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Tuesday, December 26, 2017

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.

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