A short video with a demonstration of the issue (excuse the pink vinyl and wrong-angled view)

https://youtu.be/fel7XxjjPaQ

It was not a good idea to spray lubricant, especially WD40 on the inside of the Beogram. Those turntables require a little lubricant from time to time on the tangential arm drive screw but I have seen a bunch of Beogram 400x turntables that had way too much oil. The rotating parts end up splattering the extra oil all over the place. 

Your Beogram is not detecting the end of the record but it is difficult to tell what the exact problem is. You said that playing the record from the beginning also results in the tangential arm drive motor not moving to keep the tone arm parallel right?

Remove the platter and operate the Beogram without a record. The Beogram will drop the tonearm in space where the record start would be. Make sure you have the cartridge on but with the stylus guard on. The arm should not drop the arm below where the record would be. With the arm dropped (onto what it thinks should be a record) give the tonearm a little nudge towards the center of the record and see if the tangential arm drive motor tries to move the arm. If it doesn't then your tangential sensor is not working. The lamp could be burned out or it is out of adjustment.

-sonavor 

Thanks for the reply sonavor - noted your WD40 comment, thank you.

You are correct that the same issue occurs wherever I set the tonearm down on the record; I just set it in the run-out as it obviously happens quicker than in the normal grooves. 

I've tried as you've suggested, and even with a nudge, there's no movement on the arm at all. I'm guessing the sensor is not working as it should. The lamp detects the record correctly, so I guess it's okay; how does the sensor get out of alignment, and can this be fixed by someone like me who has a fairly low level of experience with repairing electronics?

The lamp involved in the tangential arm tracking is not the same lamp as the one on the fixed arm. The lamp on the fixed arm only detects if there is a record present on the platter or not. If there is a record on the platter then that lamp triggers logic that says it is okay to set down. The actual set down position is determined by another sensor (LED driven) that is internal to the Beogram. The tangential tracking sensor and lamp are in a housing at the base of the tonearm (where it pivots). It is inside a black plastic housing but you can see the lamp glow when the turntable is on. See if you can see that glow. If not, then the lamp could just be burned out. If you see the glow then the tracking got out of alignment somehow. 

You can research various Beogram 400x turntable repairs and adjustments here.

There are some good videos of the Beogram 4002 repair/restoration on the

Thanks again - I checked the tracking sensor light and it has indeed blown. Looking at the Beolover site, I see he's created a LED replacement part, so have made enquiries there. Out of interest, is there a readily-available alternative lamp that I could source from, say, a lighting shop as a backup?

It's really fun learning about what each aspect of the turntable does as I've been fiddling with it, and it's increased my appreciation of the craft that went into creating the whole thing. Marvellous!

If you want to change out the lamp you can get a replacement from Martin (Dillen here on the forum). 

-sonavor

Ah super, I've got some spare lamps from another project he helped me with. I'll drop him a line to see if any are compatible.

A quick update - thanks to Sonavor's advice, I determined that the issue was the sensor lamp, which had blown.

I contacted Rudy at Beolover, and with the help of his excellent 3D printed replacement part with LED, and instructional video, I've been able to get the turntable running again with no issues at all! 

Thanks again Sonavor!