An unlikely topic - but who knows the depths of experience present on 
the PICList.

Short:    Anyone had any experience in autofocus repair in lenses 
and/or know if Sigma parts can be bought.

Longer:

I just bought a used Sigma 28-200mm IF f3.5-?5.6? lens. Works very 
nicely indeed, if one ignores the dire problems described below. It's 
one generation back from their latest.

It obviously had "issues" at the time I bought it, despite the seller 
saying that it had always worked perfectly. The price was low and, as 
it came with a Minolta 5000 (film) camera that fits my Minolta lenses, 
I decided that the purchase was worth the risk. The lens proved to be 
nice enough to be worth buying a new-old one on ebay, but the one I 
have is now something of a challenge (and cost enough to be worth a 
little effort on principle).

To shorten the story, it sometimes autofocuses OK but often makes bad 
grunching noises and doesn't autofocus. I now have the lens outer 
layers dismantled down to where I have the autofocus mechanism in 
pieces - quite a few layers and parts but not tooooo complex - a 
little scary as one inexperienced slipup may doom the lens forever. 
(I've already badly bent one set of grey-code encoder wipers which I 
didn't know were there until I withdrew the ring they were mounted 
under.)(Repairable- hopefully).

Fortunately the lens mechanism proper is a single module which "lives" 
inside the outer auto-focusing control shells and I have not had to 
dismantle it. The problem is that the large drive track/gear for the 
autofocus, which is formed on the inside of the focus control ring, is 
nicely stripped and generally abused. For good measure the 
housing/bearing  for the drive gear shaft is somewhat broken at one 
end, allowing the gear to lift somewhat out of engagement and, no 
doubt, hasten the stripping process. (Something may have dropped into 
the gear path originally and caused the housing to break as the gear 
tried to ride over the obstacle or the lens may have been dropped.

I *think*/***hope*** that I can 'repair' the gear by turning it to a 
new position and making a new mounting for the stop/grey-code focus 
encoder wipers mounting/... which is attached to it. Bearing housing 
can probably be persuaded to think it's OK. (drill/tap/melt/glue/...). 
I will also see if parts are available but I strongly suspect that 
access to these will be very "closed shop" and I suspect that parts 
costs will be unrealistically high.

I'd be interested in hearing from anyone who had delved into such 
things or knows of access to parts.


        Russell McMahon


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