During today’s work, I installed the Beogram 4004 platter motor, fine-tuned the position sensor’s voltage, calibrated the detector arm sensor, and addressed an issue with the arm lowering solenoid.

My previous checks indicated adjustments would be needed after installing new parts, which is typical.

I also observed that the Beogram 4004’s arm solenoid, responsible for lowering the tonearm, was not raising it correctly. There was a lag between the signal to raise the tonearm and the solenoid disengaging. This problem, encountered before, stemmed from the solenoid plunger sticking.

Before tackling the solenoid, I wanted to adjust the voltage levels of a couple of sensors on PCB 1.

The first adjustment was for the position sensor (4IC1), which I set to 5 VDC using PCB 1’s trimmer 1R88. This adjustment is made when the tangential arm is positioned where the position sensor lamp shines through a clear section of the position scale.

Next, I calibrated the detector arm sensor using the new 1R26 trimmer. I could then move the trimmer to its permanent location on the component side of PCB 1. During this process, I had to swap the initial 2MΩ trimmer for a 5MΩ one (shown below) to achieve the desired 4 VDC steady-state voltage at the 1TR3 collector.

Afterward, I de-soldered 1R26 from PCB 1’s component side and relocated it to the other side.

With the sensor adjustments complete, I installed the restored Beogram 4004 DC platter motor.

The installation of the platter motor, sub-platter, belt, and top platter went smoothly. The restored motor operates silently and looks excellent.

Although I’d typically proceed with more service manual adjustments at this point, the arm lowering solenoid issue demanded attention.

I removed the solenoid assembly for disassembly and tested the coil (reading a healthy 9.3Ω) and the solenoid itself using my 30V bench power supply.

Within the Beogram 4004, the controller applies approximately 30 VDC to engage the solenoid for less than 40 milliseconds before dropping to around 1.8 VDC to hold it in place. When detached, the solenoid engaged at a significantly lower voltage (around 5 VDC and 0.49 Amps).

However, engaging wasn’t the problem with this solenoid – it was disengaging. The plunger was sticking. I could feel resistance when manually trying to raise the solenoid plunger to its up position after disengaging it.

After removing the assembly, I cleaned all moving parts with acetone and replaced the coil’s tape, as the original was peeling off.

While it’s tempting to lubricate the moving parts when a solenoid plunger sticks, it’s crucial to avoid this. Oil and grease exacerbate the problem, hindering smooth plunger movement within the guide assembly. Cleanliness is key, hence the acetone cleaning of all parts, including the guide tube’s interior.

This sticking issue mirrors the Beogram 800x’s tonearm lowering mechanism problem, where a metal adjustment screw sticks to a metal plate. The solution there is to insert a thin plastic layer, preventing metal-to-metal contact.

Similarly, the threaded hole end of the plunger comes into contact with a metal end stop at the end of the guide assembly tube, and over time, these metal parts begin to stick. I initially suspected magnetic buildup and attempted demagnetization, which seemed successful. However, I now believe the cleaning was the more significant factor.

This time, I applied a thin piece of yellow tape (similar to the coil wrap) to the plunger’s end, preventing metal-to-metal contact when the plunger is fully extended (arm lowered).

This modification immediately yielded a substantial improvement. Previously, the plunger could be moved forward manually but resisted retraction, even when the solenoid was turned upside down. The plunger would simply cling to the guide’s end. Now, without direct metal contact, the plunger retracts smoothly.

The material’s thickness is critical. A previous attempt using a thin nylon washer as a barrier failed due to excessive thickness, preventing proper solenoid lowering.

Reinstalling the solenoid and testing arm lowering and raising confirmed the fix. The arm now raises instantly without delay.

Note: The holding clip for the lowering lever pivot post was not yet reinstalled in the photos of the lowered/raised solenoid. It has since been installed, in case anyone was concerned about the omission.

With this issue resolved, I can resume the service manual adjustments.