Beogram 4000: Carriage Returns Home Immediately After Setting Down on the Record

Oh well, the Beogram 4000 that I restored in January 2025 came back to my bench exhibiting an issue I had not experienced before:

After pressing ON the carriage went and found the LP setdown point and put the arm down. Immediately after hitting the run in groove, the carriage returned home with the arm down. It did that at a fairly high speed. Once home the unit shut off and the arm lifted.

Since the << and >> buttons worked fine with the arm up, a problem with the H-bridge was unlikely. This meant there must be an issue with the tracking sensor.

The Beogram 4000 features two photoresistors FC1 and FC2 that control the reverse and forward directions of the carriage motor (later 4002/4 models are differently set up, with only one photoresistor for the forward direction). This means the dual photoresistor tracking sensor can control the carriage in both directions, forward and reverse. The carriage is at rest when the forward and backward currents through the H-bridge (i.e. the motor) just cancel out. This condition is reached when the aperture in the tracking sensor illuminates both photoresistors with the same intensity, i.e. when it is positioned just between the photoresistors. If everything is properly adjusted, this condition coincides with the tonearm being parallel to the sensor arm. As the arm gets pulled towards the record center during play the arm gets deflected laterally, which causes the forward current to become larger than the reverse current, and the carriage starts moving forward, following the motion of the arm.

A consequence of this setup is that if one photoresistor circuit is not working properly, the carriage motor will already see a significant current even with the tonearm in parallel alignment, and the carriage will start moving. If the failed photoresistor circuit is the one that controls the forward motion, the carriage will always go reverse, independent of the lateral deflection of the tonearm.

With this in mind, I started examining the forward direction circuit. A good rule of thumb with the Beogram 4000 is that 90% of its issues are typically caused by bad contact terminals in the many switches, or by a wiring problem. The Beogram 4000 is an early 1970s design, when wire-to-board connections were still hardwired (instead of using modern wire-to-board headers and plugs, which are mechanically much more resilient). These soldered wires have a tendency to break off at the solder terminals when the Beogram is handled or transported.

This personal 'statistic' was confirmed in this unit: When I had a closer look at the wiring I immediately found the problem. The wire connecting the forward photoresistor FC2 to the H-bridge was broken off at its terminal on the main PCB (yellow marking in the photo):

I cleaned wire end up and soldered it back to the terminal. And the deck was operating normally again! I will play this Beogram a bit and then it should be time for sending it back to its owner in Australia!

Beogram 4000: An Unusual Keypad PCB

This post is about an unusual keyboard PCB of a Beogram 4000 that was recently brought to my attention. Normally, the two position scale illumination bulbs in a Beogram 4000 are connected in series between the 24V rail and GND. This shows the bulbs on a standard board.

Below the PCB is the circuit layout as shown in the service manual. The way the bulbs are connected in series creates a situation where the polarity of them is reverse.

This is perfect for the installation of my (polarized) LED boards for replacing the bulbs:

When installed, these boards need to point in opposite directions to illuminate the entire position scale, i.e. their polarity is reverse relative to each other, which matches the above standard circuit layout, and both boards light up:

BTW: The red LED is the back illumination for the 33 RPM trimmer.

However, recently I received an email from another B&O enthusiast whom I supplied with two of these LED boards for his Beogram 4000 restoration, stating that only one of the boards was working in his Beogram! Very unbeolovely! After some more communication it turned out that his Beogram 4000 had a different keypad PCB, where two 24V bulbs were connected in parallel between the 24V rail and GND, instead of the normal two 12V bulbs in series.

This arrangement resulted in the same polarity for both of the bulbs, and this caused one LED board to be connected in reverse polarity, i.e. the LEDs did not light up. This situation is shown in the top-right picture in the slide below. 

He sent me a picture of this strange PCB (shown on the left). After a bit of head scratching it occurred to me that my LED boards could still be used if a small modification is made to the board:

By (1) removing one of the jumpers that connects one of the bulbs to GND, and (2) moving the 24V terminal to the solder point where the jumper originally connected towards the bulb, the standard in-series circuit was restored.

After this change was made, both LED boards started working (bottom right picture).

Beolovely again!