RD54 Stopped Spinning

-----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

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

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