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Tuesday, June 6, 2017

Beogram 8000: Replacing the Platter Tach Disc and Cleaning the Tangential Arm Assembly

Tonight I completed the tangential arm assembly cleaning and lubrication as well as changing the platter tach disc to a new, metal replacement.

The Beogram 8000 turntables started out with a plastic tach disc containing transparent and black markings that a photo-sensor uses to send pulses to the Beogram microcomputer. The microcomputer dials in the 33 1/3 RPM and 45 RPM speeds with these pulse signals.

Unfortunately, over time these plastic discs did not hold up so Bang & Olufsen replaced them with metal discs. As I mentioned at the beginning of this project, it is quite surprising that this Beogram 8000 actually has an original, functioning tach disc. Feeling that it is on borrowed time we decided to go ahead and give it a more permanent tach disc that Martin Olsen supplies. 

Here is the Beogram 8000 center hub with the tach disc next to the replacement disc I got from Martin Olsen.





























...and here is the replacement installed.



























Before testing the new tach disc I took the opportunity to clean the floating chassis and take care of some maintenance on the tangential arm assembly. I always clean off old oil and grease from the tangential arm drive so I can inspect the parts and apply fresh lubricants.

I was surprised to discover this Beogram's tangential arm drive screw bushing has a badly worn edge. It appears to come from the stress of the arm return travel.
This picture shows the worn bushing and the floating chassis before cleaning.

















Here is a better look at the worn bushing after cleaning the parts.





































For now I decided to turn the bushing around. That gives two solid edges pressing against the black, plastic transport bracket during the return travel of the tonearm. The stop travel puts the most stress on the bushing. During play the forward travel is in very small increments so there is very little stress on the bushing.


































I reassembled the tangential arm assembly to give the new tach disc a test run. 

The Beogram speed testing with the new tach disc went great. The two speeds immediately lock in. I did some quick tests of the tonearm navigation and so far it runs very smoothly. Later, when I get to the record tracking tests I will re-evaluate how solid the tangential arm drive is working.





















Next up I will re-install these components back in the main Beogram chassis and start on the mechanical service manual checks. It is getting close to actually playing some vinyl on this baby.

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