Beogram 4002 Type 5513: Adjusting/Calibrating the Detector Arm Circuit

The Beogram 4002 turntable is almost finished. Most of the adjustments and calibrations are done.
I did say I would need to return and perform the adjustment of the record detection circuit as it measures a little low. I use Beolover's nice Blog Post about the Beogram 4002 record detection circuit.

While I am not changing the detector circuit lamp (in the fixed arm) at this time, I can adjust the circuit so it meets the specification in the service manual.

There are two components on the main board that are used to adjust the signal produced by the record detection circuit: 1R26 (a 1MΩ resistor) and 1TR3 (an NPN BC548C transistor).

This Beogram record detection signal measured a little low at  the 1TR3 collector.  Beolover's video prescribes a 1TR3 transistor with an hFE (gain) of at least 500.  Let's see what this one measures.

Very good. I will keep this transistor as the 1TR3 component.

I haven't changed the detection arm lamp yet. I want to see if the current lamp will adjust into range.

Another component in determining the detection circuit signal is 1R26.  It is a fixed resistor of 1MΩ.  I will change that resistor to a multi-turn, 2MΩ trimmer and adjust it to dial in the voltages for the detection circuit. Again, following Beolover's guide to adjusting this signal, I first place the new trimmer resistor on the trace side of the Beocord.

I put an oscilloscope probe on the collector of 1TR3 and rechecked the record detection signal when no record is present on the Beogram platter (as the tangential arm moves across it).

The measured signal was just as before...It works but measures lower than what the service manual calls for.

I adjusted the trimmer for 1R26 and remeasured the 1TR3 collector.  It took several adjustments and measurements but I finally got it set to what the service manual calls for.

The adjusted value for 1R26 was 1.667MΩ

Happy with that result I moved the 1R26 (new trimmer device) to the component side where it will live permanently.

Another adjustment completed. I may still revisit this adjustment if we decide to change out the detector arm lamp.