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99329004.JPG (226967 bytes)wpeE6.jpg (15250 bytes)Canon EF 75-300mm Zoom lens

wpe42.jpg (258707 bytes)

I recently bought this lens as faulty - I saw it as a bit of a challenge, and an opportunity to take some pics of the insides.....

wpe40.jpg (395832 bytes)Electronics PCB. Square chip is a microcontroller, which communicates to the camera body, and controls the aperture and focus motors. Other chips are power drivers for the motors.

wpeD.jpg (399591 bytes)The 'mechanical brain' of the zoom mechanism. Precisely shaped slots move the various lens elements along the barrel as the outer aluminium tube is adjusted. On the left (gold) is an encoder which tells the camera what the zoom setting is - a set of contacts on the stationary part of the body join different permutations of tracks as the barrel is rotated..

wpe3E.jpg (257015 bytes)Focus motor assembly. Motor is bottom-left. There is actually a teeny little drive belt from the motor to the first gear - I suspect this helps avoid sudden shock as the motor starts and stops. Bottom-right is the gear that engages with the focus ring.

wpe38.jpg (269876 bytes)Aperture blade assembly (also contains a lens element). On the left is the motor, which appears to be a bipolar stepper motor. The bracket at the top guides the flexi-print as the assembly moves within the lens body.


The repair.....

wpe39.jpg (261934 bytes)The aperture control was not working. Turned out that this was due to a fractured piece of the flexi-print (left, top) connecting the aperture motor. I'm somewhat puzzled as to how it got broken where it did, as (fortunately from the repairability point of view), it was not broken at a part that flexes in normal operation. The lens didn't look like it had been fiddled with before, so I can only assume that maybe it was under some stress from not being assembled correctly at the factory.

wpe3A.jpg (309716 bytes)wpe3C.jpg (389770 bytes)Fine (0.15mm dia) coloured enamelled copper (magnet) wire soldered to the end of the broken flexi-print - this was somewhat tricky!

wpe3D.jpg (399029 bytes)wpe3B.jpg (385333 bytes)(left) wires threaded through & soldered to other half of flexi.
(right) Joint taped over to reduce chance of wires fracturing during handling


99329004.JPG (226967 bytes) wpeE6.jpg (15250 bytes)

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