To be honest, if you are not VERY good with servicing electronics and B&O in particular, it's not a job for you.
I've seen too many sad cases where what was actually a simple fault had turned into a complete mess because
the owner, wholeheartedly, well-meaning but unable, tried to service his Beogram himself.
Not knowing your tech skills, the Beogram 4000 is easy to get into but definitely not the easiest machine
to service. When I write that it's probably a simple fault, I mean for a trained tech guy who is familiar with the
BG4000 and it's peculiar ways. Worth getting it serviced prof. in my opinion.
If you feel like having a go yourself, put an oscilloscope to the power supply rail(s) and look for ripple.
Finding bad power supply capacitors is quite common in these wonderful machines. Note that one of the large
capacitors is a bipolar type and used in the platter (AC-) motor circuit rather than the power supply.
Then put the scope to the signal path from the tracking arm sensor and
look for the reflection pattern caused by the radial platter streaks. Should be a fairly clean and well-defined signal.
Martin