FED Etude stereo - left lamp issue

Hello!

I now have two FED Etude stereo projectors (gray and orange), however in both, the left lamp is not working. I just cannot find the problem despite taking them apart to the last screw, bulbs are fine. Since both projectors suffer from exactly the same issue maybe I am doing something wrong?
There are only 2 buttons for the lamps, but the left does absolutely nothing.

One projector will be converted to use modern CDM Lamps but I'd also like to have a working original one.

Hope you can help, I am running out of ideas.

Regards,
Christian

Noone?

I checked the lamp sockets and there simply is no current on the left one. Anyone got this projector in operation?