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Tuesday, January 8, 2019

Beogram 8000 Type 5613: Service manual checks and solving an old tonearm lowering problem

This Beogram 8000 project is getting close to complete. The electrical work has been done and a sensor problem was solved in the last post. Now it is time to do the service manual checks and adjustments.

Similar to the Beogram 4004 the Beogram 8000 calls for the top surface of the platter to be 23mm from the top of the fixed arm. I made the check with my usual measuring tool.

Note that to operate all of the Beogram 8000 functions other than the turntable platter you can disconnect the P4 connector on the main board. That will leave the platter disabled during the testing.

No adjustment necessary here.

Before the next service manual adjustment I wanted to check the Beogram 8000 arm lowering function. With Beogram 8000 and 8002 turntables the damping of the arm lowering mechanism is accomplished electronically. Various owners (myself included) have experienced weird occurances of the tonearm dropping without any damping. The click of the solenoid could be heard in these cases, then after a short delay, the arm suddenly drops all the way down to the record. The first thoughts about this was there must be some defect in the electronic control for the arm lowering. Afterall, the electronic circuit is in control. While that is a possibility it is far less likely than the problem being mechanical.

The last time I ran into this problem I discovered some FOD (foreign object debris) in the lowering mechanism that was causing the problem. This turntable did not have any obstructions. I had made a point to check that earlier in the project. So I was surprised when I discovered this turntable indeed suffers from the sudden tonearm drop problem. I was kind of happy about it as maybe I could finally get to the bottom of this.

Here is evidence of the problem.

At the arm raised position you can see that the arm raise/lowering solenoid is to the right of its housing. The raise/lowering bar is in contact with the tonearm connection point (to the bar). This is all proper.

Here is the problem though.

The arm raise/lowering solenoid is now all the way to the left of the housing...in the arm lowered position. The arm raise/lowering bar has raised up to allow the tonearm to lower...but look, the tonearm connection point remains down at the arm raised position.

The way this should work is the gravity from the tonearm phono cartridge tracking force setting (typically 1 gram) should keep the connection point on the arm raise bar as it goes up.

After a short delay the arm finally does drop down suddenly. Why?

After checking the tonearm assembly pivot pins for lowering I discovered that they were operating fine and without any obstruction. I couldn't feel anything preventing the arm lowering when the lowering bar was moving to the lowered position. Very strange.

I was able to operate the arm raise/lowering bar manually and duplicate the delayed tonearm drop. Exercising that several times I increased the cartridge tracking force to 2 grams. Now the tonearm lowered correctly every time.

So the problem was that at 1 gram there was some force it had to overcome to lower.

The only mechanical suspect left was the adjustment screw that sets the height of the tonearm relative to the fixed arm. That screw physically touches the bottom of the tonearm counterweight.

My guess is the metal to metal contact develops into some sort of delayed release for the tonearm lowering. As a test solution I cut a small rectangular piece of Dura-Lar and glued it to the base of the tonearm counterweight where the adjustment screw contacts it.

That appears to have fixed the problem. Now the arm lowers gently when the electronic damping control moves the solenoid.

You can see that the tonearm connection point remains in contact with the arm lowering bar.

When the Beogram is all assembled the tonearm will contact and be playing a record before the arm raise/lower bar is at its full travel point. So in real operation the bar will have some separation from the tonearm contact point during record play.

Whew!  I think that mystery is solved now.

On to the cartridge tracking force check.  No problem there.

The last service manual adjustment/check before trying to play a record is the tangential tracking sensor.  As the cartridge stylus travels the record grooves the Beogram must sense the movement and keep tangential arm assembly moving to remain tangential to the stylus tracking path.

Like the Beogram 400x turntables the sensor is an optical sensor with an adjustment for the sensitivity of the tracking. The service manual calls for the adjustment to be made where the tracking motor first advances after two revolutions of the platter (± 1 revolution). After that the tracking motor should advance on every revolution of the platter.

Here is the adjustment point.

Here is the check of the adjustment.

Now I can hook up the P4 connector so the platter motor is enabled.  Note: Always disconnect and connect the board connectors with the Beogram unplugged.

There it is. Another Beogram 8000 turntable ready for reassembly and real record play.

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