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Saturday, August 5, 2017

Beogram 8000: Tonearm Lowering & Raising Components

In my recent Beogram 8000 project I had to investigate the tonearm lowering and raising functionality. I have also had to look at the part of the Beogram 8000 on one of my own units (where the lowering was sticking, then suddenly dropping). I thought is would be useful to post some details of the Beogram 8000 components involved with the tonearm lowering and raising control.

First are a couple of pictures of the tonearm in the raised position to get familiar with some of the key players.

There is a solenoid involved that is controlled by the Beogram microcomputer and an op-amp circuit that generates a ramp signal to lower and lift the arm via a lever and control bar.

The above pictures show the control bar pressing down on a tonearm linkage component. There is a slot in the exposed end of the linkage to adjust the (vertical) parallelism of the tonearm. Adjusting the linkage position affects the gap between the tonearm and fixed arm.

When the control bar is pressed down on the linkage the tonearm is at its fully raised position. There is a metal spring mechanism that the normal position is designed to keep the arm raised. The solenoid plunger moves a lever that presses against the spring to raise the control bar (and allow the tonearm to lower).

The next picture shows the Beogram with the tonearm in the lowered position.

This picture was taken with the Beogram actually playing a record. You can see the control bar was pushed away (by the solenoid) to allow the tonearm linkage free movement. When the control bar is raising it does so with a controlled motion so the arm slowly lowers to the vinyl record. When the stylus touches the groove the tonearm obviously reaches the limit of its drop. The control bar continues to raise a bit more so it is no longer in contact with the linkage.

At this point the tonearm movement is controlled by the record groove and the Beogram tangential tracking detection.

Here are a couple pictures of the arm lowering/raising components on a stripped down Beogram 8000. The tonearm is removed in these photos. Only the pivoting base remains. These pictures give a good view of the control bar. You can see that the bar travel is managed by a guide slot in a metal bracket.

WARNING: Removal of the tonearm is not part of any adjustment procedure of the Beogram 8000. Attempting to remove the tonearm can cause serious damage and render your Beogram unusable. This is because of the delicate phono wires that travel through the arm and pivot base. These pictures were available for me to make because the Beogram pictured was a damaged unit where the tonearm wires were severed.

Here is a photo from the back of the Beogram with tonearm installed showing another angle of the control bar.

One more photo of the control bar and related components from underneath the tangential arm assembly.

The above photo also shows the location of the adjustment screw for the other tonearm parallelism (horizontal) adjustment. This is for aligning the top surface of the tonearm with the fixed arm when the tonearm is in the raised position.

Now for a look at the electronic part of the arm lowering and raising. Here is the relative circuit as taken from the service manual schematics. The phono cartridge muting circuit is also shown as both the muting and arm lowering/raising are controlled by the same signal from the Beogram microcomputer.

The part of the circuit shown as my test point (TP) for the lowering/lift integrator signal is the drive signal for the solenoid. When the arm is being lowered that signal is a slow ramp signal from 0V up to around 12.6V. When the arm is being raised it is a fast signal from about 12.6V down to 0V.

Here is a view of the arm lowering and raising signals at the test point relative to the microcomputer control signal.

So far in my encounters with these Beogram 8000 turntables, any issues with the tonearm lowering and raising have been mechanical problems. Something interfering with the mechanical movement of the arm lowering/raising components.

If you are having problems with this area I recommend putting the Beogram in service position and studying the movement of the control bar in relation to the arm linkage as well as the movement of the solenoid.

I observed on one of my troubled turntables the solenoid and control bar operating correctly but the arm linkage was sticking, then dropping all at once. I took apart most of the involved components and made sure everything was clean and nothing was interfering with the movement. Once re-assembled the arm lowering worked perfectly again.

If there is a problem where the solenoid isn't working, here are some pictures of the Beogram 8000 solenoid components disassembled. I mention again that I haven't encountered a failed solenoid yet and these pictures were taken from one of my spares just to have as a reference.

I hope the pictures are useful to you Beogram 8000 owners wanting to trouble-shoot any tonearm lowering problems.

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