Help with a elusive Disk ][ drive problem

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Last seen: 3 months 2 weeks ago
Joined: May 30 2022 - 23:04
Posts: 7
Help with a elusive Disk ][ drive problem

I have a Disk ][ drive that I brought back from the dead.  It seemed to be stored in a moist area that was not conducive to it’s good health (cast aluminum oxidized, dirt, other tell tale signs).  After cleaning and lube, it started working again and i used it a couple times to read some disks.  Then it seemed to stop and there was no sign of the normal head banging when booted when hooked to Drive1 on the controller.  I opened it up, and recleaned the head rails/slide, removed all the grease, and changed the pressure slightly of the tab that rides in the track of the plastic cam.  It works again, although short lived.  Now, even with the head rails lifted away, it can no longer turn the cam wheel under it’s own power.  It seems if you help it, it can maybe turn a little but seems to find it’s way back to what apparently is a bad spot in the winding.  Because this exact symptom could have potentially been a problem with the darlington chip, I tried a new one, and even swapped the whole analog board with a working one - No Go.  The working drive’s analog board would not work in this one, and this analog board did work in the other drive - so not the analog board, and not the controller.  Board on the back has nothing to do with the stepper, so we are down to the stepper.

I can’t find the exact model on fleabay, but I did get one with all the important numbers matching.  One number is different-  Orig stepper is a Copal Electrica, dia4/35ohm/12volt7.5degPerStep.  After the 7.5 degree it has a number “-18”.  The new one has -85.  The wiring colors are different so I’m thinking it could be the code for the wiring colors.    Using an ohmmeter, I found the centertap of the windings.  Old motor it is RED on both sections, new one is BROWN on both centertap sections.  This brings me to something odd I discovered while checking.  Step motors generate power when turned.  The new one did on all phases (center -> both ends, and end to end).  Oddly, so did the supposed bad one!  The ohms are identical, center to end (36 of course), and double that (72 ohm) end to end on the old and the new.


Now in past, I’ve made automated window shades for home automation out of old HP printer stepper motors.  I know well how to find the coils, wire them with a darlington array,and how to write programs to drive them.  I’ve never seen a bad stepper that passed the ohm check on the coil AND was able to generate power on all the windings when spun.  I'm suspicous about how this motor can be bad, but what else could it be?  The connector pins look perfect, and I even took the ohm measurement off of them to check voltage generation. 


Two questions - 

  1.  Am I missing anything?  It about has to be the stepper motor as I changed everything else, and it absolutely cannot be a mechanical problem as I have the whole head assembly removed and it still will not spin.  It has all the symptoms of a motor that got weaker and weaker until it died.  It is not difficult to spin either, so not a bearing problem.

  2.  Any clue what the -18 and -85 mean on the label?  (search any picture of the bottom of the disk ii on google, you will see they all have 18)  I’m almost sure it’s wiring arrangement maybe for different manufacturers so I’m doubting it’s very important

Temptation is to break out an Arduino and write some programs to step it/micro step it and see if it acts up.  I’m thinking I may have to do that anyway to find out which is the lead and tail of each phase block.  

In the worst case, I’ll use the case for the floppy emu ][ project and use the drive for spare parts to save another, as everything else seems working.  I just hate to butcher it as they arent making any more.  On the other hand, I won't loose too much sleep over one of these, although the clowns on fleabay seem to think they are "RARE" and list them for $150 now.

MacFly's picture
Last seen: 10 hours 55 min ago
Joined: Nov 7 2019 - 13:49
Posts: 446
Seems you already narrowed it

Seems you already narrowed it down pretty well. Maybe the internal magnet/some of its teeth have rotted away? So it's not making proper steps, or the magnet has lost its strength (while still being good enough to induct a bit of voltage when you turn it)?

When the machine is booting, it tries to move the head all across the disk to track 0 (which triggers the famous brrrrrt sound, since the head often already is at track 0 and movement is blocked). Wy not just wire up the new motor (while keeping it separate) and see if it turns with proper torque? That would be the final proof, it's only the motor - not electronics/something else.

Also, the disk II controller allows you to directly control all four phases - and has a very simple interface (there are 8 addresses, each turns one of the four phases either on or off). So, you could even use the Apple II if you wanted to test the stepper. But it sounds like you already have a setup for your arduino to drive stepper motors, so in your case that may still be the more convenient option... :)

Concerning the part number: without the datasheet it's almost impossible to tell, what the difference could be. Copal is still making steppers today (now Nidec-Copal). According to their current datasheets, they nowadays assign different dash numbers for each combination of all motor properties. So, you have to check the datasheet find out what each number means (resistance, number of steps etc). One obvious property not listed separately on the back of the SP57 is the strength of the permanent magnet. At least their current steppers are being sold with different magnet strengths - which also influences their current dashnumbers.


Last seen: 1 year 7 months ago
Joined: Sep 21 2020 - 04:11
Posts: 8

It's an interesting issue. I have found when working on similar issues it's easy to go down a rabbit hole and when it gets too detailed it can be good to zoom out a bit. Could you simply swap the stepper from a known good drive and see if that restores the functionality? If so, throw the old stepper in the bin, or even better use the Bob Pease approach, he kept a hammer in his lab for the sole purpose of pulverizing such time wasters, the hammer had a name but I can't remember it. As to new steppers there is no harm in ordering some to try, but if it gets too involved you may have to simply buy a parts drive or parts from one, sad I know but that's life. Of course if you are set up to recondition them or know someone who is, great, but keep in mind the cost of your time. As you are an engineer with troubleshooting skills someone will pay you $50 to 100/hr and its nearly impossible to get anyone at present. So you maybe should not be worried about $150 for a new stepper, if that is what you need to solve it.

cheers, Nick

Last seen: 13 hours 23 min ago
Joined: Apr 26 2016 - 08:36
Posts: 678
I am sure that someone has

I am sure that someone has see this happen, but this might be the first time I have ever seen the stepper motor itself actually fail.

Does it turn freely (you will feel "cogging" if you turn its shaft manually - that's normal) but perhaps it might need lubrication?


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