Sigma 170 - 500 repairs ?

[Doughnut]

Have asked this question before, but think the thread got lost.....

Amongst many lens I use I have a rather old, but still very good, Sigma 170-500 APO and I use it on Canon 400D. The lens seems to work OK at F5.6, with good aperture / shutter read readings and well exposed images, however as you stop down, although the camera is giving good readings ie the shutter speed is reducing in proportion to the increased aperture setting, the images become over exposed until at F36 they are totally over exposed. My other lens are still working OK with the camera. My assumption is that the diaphragm is not stopping down as pressing the depth of field preview button has no affect. Is there any manual way to 'reset' or check that he diaphragm is not jammed fully open ? Sigma want £99 + parts to service / repair, does anybody know of others who might service the len ?

[waddojohn]

Bligh Photo Digital,
230 Harrogate Road
Chapel Allerton
Leeds
LS27 4QD

Tel: 0113 237 0833
Fax: 0113 237 0833

Have used them before, they serviced a Tamron lens for me, came back better than when I bought it new.
Give them a call

Hope this helps

John