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Beolover SyncDrive: DC Platter Motor Replacement for Beogram 4002 and 4004 (Type 551x and 552x)

Late Beogram 4002 and the 4004 (Types 551x and 552x), which have DC platter motors instead of the earlier synchronous AC motors usually suff...

Wednesday, December 30, 2020

Beogram 4002 Type 5521: Restoration of DC Platter Motor, Main PCB, and RPM Panel

I recently received a DC platter motor, the main PCB, and the RPM panel from a Beogram 4002 (5521) for restoration. The parts reached me via a personal courier from the Netherlands. As usual I started working on the motor first since it can take a few days until the motor bearings are infused with fresh oil. This shows the motor as received:

I took it apart to get at the bearings:
The bearings are the two small donuts on the black pad upfront. I submerged the bearings in motor oil and pulled a vacuum. Immediately, bubbles emerged from the bearing. This indicates that the vacuum draws the air from the pores of the bearing, making room for oil to inter diffuse:
After about 48 hours the bubbling stopped and I extracted the bearings:
I reassembled the motor and put it aside.
The next step was to rebuild the main PCB. This shows the board as received:
This shows the original 'RPM section' of the circuit in detail,
and the original amplifier circuit of the record detection circuit.
I replaced all electrolytic capacitors and installed a new RPM relay, RPM trimmers and a new high gain transistor for the record detection circuit.
This shows the rebuilt RPM circuit with modern encapsulated trimmers and relay:
The next step was to replace the incandescent light bulbs in the RPM panel with LED boards. They are the two green squares:
Before I tested everything, I adjusted the new gain trimmer for the record detection amplifier to yield the specified 4V at its collector:
This is an important adjustment to ensure that the absence of a record on the platter is properly detected to prevent damage to the stylus in the event that start impressed without a record on the platter. After the adjustment I installed the trimmer 'below deck' next to the new transistor:

And now it was time to do some tests. First I measured the sensor response above an empty platter and I got a nice ~5Vpp signal as specified in the manual (each dip corresponds to a black 'rib' passing by under the sensor arm - this is how the Beogram detects that there is no record):

So far so good. The final step was to adjust the RPM and measure the RPM stability with the BeoloverRPM device:
It allows logging the RPM in 10s intervals. This is the curve I measured over about 24 hrs:

This is about as good as it gets with the Beogram 4002 DC Platter motors. It appears these components are ready for duty again. Soon they will be sent back to the Netherlands!

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