Repairing the focus assembly of a Canon EF 500mm f/4L IS USM

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tedlin01 New Member • Posts: 2
Repairing the focus assembly of a Canon EF 500mm f/4L IS USM

A couple of weeks ago I started having issues with my lens that I bought second hand, half a year ago. I started having issues with the IS and the autofocus. The IS stopped working randomly and the autofocus was missing it's target a lot.

Canon no longer services this lens and there is no spare parts available anywhere so I decided to buy a lot of equipment/tools and have a look inside myself.

Shutter and motion sensors.

I started with removing the top part and everything looked fine, putting the IS aside for now..
I then dissembled the focus assembly and realized that there was a crack on the sensor strip. I tried to repair it with a special glue made for acrylic but I just managed to make a mess out of it and destroyed the strip totally. (Don't clean acrylic with regular rubbing alcohol, it's to strong).

Focus assembly attached to lens barrel.

Focus assembly inside.

Focus assembly outside showing the ultrasonic motor etc.

Ruined sensor strip.

The sensor strip is laser-engraved with some sort of encoding that tells the position of the barrel so It can't just be replaced with some random acrylic glass.

In my hunt for replacement-parts I have read up on the focus motors that Canon use. The most frequent one used is Ring-type and there are two versions of it: The 62mm M-1 that is used for most lenses and 77mm L-1 which is used for the heavier lenses such as mine.
I am now investigating if I can take the sensor strip from the focus motor of a different model. It would probably have to be a lens that has the same size on the USM. It's hard to come by information about the focus assemblies and/or USM for different lenses,  It's also hard to find spare parts for any of the "heavy" lenses, especially since I started to look at lenses that is about the same age as mine. I don't know if it would be possible to use a focus assembly from one of the newer lenses? Could they possibly still be using the same sensor strip and/or focus assembly?

Perhaps someone can shed some light into this project? Any help with repairing my lens would be greatly appreciated.

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