Beogram 4004 (5526): Restoration of Arm Lowering and Tracking Mechanism, Restoration of Original Solenoid Wiring and a New Reservoir Capacitor

The first focus on the Beogram 4004 (5526) that I am working on right now was, as usual, the restoration of the arm lowering and tracking mechanism. This restores in most cases the proper working of the solenoid/damper mechanism.

This Beogram came with a host of modifications that suggested that the previous owner had an issue with the solenoid function. He/she implemented a bigger reservoir capacitor and a thicker wire between main PCB and the solenoid board, obviously in an attempt to boost the power of the solenoid. All this was in vain, since the root-cause of the malfunction, hardened lubricants, was not addressed in the process.

This shows the modifications: Exhibit A, the added reservoir capacitance

and Exhibit B, the thicker (white) wire soldered directly to the board:

So the first step of this restoration was to reverse these efforts to restore the original wiring. Unfortunately, the previous owner cut the original control wire for the solenoid function pretty close to the plug insert and at the opposite end close to the solenoid board. Luckily he/she just left enough of the wire peeking out that I was able to remove some insulation with a razor blade for adding some more wire to restore the connections.

This shows the wire to board connector pin with cutoff wire.

I extracted it by pushing in the tab that blocks it from sliding out in the header. A bit of microsurgery with a razor blade allowed me to install a short jumper to be soldered to the solenoid wire:

Then I pushed the pin back into the header housing and soldered the wire in and insulated it with a short piece of almost color matched (always in style! - that is Beolove!) shrink tubing:

Then I did the same on the other end:

And so the connection was restored to its original design.

The next step was to remedy the capacitor mess. I removed everything and cleaned up the solder points on the board, and then installed the replacement:

At this point the mechanism still malfuctioned, i.e. the solenoid would briefly twitch when the deck initiated arm lowering at the 12" run in groove position. I measured the solenoid signal with an oscilloscope, just to make sure there was no electronics issue. The signal turned out to be a perfect close-to-30V-40ms pulse decaying into a ~2V constant voltage:

The purpose of this signal is to briefly power the solenoid at (or probably above) its limit to be able overcoming friction and the resistance of the damper etc...before settling into a much more 'comfortable' low power state that is just enough for keeping the springs in the arm lowering setup tensioned. At this point it was pretty much certain that hardened lubricants were the root cause of the problem.

This shows the solenoid/damper mechanism:

I removed the linkages and the damper for cleaning and relubrication:

During this procedure it became clear that the main cause of the issue was the solenoid linkage where the grease was so hardened that I needed to use a big screwdriver to carefully pry the linkage from its pivot point.

After reassembling the mechanism, the arm lowering process worked again. But I went on to complete the restoration process by removing the sensor arm to get to the linkage that links the damper and the tone arm. This shows the arms from the back:

and after taking out the sensor arm assembly:

I removed the linkage (careful if you try this at home - there is a small spring under the locking washer that can easily jump off to a four-dimensional coordinate point in the time-space continuum where it cannot ever be found again..;-):

I cleaned and lubricated the pivot point and put everything back together and reinstalled the arm assembly:

The final two steps in this restoration chapter are the replacement of the tracking system light bulb with an LED assembly, and the installation of an aluminum pulley to replace the cracking-prone original plastic wheel.

This shows the original tracking sensor setup:

I removed the light bulb housing

this reveals the aperture that controls the light amount that falls on the light sensor in the lower half of the sensor housing. This signal is used to form a feedback loop controlling the carriage motor during playback of a record in order to advance the arm as the needle is dragged towards the center of the record.

This shows the original bulb set-up with the Beolover LED based replacement:

The assembly is designed in a way that the SMD LED is in the spot where the filament of the bulb is located.

One weak spot of the aperture design is the sheet metal screw that is used to clamp the ring holding the aperture to the tone arm bearing. It is frequently loose and one cannot really tighten it. I usually replace it with a stainless M2x12mm flat socket head screw and a nut, which gives this setup a much better long term stability. 

This shows the LED assembly installed together with the new bolt:

On to the pulley. This shows the original one:

And the aluminum replacement (together with a new belt):

Allright...all good again in the arm lowering and tracking department! On to rebuilding the PCB.