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Friday, December 18, 2015

Beogram 4002 (5521): Adjusting Platter and Chassis

The looks of a Beogram 400x strongly depend on the platter being properly aligned with the aluminum panels surrounding it. When correctly done the platter aluminum surface is flush with the surrounding panels, i.e. the black 'ribs' on the platter protrude above a flat aluminum surface. This adjustment is pretty tricky since several parameters must be brought in alignment. Over time I found that this adjustment is often complicated by a sensor arm that is not completely parallel to the floating chassis plane. The adjustment procedures outlined in the service manual and featured in my videos is based on the assumption that the sensor arm is parallel, i.e. if it is not the process gets 'distorted' and one ends up with one or more parameters out of spec once the platter is flush with the aluminum surfaces (most often the resulting problem is that the chassis is tilted after the adjustment in such cases and therefore chafes with the transport lock bushings).

So my most recent approach is to first get the sensor arm as parallel as possible to the floating chassis that holds the platter. The arm is on the carriage which travels on two precision rods that are held in fixtures where the rod height can be adjusted with set-screws from below. This picture shows one of these fixtures:



Unfortunately, the adjustment of these screws would require to remove the floating chassis, which is a bit painful.

This led me to a different approach: First I disconnected the floating chassis from the leaf springs that hold it up that it sat flat on the 'floor' of the enclosure. Then I put in the aluminum panels and measured the parallelism of the sensor arm relative to the panels at the lateral locations of the two rods. In this particular case I identified a 1.5 mm difference between the back and the front rod locations. Then I designed 3D printed spacers to lift the front rod up by 1.5 mm:



I did the same for the other end of the rod. Now the arm was parallel:

The next step was to get the platter parallel to the sensor arm in a 23 mm distance between top of the arm and the platter aluminum surface. Essentially I did the procedure shown in my video. Basically, what you should find is that the platter should be pretty close to parallel to the arm with the bearing fully screwed down if the arm is really parallel to the chassis. After this it was time to float the chassis again and adjust the springs until the platter was flush with the aluminum plates:


Ahhh...so beautiful! This is Beolove!






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