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Monday, January 7, 2019

Beogram 4004 (5526): Replacement of Sensor Arm Light Bulb with an LED Assembly

When I restore a Beogram I usually exchange the incandescent light bulbs with LED assemblies. LEDs last much longer and yield a constant light output over their lifetime. This post discusses the replacement of the sensor arm light bulb of the Beogram 4004 (5526) that I am currently restoring. The bulb and sensor compartment can easily be pulled out of the aluminum profile (if your fingernails are not cut too short..;-):
The flex-PCB based LED assembly is shown on the right. It uses a warm white LED which produces enough red photons to make the B&O logo at the end of the sensor arm light up red. I unsoldered the bulb and extracted it. The LED assembly folds into the vacated bulb compartment:
This shows the assembly in action:
Whenever the light source is replaced in the sensor compartment, the record detection circuit needs to be checked for proper functioning. A stylus crashing onto the platter without a record present can get expensive...
The first step is ensuring that the collector of the sensor amplification transistor TR3 has the prescribed 4V DC without a sensor signal present. Usually, it shows a lower voltage due to the design of the circuit that relies on biasing TR3 via a single 1MOhm resistor connected to the collector. Due to variations in the transistor gain Hfe between individual transistors of the same type, this biasing scheme is not reliable since the base bias will vary with each transistor. This can be remedied by replacing the biasing resistor with a 2 MOhm multi-turn trimmer and then adjusting it for 4V at the collector. This is shown here:
I usually install the trimmer on the solder side of the PCB so one can adjust it while the board is powered up, and then I unsolder it and install it on the component side.

Once 4V are established, it is time to measure the sensor signal at the TR3 collector when the arm is over the rotating platter. For that a jumper wire needs to be soldered to the collector and the platter installed. This shows the oscilloscope trace measured after the LED was installed:
The amplitude should be between 5 and 6V, and the valleys should go all the way to 0V. This Beogram passed with flying colors. If the trace does not hit 0V, then usually the LED (or light bulb) does not sit in the right spot. This can sometimes be a bit tedious, but after a few trials one usually can achieve a trace like the one shown here.





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