chickenplucker Offline Upload & Sell: Off
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Alright, good news and bad news. The good news is I ended up being able to fix the motor on my lens after taking it apart and putting it back together for the 6th time. Bad news, I'll not be ordering some random parts now to see if swapping the USM drive out of another lens would have worked.
But going back to the swap idea, from what I could tell the USM stators and rotors appear to all be the same on any 2.8L based lens. Keep in mind this is just from pictures I was looking at, the actual dimensions could be different but I just can't see Canon making different motor parts for every lens. It doesn't make sense from a design perspective to keep redesigning every lens motor when they would have parts in stock that would already work.
The dimensions on the 28-70mm 2.8L's stator are 5.4cm for the inside diameter of the stator, and 5.8cm for the inside diameter of the rotor.
The 28-70's USM drive. Top left is the retaining washer with spring that keeps pressure on the motor, its glued together. Top right is the felt washer that goes beneath the retaining washer/spring to keep pressure down on the stator. Bottom left is the stator and its ribbon. And on the bottom right is the rotor, which as far as I can tell is just a big flat washer that acts as a surface for the stator to do its magic on, and as a drive transmission to connect the auto-focus to the actual lens. If you have never read up on how a USM drive works, you really should. Its quite amazing stuff.
From the side, top to bottom; Retaining washer, spring, felt, stator, rotor.
I was just getting ready to start pestering Ebay sellers about the inner diameter of newer lenses focusing motor assemblies, but mine started working before I did. I did narrow down some potential swap candidates in case anyone else wants to chase this down further. Note that most of the auto-focus assemblies come with a portion of the barrel
Canon EF 16-35 mm 1:2.8L Ⅱ -Part# YG2-2332-000 Cheapest at about $85.00
Canon EF 24-70mm F/2.8 L USM -Part# YG2-2064-000 Looks closest to the 28-70's.
Canon EF 70-200mm 2.8 L non-IS USM -Part# YG2-0212-009 Very, very long ribbon. May not be the best.
Assuming the above stators and rotors are the same diameter, they all seem to have a 4 wire ribbon attached with various lengths and bends. The Canon EF 24-70mm F/2.8 L USM appears to have the same bends and length though, so it may be the best candidate. I think that lens even operated the same as the 28-70 with both being non weather sealed and having barrels that extended when zooming wider and retracted when going long. Here is a pic of the 28-70's stator cable plugged in, so you have a rough idea of how far it has to travel to the lenses board, note that the stator is about level with the distant numbers you can see on the barrel.
So if anyone takes this further let us know how it works. If mine ever dies again I'll do a follow up on the attempted swap. But for now I managed to fix mine by lightly oiling it, removing the oil, squashing the ribbon connector to the stator back down hard, rotating the felt washer a quarter turn, and then randomly taking it apart and reassembling it six times. No more jumpy erratic auto focus! Good luck everyone, keep those 28-70's alive.
Edited on Mar 10, 2015 at 02:59 AM · View previous versions
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