The new LED replacement light source for the tangential arm position sensor was a little weak for me to leave it installed. It appears to work fine but with the 1R88, 25K ohms trimmer set for the highest LED output, the maximum voltage I could get on the 1TR17 base was 0.695 VDC. I would prefer for the 4D1 light source to have enough juice to max out the sensor then use 1R88 to dim the LED down where the base voltage of 1TR17 is 0.7 VDC.
That meant ordering a brighter LED. Here is the circuit as shown in the service manual. The 1R88 trimmer controls the current to the lamp source, 4D1. That increases or reduces the LED light output to the sensor, 4IC1. The sensor controls the base of 1TR17.
With a new amber LED installed (Cree C503B-AAN-CY0B0251) for 4D1, there is plenty of light on the sensor now.
In operation the black bars on the tangential arm transport scale affect the light source on the sensor causing a change in voltage at 1TR17. The output of 1TR17 drives analog control logic for the operation of the Beogram. It is quite interesting how the control system works on these analog 400x turntables compared to the 800x series which use a microcomputer for their control.
Here is a picture of the tangential arm position scale.
Here everything is put together.
Next up is the voltage adjustment on 1TR3 for the fixed arm sensor - the detector arm circuit. Two reference posts by Beolover show the relevant circuit and the installation of a 2M ohm trimmer in place of the 1R26, 1M ohm resistor (between the base and collector of 1TR3).
Here is the 1R26 resistor on the main board before switching it out with the 2M ohm trimmer.
Following Beolover's same procedure I temporarily installed the 2M ohm trimmer on the trace side of the board to make it an easier task of adjusting the trimmer setting while operating the Beogram.
Measuring the collector of 1TR3 to ground I adjusted the trimmer until I got to 4 VDC.
After the adjustment I de-soldered the trimmer and re-installed it on the component side of the board so it will be out of the path of the platter.
As a final confidence check of the detector arm circuit I connected a probe wire to the 1TR3 collector so I can view the resulting signal when the detector arm is over the platter.
I get a healthy 5.66 Vp-p minimum voltage when the arm is stationary over the platter and 6.16 Vp-p when the arm is moving across the empty platter.
These two sensor circuits are ready to go. The next tasks will be to run through some of the arm and platter mechanical service manual checks. After those I will adjust the tangential arm tracking of a vinyl record.