Canon FDn 300/4 stuck aperture wide open

[Zulu Uluz]

I have a Canon FDn 300/4 with an aperture that is stuck wide open. Any hints on how to approach fixing it would be very helpful. Would it be relatively easy to disassemble from the back and open it up to release the iris? I've done some lens disassembly in the past so I'm ok with trying it out.

[Jesito]

[calvin83]

Here is a link for the L version(the non-L should be mechanically the same as I recall) on the famous lens modding site in traditional Chinese http://www.bigeye.url.tw/big5/d_ca300_4.htm .

The aperture block is controlled though two long metal rod connected to the lens mount. The two metal rod must be placed at the exact position for the aperture to function. There are two slots on the front of the aperture block which is responsible setting the full open position. The aperture need to be set to full open, adjust the two slots and lock the aperture in place before putting them all together.

I don't have time and ability to translate the whole page to English. It will be quiet difficult to fix it if one does not have experience on repairing lenses.

[Bah]

I've just fixed the stuck apature on my 300 f4, they are acually pretty easy to take apart from the back, once you get it out just flood the hell out of it with electrical contact cleaner while working it to remove all the oil, leave it to dry and it'll be right as rain.

That bigeye page is quite good.

After you take out the screws at the front of the focus ring (under the rubber) you unscrew the rear of the lens from the front; it will be hard, but you just have to get about it and it'll come off.

Good luck.

I am trying to get to the front group to remove fungus on the 2nd and 3rd element; that is proving hard...