Old (1988) FUHR 8820 multi-point won't lock, help please

[fourtytwo]

I am having trouble locking my front door, sometimes it will but most often it's a struggle especially from outside.
The euro-cylinder was replaced 2 years ago for security reasons no trouble but in the last few months it has become more & more difficult to lock.
If I lift the handle gently with gentle pressure on the key sometimes it will lock, otherwise it's handle down and try again. When pushing the handle back down sometimes there is a quite loud clunk seemingly from within the gearbox and this is a sure sign the next attempt at locking will be successful.

It always locks if the door is open.

I have tried lubricating the rollers & slides, made sure nothing is binding. I looked inside the gearbox without taking it apart and it is lubricated however I can see some parts are worn.

I have tried removing a bit more plastic allowing the euro-cylinder to align freely, tried it with the handles loose, all no joy. I know nothing about these gearboxes is the clunk some magic cam release that must happen before it will lock ? I fear taking the thing apart in case I cannot get it back together again and am completely unable to lock my door.

Any suggestions anybody ?

 

[ronniecabers]

If the locking works everytime with no clunks etc when the door is open, then it is an alignment issue. One of the cams or even the deadbolt are hitting the keeps on the door frame

 

[crank39]

As Ronnie says if it works when the door is open then likely it's an alignment issue, looks like it only has 4 roller cams and the dead bolt, try taking each keep off the frame one by one to isolate the problem, it could the centre latch keep though and the deadbolt fouling the slot in the keep, remove and try

 

[fourtytwo]

Thank you very much for your replies, I had not heard of taking the keepers off before so as soon as the weather warms up a bit next week I will do it that way. Brilliant idea, thank you.