Film-Tech Forum ARCHIVE: just repaired platter motor not running at correct speed?!

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Posted by James Waite (Member # 4536) on 05-05-2009, 10:34 PM:

The middle one of my Potts platter motors was making a bit of a clicking sound so I took it into a repair shop who changed the bearings. I brought it back and reinstalled it. I put a movie on the platter with it from the makeup table and everything seemed fine. However when the projectionist started playing the film that night the platter wasn't going fast enough and film started to wrap around.

I looked at it later and the motor is now visibly running slower than the other motors. When the lever on the control plate is all the way 'on' it's running a lot slower than the other platters. If I put two of the platters on rewind and play with the yoyo it's obviously running slower and stops turning before the other one as I bring the yoyo to the top.

I took it back to the shop who said they didn't do anything except change the bearings and didn't see anything that could be different about it.

I tried plugging the bad motor into the other platters and got the same result so it's not a coincidental problem with the control system for the middle platter.

I'm baffled. The shop wants me to bring in one of the good motors to compare but I'm loathe to do that as I would be screwed if it gets buggered up as well.

Anyone have any ideas?

Thanks in advance.

Posted by Monte L Fullmer (Member # 2797) on 05-06-2009, 01:55 AM:

What these platter motors are - simple vacuum cleaner motors.

If he replaced the bearings, they're probably a bit stiffer than what should have been replaced with - why the lower spin rate at a certain voltage rate.

Also, when he put the motor back together, the rear brush cage assembly (which holds the rear bearing) could have been put back on the field winding not fully aligned to allow full and free armature rotation..

Plus, on disassmbly, wonder if he got the brushes back in a correct place so they can be reseated on the "com" of the armature..or did he change out the brushes as well...

..if so, then the brushes needs to be seated in to be fully operational with the correct spin rate that you need.

-Monte

Posted by Mark Gulbrandsen (Member # 72) on 05-06-2009, 08:12 AM:

Have you timed the decks? Doesn't sound like it to me...

Mark

Posted by Jarret Chessell (Member # 5077) on 05-06-2009, 05:39 PM:

Hi James,

Let me know if you want your platters re-timed. I have a little time off at the start of the week, I can pop up there.

Posted by James Waite (Member # 4536) on 05-06-2009, 06:28 PM:

I'm not completely sure what timing the decks means but I don't think that is the issue. I'm using made up numbers for comparison only. For example if I turn all three platters to payout and pull the control plate levers to full 'on' the platters that weren't fixed run at 25 rpm but the platter with the motor that was fixed runs at 10rpm. it's a very noticeable difference.

Posted by Jarret Chessell (Member # 5077) on 05-06-2009, 06:39 PM:

The two bolts you likely loosened to take the motor off in the first place... loosen those up, try moving the motor down about a quarter of an inch, tighten it up, and see what happens. If the platter is spinning faster than you likely just have the motors out of wack.

Posted by James Waite (Member # 4536) on 05-06-2009, 07:07 PM:

The motor is running slower than the others even when not engaged to a platter.

Posted by Gordon McLeod (Member # 33) on 05-06-2009, 08:14 PM:

The bearing alignment is often an issue
Those motors can be a pain
buy a new one