Beogram 4004 (5525): Replacement of Sensor Arm Bulb with LED Assembly and Calibration of Sensor Amplification Circuit

A Beogram 4004 that I restored a few years back returned to my bench for some TLC/adjustments. Along with the adjustments it was decided to also update the restoration to the latest Beolover standard, i.e. the sensor arm bulb needed replacement with an LED assembly, which was not yet available during this Beograms's first visit.

This shows the sensor arm bulb cabinet pulled out together with the flexPCB based LED replacement circuit:

After extracting the bulb I installed the LED assembly and fired it up:

The LED that is used on the board is a warm white LED, i.e. it has enough red emission to make the B&O logo light up nicely.

The next step of this procedure is to adjust the biasing of TR3 properly. TR3 is responsible for amplifying the weak signal of the photo diode that is in the sensor arm. By design the base of this transistor is biased with a 1MOhm resistor (R26) pulled up by the collector. Unfortunately, this biasing scheme depends on the transistor gain Hf, which can vary quite drastically across several transistors of the same production run.

As a consequence one rarely finds a Beogram 4002 or 4004 that has the proper manual-specified 4V at the collector. This issue can be corrected by adjusting R26 until 4V are present. This can be done by replacing R26 with a 2MOhm multi-turn trimmer, and adjusting for 4V. This shows the trimmer installed from the solder side of the board that it can be adjusted while the Beogram is running:

After adjusting the collector voltage to be 4V I unsoldered the trimmer, and installed it from the component side:

After this it was time to measure the sensor signal at the collector of TR3:

The amplitude of the signal is 6.3V, exceeding the manual specified 6V, which is good. It should not be smaller, but a bit larger is o.k. Important here is also that the signal drops (that correspond to the passing of a platter rib) go all the way down to 0V. 

This result indicated that the sensor circuit was working properly, i.e it was time to test with and without a record present. All went well. I will play it a bit more and then it will be time to send it back to its owner!