I hope some of the Beogram brains here will have an educated guess for what's causing this problem. . . .

As the subject line says, my friend's Beogram 1800--which I had previously brought back to life with the help of this forum--is playing noticeably off pitch. At first I thought that the platter might be rubbing against the chassis, throwing off the speed. But I adjusted the leaf springs and the problem is still there. Belt seems fine; it was a replacement from Dillen. I also checked and adjusted the speed for both 33 1/3 and 45 using a very accurate strobe disc and light (the Speedstrobe from KAB Electro-acoustics). Any ideas?

Usually, the speed must be right, when a strobe disc indicates no drift. But You watched the right hertz marking and used a regular light bulb (not those with electronic transformers etc) then?

The Speedstrobe uses a quartz-locked light that doesn't depend on the AC line frequency, so the hertz isn't an issue.

I had checked and adjusted the speed with the chassis plate off (so that I could access the speed adjustments). I'm going to check the speed again with the plate on--maybe it's interacting somehow or something is rubbing against it.

The speed trimmers can be accessed from below the Beogram. 

Martin

Really? I only saw the adjustments on top. Where are they located underneath?

Along the right edge, close to the front is a pair of holes in the bottom plate.
Place the Beogram so that its right edge is 5-8cm out over a table edge
and you can access the adjustments while it runs.

Martin

Great! I'll check the speed that way and see if it tells me what's going on.

Ok, here's an update: Even after adjusting the speed, I'm still getting a weird pitch problem. It's almost like something is slowing down the platter at a specific point in each revolution, a speed "hiccup" if you will. The last thing I tried was adjusting the platter height so that it's high enough to avoid hitting anything underneath but not so high that the belt would slip off (which I found can happen), but the problem is still there. Now I'm thinking it's either something with the belt or the motor spindle. Any thoughts?

Check that he belt pulley on the motor is properly fastened, if not fasten it with Blue Lock Tite or with a little crazy glue, the right way is lock tite. Check your platter bearing is free from damage and wear and oiled, check the platter shaft is 100 % straight and without wear.