One of my 500 c/m bodies which I bought on Catawiki auction site - stay away from that site, they are not serious and have no control what is put on sale, and don’t care either - have the same symptom.
You have to open the body. Guidelines you find here at
1984 | SPT Journal | Learn Camera Repair
I have had to dismantle everything. Started to wash old grease inside the body for the shutter mechanism. Still did not work to fire and advance.
The problem lies with the connection between the film advance mechanism (turning knob/crank) and the back shutter. I have washed all old grease away and put new on, still it tends to jam.
Second problem is that the upper flap does not open fully which means with a A12 magazine there is a black stripe. I can not get it to open fully.
Maybe we should start a new thread but I think we can benefit from our mutual experiences. On example, do NOT open the screw where the spring on top on advance for the film magazine. It is very tricky to get back.
BR /Mats