Help needed - backing cap jammed on a SAMYANG 28-70mm Zoom lens

Hello,

Recently I inherited my late father's Canon T70 SLR with a number of
different lenses. I'm having problems with one
of them and being a newcomer to SLR cameras, am in need of assistance..

I hope someone may know a trick to removing a backing cap on a SAMYANG
29-70mm Canon FD lens. When I placed the cap on, I lined up the silver
"pin" with the corresponding "gap" in the flange of the backing cap and only
tightened it a little. However, I must have accidentally turned it too far
and it has somehow "clicked" or locked into place. Now the retaining ring
won't turn back to
release the cap. Its all stuck quit firm - no amount of force will budge
it.

This has happened to me before and I took the lens to a repair shop and they
removed it for me but didn't show me how. That store is now closed for a
couple of weeks and the cap has jammed again.

Maybe I have to remove the locking ring itself, but don't know how.

I'm sure there's a trick to this but can't figure it out. Its probably a
case of the wrong backing cap for that lens,
but now that its stuck on, its a bit academic!

If you know how to do this, I'd appreciate some assistance.

Thanks in advance

Paul Peacock

Brisbane
Australia