Beogram 8000 (Type 5613): Replacing the Tracking Sensor Light Bulb with a LED Assembly

Recently I was able to buy a Beogram 8000 in fairly decent cosmetic condition. I thought I would restore it and then offer it for sale. After completing the basic functional restoration tasks (stay tuned for my report on the work I did) I had to realize that the tracking sensor light bulb had a broken off wire and so the unit did not track anymore.

So be it I thought. I do not really like incandescent bulbs anyway. They change their emission characteristics continuously over their short lifetimes, i.e. they are always good for some trouble. So replacing them with LEDs is usually a good idea. 

Since this was the first instance of a broken tracking bulb in a Beogram 8000 for me, I did not have a ready to go replacement. So it was time for a bit of creative development work!...;-):

This shows the compartment under the arms where the tracking sensor resides:

After a communication with co-Beolover Sonavor I understood how to get in there. He had done it before. Standing on the shoulders of giants!...;-). One can simply lift up the cover plate a bit until it clears the pin that holds it in place and then slide it out. This shows the compartment opened up:

Here more magnified: 

Photoresistor and bulb are mounted on a small PCB in the back, which connect s them through four wires to the rest of the circuit. These wires come out at the bottom of the compartment and are pretty tight. Luckily one can remove a couple rubber washers that hold the wires in place and then they get a bit looser:

This allows pulling out the small PCB just enough so one can access the solder side:

Here a more detailed view of the backside (bulb already removed):

The red and black wires power the bulb, with the black being GND and red 15V. The hole is sized to hold the bulb base whose wires are then soldered to the two solder pads around the hole.

First I thought I would build a prototype LED board using protoboard to fit into the orifice to replace the bulb. Out came the Dremel and I made a little board that fit:

The plan was to mount a LED and an appropriate resistor on this board and then make the connections to the small PCB. When I debated which LED from my stash to use, it dawned on me that the LED circuit on my standard Beolover Tracking Sensor LED Light Source for Beogram 4002 and 4004 would fit the bill perfectly if the adjustment trimmer connected in series would be removed. In Beogram 4002/4004 the bulb runs at 21V, while here in the 8000 it is powered from the 15V supply. Without the trimmer I thought I would probably get a very similar light output from the LED! So I Dremeled one of the 4002/4004 boards to have the right form factor while still sporting both LED and current limiting resistor. I used magnet wire to make electrical contact. Here you can see the end result in comparison with the bulb:

The narrower end of the board on the right is just about as wide as the bulb base.

This shows the board stuck through the orifice and soldered to the pads:

A view from the other side:

And everything pushed down into the compartment and powered up:

The LED is located in front of the lens at a similar position like the original bulb filament.

The final step was putting the wiring back into place so it would not chafe with the floating chassis base during carriage movements. I pushed the wires back where they were and installed the two rubber discs that hold them in place:

Then I put my test cartridge on the arm and the record that I use for tracking feedback adjustment on the platter. I dialed in the tracking aperture to get about 2-3 rotations of the platter before the carriage starts moving after putting the arm down. Everything worked perfectly! A good afternoon at the Beolover bench!...;-).

This shows the tracking sensor in action:

Records played perfectly with it! 

After this successful prototype development I sat down and designed a more professional PCB that should make the bulb replacement a snap. It is currently in production and I hope there will soon be a new Beolover part available via my store.

Stay tuned for the description of the other restoration task that I performed on this lovely Beogram 8000!