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Monday, March 7, 2016

Beogram 4000: Rebuilding the Arm Lowering and Tracking Systems

After fixing the broken off MMC cartridge mount in the Beogram 4000 that I am restoring right now, it was time to straighten out the not working arm lowering and tracking mechanisms.
This shows the solenoid and damper setup that controls the arm lowering process:
I extracted the damper and the linkages for cleaning and lubricating:
Unfortunately, the damper unit showed signs of human creativity:
The rubber washer that acts as a valve seal for the air release mechanism was missing. Instead a crudely shaped piece of transparency foil had been implanted preventing the damper from working properly:
Furthermore the damper cylinder was damaged causing the piston to get stuck. This meant that a replacement damper needed to be installed. Luckily, I recently bought a few parts units on ebay and craigslist, so I was able to extract a damper from a Beogram 4004. Here is a picture of the proper setup with a rubber washer at the end of the piston:
I lubricated everything and installed the new damper:
Then it was time to extract the damper to arm linkage. For this the sensor arm needs to be extracted. This shows the arms from the back:
The sensor arm is held in place by two screws and can easily be taken out to get at the linkage:
Here I found more signs of human interaction: The retaining washer that holds the linkage on its pin was missing. Luckily the spring that sits underneath was still there. I extracted the arm after pushing the sensor arm tube a bit out for access:
After cleaning and lubricating the bearing I put everything back together and added new retaining washer:
Then I put the arm back in and aligned everything that the arms were both perpendicular to the back of the enclosure.
The next step was to replace the tracking sensor light source with one of my SMD LED based light sources. This ensures long term stability of the tracking sensor sensitivity. Light bulbs tend to break and also change their light emission over time. This shows the part:
It is available to other enthusiasts. Just send me an email if you need one.
This shows the original tracking sensor light bulb housing installed:
And after extraction:
The tracking sensor aperture is visible. I installed the LED replacement part:
And then it was time to adjust the tracking mechanism, or so I thought. When I started the deck it turned out that the solenoid was not able to lower the arm. All it did was emit a loud hum while doing some rattling motions. Not good...this suggested that this deck has a bad reservoir capacitor for the 30V solenoid supply. So the next step is replacing the capacitors and get the power supply straightened out!

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