Infinity focus problem 50mm 1.7

I have an M version of Pentax 50mm f1.7.
It works great in general, but will not focus on infinity. The close distance is not a problem, but the longest distance I can focus on is approximately 4m, and that is when the focus ring is set to infinity.

Any ideas?

Thanks

It might have been taken apart and incorrectly assembled.

hm, any idea how to solve it?

Disassemble it and put it back together the right way

Easier said than done, I know. I usually don't disassemble lenses (currently busy with my second one) so not much experience and definitely not with the M50/1.7

Else contact a local repair center (others can advise as I'm not familiar with your area).

It may be the front element needing adjustment. I took a tamron 80-210 apart to clean off some fungus and when put back together could not get infinite in focus. I had to screw the front element further in and it sorted the problem out.

Not sure it they are identical, but here's a link to an article involving repairing a 50mm 1.2 M lens> http://grantharg.tripod.com/CameraRepair/M50mmfixa.htm

Thanks John and all others. I think I will give it a go.