, Robert Jarratt wrote:
> >
> >> -----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
> > sensors
> >> 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
> > will
> >> 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
> > few
> >> minutes run time before the sensors go more flaky and the drive shuts
> > But,
> >> 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.
> >>
> >> Jon
> > I checked again this morning, because something was bothering me. I
> > realised that the pins I was observing with the scope were the D pins
> > on the FETs and not the hall effect sensor outputs. When I went back
> > to measure the sensor output properly the outputs looked fine and
> > perfectly stable. The outputs from the Z8 also looked fine. The
> > outputs of the inverters into the FETs looked stable, although not
square (pics
> here: http://1drv.ms/1a39cwz here:
> > http://1drv.ms/1a39kfs and here: http://1drv.ms/1a39pju). It is only
> > the output of one of the FETs that looks a bit unstable, but that
> > could be a triggering problem because the signal is a bit irregular (pic
> > http://1drv.ms/1a39ymO), the other two FETs have similarly shaped
> > outputs but do not wobble in the same way. I wonder if replacing that
> > FET might do the trick? Or is the inverter the problem as the inputs
> > are perfectly square?
> >
> >
> Do the gate signals not have the wobble?  (Wobble could be the PWM of the
> speed control logic, so could be normal.) If so, then the FET may be bad.
> fact, the FET may be entirely OPEN, and the waveform you see is coming
> the other two driven phases.
> Kind of strange that one gate signal is a smooth ramp, the others have
> wave on the top.
> But, the drive probably has a current sense resistor in the common source
> of the low-side transistors, so that is feeding some of the current
> back to the gates.  So, it may well be the one with the linear ramp on the
> indicates the drain is open.
> Jon

I tested each FET using a component tester (Peak DCA55) and it appeared to
show that they were all good. I believe they are FETs because the XT-1140
schematic someone posted shows FETs and the component tester also reported
them as FETs. I do wonder if one of them is only partially working, I seem
to remember that the one you point out as having smooth ramp was indeed the
one with the wobble. If I have further trouble I will follow Tony's
suggestion and swap them around to see if the wobble moves, but detaching
them from the heat-sink looks to be difficult.



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