Slow spindle on drive II

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GeekDad's picture
Last seen: 1 year 2 months ago
Joined: May 5 2022 - 13:04
Posts: 8
Slow spindle on drive II


Hope I can get some help from the group.

I am debugging a Drive ][ I have where the spindle appears to the running at about half speed.

I haver verified that

               the belt is not slipping

               the spindle (big flywheel) spins freely withe belt removed

               the motor spins freely when not powered

               Adjusting VR1 (to both end points) changes the speed, but never lets me the speed correct (not even close)

So starting thinking electrical, looking at pin 7 of the speed control chip the voltage was reading kind of low like 10.5 volts vs 11.8 and the motor seemed to not have alot of torque.

I had a spare speed control chip and swapped that out but with no change in performance.


So I am about to start really digging and tracing through the whole speed control board, but thought I would ask if anybody had any advice first. 




Last seen: 11 hours 31 min ago
Joined: Apr 26 2016 - 08:36
Posts: 650
Check the following:- that

Check the following:

- that your power supply is truly outputting +12V

- that those 12V are reaching the disk drive

- that the spindle motor is turning freely - a single drop of light machine oil (like 3-in-1) on the spindle motor shaft at both ends works wonders sometimes.

- that the drive chuck is turning freely - same comment on lube


Next check the electrolytic capacitor on the speed board, and the switching transistor, an X45C683 (obsolete, of course, and no datasheet that I can find).  That is, if your drive is Shugart based.  Also, you touched on the speed control IC.  If it is a DIP 14 then it's for a Shugart.  If it is a SIP package, it's an Alps.

If your drive is Alps based then some of the speed control comments may need to be modified.  However, if it's an Alps based mechanism, you can cannibalize a Commodore 1541 drive - they also use standard APLS mechanisms with identical speed control circuitry.


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