I am making good progress with the restoration of the Beogram 4000 that I recently put on my bench. While it seemed on first contact that this unit had escaped 'creative human interaction', I had to learn that it had been worked on by a tinkerer, and that a little bit of a mess had been made of the sensor arm insert that houses the light bulb and the photocell that measures the presence of a record (or rather its absence by detecting the intermittent reflection caused by the black ribs on the platter). My sensor arm LED installation video for Beogram 4002/4 explains in more detail how the circuit works if you are interested. It is pretty much the same circuit in the Beogram 4000.
When I pulled out the sensor arm compartment it immediately became clear that someone had been 'in there' before me: One wire of the photocell was not connected, the other was mangled (from not being careful when pushing the compartment back into the aluminum profile)
installing a modern phototransistor averted. But when I tested the sensor signal at the collector of TR14, I saw this signal, which looked inverted compared to the expected trace shape: