Beogram 8000: Sensor Checks and Adjustments

The problem with the Beogram 8000 detector arm not detecting an empty platter turned out to be a simple adjustment to the sensor source lamp positioning.

The Beogram 8000 schematic shows the detector arm circuit and what the expected sensor signal should look like.

Measuring at C19 I could see the signal was not as strong as it needed to be.

I used a flashlight to apply more light and saw that the sensor output increased up to a working level.

The easiest thing to adjust and check at this point is the sensor source lamp position. I opened the lamp and sensor housing to move the lamp down a little inside the housing.

That did the trick. The detector arm now functions normally with the correct output voltage on the detector circuit.

While I had my Beogram test setup in place and my scope warmed up I checked the tangential arm position sensors. Here is the circuit diagram.

Both sensor outputs look good and the Beogram has been working as it should with regard to the tangential arm movement.

Another sensor output to check is the platter speed. This Beogram still has the original, plastic printed tacho disk. Let's see how it measures. Here is the circuit and measurement point.

Here is the sensor output for 33⅓ RPM and 45 RPM.

Last are the forward/reverse scanning LDR sensors. This is the adjustment I always add my custom connector for so I can check the LDR voltages at any time without having to completely open up the Beogram again.

Here is the circuit.

Here is a photo of the test connector I always add for the this LDR adjustment. This photo is from an earlier Beogram 8000 restoration project. It is the same on this current project. The wire colors used on the test connector were chosen to match the Beogram ribbon cable for those signals.

Here are the forward and reverse LDR measurements after adjusting them to be between 0.6 VDC and 0.7 VDC.

I believe this Beogram is working correctly electronically. Now I need to check the tonearm balance and the distance to platter surface. After that I will check the cartridge tracking and, if necessary, adjust the linear tracking sensor. I should be playing records on this Beogram real soon.

Beogram 4000: Final Adjustments, Some Odds and Ends and a Test Drive with Cal Tjader

After exchanging the black painted plinth with a teak plinth from a Beogram 4002, it was time to do the final adjustments and give this beautiful Beogram 4000 a first test drive. After adjusting the platter height, tilt and alignment with the surrounding aluminum panels, I adjusted the arm lowering limit to ensure that the needle will miss the black ribs on the platter should there ever be a malfunction in the platter detection mechanism. The limit needs to be adjusted that the needle misses the lower parts of the ribs by about 1 mm:

The next step was to adjust the tracking sensor feedback that the arm would track parallel to the sensor arm:

Then came the motor waveform adjustment. Essentially, the trimmers need to be adjusted until a nice sinusoidal is achieved at the highest possible amplitude. An oscilloscope is helpful, but in absence of one it is possible to adjust the motor simply by listening to it. Increase the amplitude until it starts getting noisy, and then reduce it until the noise goes away. This is the measured waveform:

While I still had it open, I also installed a grounding switch that allows connecting signal and system grounds. This can be helpful for hum reduction, especially if RCA cables are used like in this Beogram (RCA does not carry the system ground, that is why there is usually a separate wire to connect the system grounds of amplifier and turntable when RCAs are fitted). This is how the output terminals looked like before implantation of the switch:

And after installing the switch:

Then I installed a nut on the screw that holds the counter weight of the arm in place. Originally, this screw is only held in place with a flimsy retaining washer allowing it to move a bit:

I took the retaining washer out and replaced it with a M3 nut and a washer:

This allows fixing the calibration in place in a more permanent way, which is a necessity if it is to survive the shipping process intact. Now it was time to calibrate the tracking weight:

This is done best with a digital gauge since they are very accurate and do not cost much money anymore. One last thing: One of the mounting bolts for the hood had a 'custom fashioned' spring that did not work very well. I replaced it with a new one to ensure trouble free hood installation:

And then it was finally time to give this lovely Beogram 4000 its first post-restoration spin!

I selected a recently obtained record by Cal Tjader from 1971, simply called "Tjader". His output from the 70s usually earns less raving reviews than what he did earlier, but I really like this record. Especially the tune "What Are You Doing The Rest Of Your Life" (one of my big questions, too!...;-) is a very nice and smooth tune. Here is an impression of Cal together with the Beogram:

A lovely combination! I will now play this Beogram a bit longer and then it will be time for it to travel back to Germany!

Beogram 8000: Display Segments Working Again

When I was able to finally check the recapped electrical work I discovered the bottom segment of the seven segment displays was not working. That usually is a connection problem (like a bad solder joint) but it could also mean a problem with the 4.7nF capacitors on the display driver circuit. In the case of the bottom display segment, C41.

Here is the failed segment on the Beogram display.

Here is the schematic portion for the Beogram display.

The display driver circuit is located on the microcomputer PCB. That board is small and the solder pads are very close together.

You can see that there are several solder pads on the component side. The 4.7nF capacitors here have one lead soldered to the trace side and the other to the component side. The pad on the component side is very difficult to get to with the surrounding components in place. The best thing to do is to remove all of the components in the area of the 4.7nF capacitors including the display cable.

On the trace side of the board I found some failing solder pads. One damaged pad is on R21 which corresponds to the bottom display segment (d).  The solder pad immediately to the left of it is also starting to come loose so I have two solder pads to repair.

Since I have the display driver components removed I will go ahead and replace the old 4.7nF capacitors with new ones while I am repairing the connection at the failed solder pad.

Here are the new capacitors, the original resistors, the TR1 transistor and the display cable re-installed.

The trace side shows where I bridged the failed solder pad connections.

Now I can re-assemble the Beogram components and check the display again.

The bottom segment is working again.  Now on to the platter detection circuit problem.

Beogram 8000: Initial Voltage Checks

Now that the required electrical work is done I am able to connect the Beogram 8000 components back together and see what the main voltages look like.

Here is the test set up I always use for Beogram 8000 and 8002 turntables. It is nice that every component I would need to get to is easily accessible for measuring.

Plugging in the power cord the Beogram standby light came on as expected.

Next I tested the 33 ⅓ RPM and 45 RPM speeds. The turntable speeds look like they are working but the bottom segments of the display is not working. Usually that indicates a problem with one of the 4.7nF capacitors on the microcomputer PCB. I will have to dig into that after these initial checks.

I observed one other functional problem I will have to investigate and that is with the record play. The tonearm moves to the start of a record but the fixed arm sensor is not detecting an empty platter.

Other than those two things the electronic functions look to be in good order.

On to the voltage checks for the Beogram power supply.  Here is the schematic for the power supply.

I checked the C24 capacitor on the main board for the voltage supplied to the Beogram +5 VDC regulator and the +5 VDC voltage at the microcomputer IC. Both measured good.

The C27 main board capacitor and +15 VDC supply voltage also look good.

So do the C29 main board capacitor and -15 VDC supply.

Overall good news on the electrical parts of this Beogram. Next I will look for the display segment problem and check the signals coming from the various sensors. That should point me to the culprit on the platter non-detection.