Beogram 4000: An Interesting Tracking Issue and Installation of a Beolover Efficient 24V Power Supply and Main Capacitor Array

I recently received a Beogram 4000 back that I restored in February 2024 for a customer in Massachusetts due to an issue with the tracking system.

The phenomenon described was that the carriage occasionally would not track after the needle was lowered into the run in groove. I tried for some time unsuccessfully to reproduce this issue until it finally happened after I put a new record on the deck. It became clear to me at this point that it depended on the particular record played since the problem was fully reproducible with this particular record, i.e. happened every time I started the deck.

After a bit of head scratching I figured out that when facing a 'fast' run in groove, the arm would move so fast to the left, that the carriage motor was not able to keep up moving the carriage along before the tracking sensor aperture had already completely passed past the photo resistor opening in the sensor, in effect turning off the motor. The motor voltage is directly controlled by the resistance of the photo resistor and if it is dark the motor stops running.

The big question at this point was why this would happen with this particular Beogram 4000 and not with all the other ones I restored so far. 

I set out to measure the motor voltage with my oscilloscope. This trace shows the evolution of the motor voltage during regular play of a track on this Beogram 4000 as I received it:

What we see on this graph is that the motor voltage 'oscillates' and that there are larger jumps every few oscillations. It turns out that the oscillations correspond to the rotation of the platter: Note the 4s per unit time constant and that there are two oscillations per scale unit. It takes about 2s for a rotation at 33 RPM...

The voltage fluctuations are essentially a result of the eccentricity that most vinyls have to varying degree. The graph means that the carriage only weakly moves during ~6 or 7 rotations and then in one large burst after the voltage gets big enough for the motor to overcome stiction. Then the carriage drives forward until the arms are almost parallel and then the process starts again.

This suggested that the amount of light falling on the photoresistor was not enough to generate a high enough motor voltage through the H-bridge.

I decided to modify one of my standard Beolover Tracking Sensor LEDs to give the LED a bit more current to make it light up more:

After I installed the modified light source I re-calibrated the tracking feedback gain to start moving the carriage after about 3-4 rotations and measured again:

This time the curve looked like this and the carriage reacted speedily after hitting the run in groove of the record that caused the above issue. During play of the same track as above it gave me this trace on the oscilloscope. This shows that the carriage is now basically moving every rotation of the platter.

There are still a few variations between the peaks, which probably indicates a variation of the friction the motor encounters as the spindle rotates. Most spindles are not completely straight and so there is some change in friction during a rotation. It appears that each rotation of the spindle corresponds to maybe 4-5 turns of the platter. Of course this depends on the particular record since the groove density varies from record to record.

It seems this issue is fixed, but of course I am wondering why this particular Beogram 4000 needs a higher light intensity. It may be that the carriage motor itself is a bit different and turns less easily than others at a given voltage. Another possibility is that the photo resistors in the sensor are different, or from a different batch. The Beogram 4000 has two photo resistors in the tracking sensor that allow the carriage to actually track both forward and backward (this was discontinued in the later 4002 and 4004 models which only have one resistor for forward movement).

This unit seemed to track about the same in both directions when the arm was moved to the right or left manually, so I think it is not an issue with one of the resistors. If it were, I would expect that only one direction be affected. B&O has a history of making small circuit changes over the manufacturing run of a design. Maybe they simply used somewhat different resistors or there was variation between batches of them. Possibly the potentiometer for adjusting the voltage into the bulb that is normally found on the original configuration of the tracking sensor of the 4000 is an indicator that they already had this issue when they produced them, and someone calibrated the bulb intensity before Beograms were shipped out.

We may never know!...;-). Anyway, let's hope this tracking mechanism is fit for duty again!

While the unit was on my bench my customer decided to let me install one of my new efficient 24V power supply and main capacitors boards for Beogram 4000:

These boards elegantly replace the big reservoir and motor capacitor mess that is normally found in Beogram 4000s. It also updates the old-fashioned linear regulator based 24V power supply that causes a lot of power loss and heat emission in Beogram 4000s. This board will make any 4000 run more efficiently and the unit will get much less warm.

This Beogram of course already had my previous capacitor replacement kit implemented:

Out with the old

and in with the new! This shows the board bolted in. The solder pads are clearly labeled and in the right spots for easy connections:

This shows all the leads soldered to the board and the motor re-installed:

All good now with this Beogram 4000!

I will play it a bit more and then it will be time to send it back to my customer!