Beogram 4000: Installation of New Beolover Sensor Arm Photocell

The Beogram 4000 from Australia that I am restoring right now kept on giving! After I fixed the tonearm wiring to get both stereo channels working further tests revealed that it would not recognize the absence of a record and eagerly put down the arm onto an empty platter. Luckily I had already adjusted the arm lowering limit and so nothing happened to my MMC20 EN cartridge.

Ignoring the absence of a record is a telltale sign for a dead photocell in the sensor arm. The circuitry recognizes a dead sensor arm bulb and shuts down arm lowering if this is detected. However, a damaged photocell is not recognized. It simply simulates a black surface on the platter, and so the arm lowers.

I pulled out the sensor compartment and closer inspection revealed that one of the two electrodes on the photocell had disconnected. It looked like this:

This photo is from an earlier post about this type of issue, where the photocell of a Beogram 4002 had come apart. This earlier post reports about my initial development efforts aiming for a suitable replacement of the original photocell. 

I removed the photocell fragments:

And then it was time to install the newly designed Beolover Sensor Arm Photocell for Beogram 4000, 4002, and 4004. It is available via the Beolover Store. This shows the business side of the part:

At the front end (left) there are three photodiodes in parallel (to ensure sufficient current generation). On the back end (right) a 3D printed alignment piece is installed that helps keeping the cell properly oriented and in its proper spot relative to the lens in the sensor arm compartment.

This shows the backside with the two contact leads:

This is a real photocell like the original. It puts out ~0.45V photovoltage when exposed to sufficient light:

This means that unlike with other photocell replacement schemes no circuit modification is necessary. This is a direct drop-in part that replaces the original photocell (B&O part 8760002)

Installation is simple: Just stick the leads into the holes in the small circuit board attached to the sensor arm compartment (it is a good idea to use a solder sucker to remove the solder from the solder pads on the backside of this board to ensure the holes are open after unsoldering the original cell):

Then insert the photocell into the small compartment where the original cell was located and align the small protrusion on the alignment piece with the hollow rivet that holds the PCB to the plastic part: 

Then press the back end down until the frontend with the photocells hits the roof of the compartment:

Then the leads can be soldered to the pads on the underside of the PCB:

This shows the underside with the red and blue photo cell leads attached:

I tested the new cell with my oscilloscope, which was connected to the collector of the sensor transistor 1TR14 whose collector DC bias was adjusted to be 1.8V:

 

The photocell yielded a strong 2.5V amplitude signal over the rotating platter. More than spec. So all is good again in the record detection department.