Drive RPM too high at min setting

[llopis]

Short story: I have a 6128 drive that's running at 330 rpm (probably the high limit that still works) even at the minimum level in the potentiometer. How do I fix that so it's back within spec?


Long story: I started with a perfectly working 6128. I checked the rpm and saw it was running at 308. I decided to bring it to an even 300, and as soon as I touched the potentiometer with with the screwdriver, it went haywire. I started showing 360 rpm and even higher. I was only able to trim it back to around 330.


Then, I decided to open up the lid to the motor to see if I had accidentally snapped something loose, but everything looked good in there. The potentiometer was in place and it read correctly between 0 and 9.2KOhms. I put the lid back incorrectly, causing one of the transistors that controls the 12V line on the main board to blow :-(. Only then I realized I was supposed to use the two holes on the lid to pull the contacts apart while I closed the lid.


Rough day at the office! 


The motor itself was an MMA-6U2LEP. The kind with the potentiometer facing down.


So, having managed to fumbled my way through that and screwed up a perfectly working computer, any idea what might be the problem? Did touching the potentiometer with a metal screwdriver cause all this problem? I don't see anything in the motor circuit itself that could cause that, but I also don't fully understand that circuit. Does anyone have a good reference how that works and why the potentiometer changes the rpm? If I had a better understanding of it I would probably be able to diagnose it better.

[Bryce]

This may sound like a strange suggestion, but.... Check whether you have dry joints on the motor connector on the main drive PCB.

Bryce.

[llopis]

This may sound like a strange suggestion, but.... Check whether you have dry joints on the motor connector on the main drive PCB.

They looked fine, but I just reflowed all the cable and power connections on the drive PCB and on the main PCB. Still the same.


I also replaced the blown PNP transistor that was causing 12V to be delivered to the drive all the time. Still nothing.


So, maybe we're thinking about this all wrong. The drive now reports 340rpm (no idea why it went up a little), and yet it loads perfectly fine. It makes a slightly squeaky sound as the disk spins. Could it be a problem in the logic that detects the spinning rate? Would that make any sense?

[Bryce]

It's definitely not spinning at 340 as nothing would load and it would be pretty obvious from the sound that it spinning way too fast.

The problem is, I've had this issue before, but I just can't remember what was causing it. It wasn't a broken component, it was something stupidly simple.

Bryce.

[llopis]

All right, well... the problem is gone. The problem is that I don't know *exactly* why :-b


After testing a good drive in that computer (worked fine), I decided to re-open it again and re-fit the belt. I noticed there was a *tiny* bit of leftover gunk in the drive shaft. We're talking tiny bit here, just some black left stuck, but hardly any depth to it.


While I was there, I decided to put a drop of oil in the shaft itself since the drive had that squeaky noise I mentioned earlier. I had already lubed the large wheel moved by the belt, and the small cap that goes over the disk itself and that had no effect.


I reassembled it, tested it, and... it worked fine, no noise, and the RPM test reported 288rpm (which is expected since I had left the potentiometer on the lowest setting). I carefully tweaked the potentiometer half-expecting it to go haywire again, but I was able to slowly bring it up to 300 rpm. Now it's working like a charm!


Moral of the story: You can't clean and lube a drive too much :-)

[Bryce]

Actually the moral of the story was already in your first post: "I started with a perfectly working 6128." - Moral: IF IT'S NOT BROKEN, DON'T FIX IT!! 

Good to hear it's working again though.

Bryce.

[llopis]

Actually the moral of the story was already in your first post: "I started with a perfectly working 6128." - Moral: IF IT'S NOT BROKEN, DON'T FIX IT!! 

Oh come on, like you wouldn't have tweaked a drive running at 310 rpm 

[Bryce]

I wouldn't have checked the RPM unless it wasn't loading disks So I wouldn't know that it was at 310. 

Bryce.