Beogram 4002: Replacing the Detector Arm Light Bulb, Discussion of the Circuit and Relevant Oscilloscope Traces

The Beogram 4002 that just arrived needed a new light bulb for the detector arm. I replaced it with a new one. While it is a fairly simple operation to exchange the bulb, it can be a bit tricky to get the deck working again if the new bulb is not in the correct location. I made a short video about the progress that shows how to implant the new bulb, and also discusses the adjustment of its position. The video also discusses the relevant parts of the circuit and shows key oscilloscope measurements on a working detector set-up:

Here are high res pics of the oscilloscope traces. This shows the  weak signal at the base of TR3 while the platter spins under the detector:

This shows the signal after amplification through TR3 at its collector:

The signal is now about 6Vp-p. I have seen values between 5 and 7V in different 4002/4 units. This shows a representative trace at this point for a light bulb that is not properly adjusted, i.e. does not put enough light under the sensor lens:

Note that the Vp-p is now less than 1.5V due to the reduced light intensity. This shows the resulting signal at the base of TR4 after it passes capacitor C16:

the signal is now 1Vp-p but at a 21V offset to pull TR4 down and turn it on. When it is turned on capacitor 18 charges up with a sawtooth in sync with the platter ribs as they pass by under the detector:

Finally, the charged capacitor pulls the base of TR6 up via R34 (1M) and with this the tone arm lowering circuit is deprived of voltage and the arm cannot be lowered anymore. What a lovely analog control system!

Beogram 4002: Replacing the Detector Arm Light Bulb with a LED

The Beogram 4002 that I am currently rebuilding also needed a new light bulb in the detector arm. The previous owner had done a poorly executed 'repair' attempt, and ripped out two wires from the detector arm lamp assembly, while also bending the photo cell backwards - unbelievable what some 'experts' are coming up with. Here is a picture of the assembly as I saw it the first time:

For comparison I pulled the compartment out of the detector arm of my Beogram 4004, which is in a pretty good condition:

These light bulbs are hard to come by and I also believe in LEDs as a way to get these units stable for another 30 years. So I decided to replace the light bulb with a small 3 mm LED.

This effort turned into an interesting experience, since on first try, I was not able to get the record detection mechanism working after I put in the LED with a 220 Ohm resistor. The turn table simply ignored any record that I would put on its platter.

Study of the circuit revealed that the original designers of this gorgeous piece of analog control electronics proofed the circuit against failing light bulbs (a failed light bulb would result in always detecting a record whether there is one or not, with potentially fatal consequences for the pickup). The much lower current in the LED resulted in a partial activation of this fail safe mechanism, which prevented the Beogram from auto-lowering its arm. This was fixed by replacing R36 with a 560 Ohm resistor, which resorted the original balance in the circuit.

I made a video about the entire process. The video discusses the detector arm circuit and explains how the LED exchange was performed:

Here is a photo of the final result: