Beogram 4002 (Type 5513): Replacement of Carriage Position Sensor Photocell

Having worked on quite a few Beogram 400x in the last 10+ years I started thinking 'I saw it all'. But if there is one thing one can rely on with these beautiful designs: They will always come up with new challenges. This time it was a failing photocell in the carriage position sensor. Usually, this sensor fails due to a dead light bulb (or IR LED in later models). Or due to a broken photocell housing, causing the photocell to bend away from the 'plexiglass ruler', which can cause reliability issues.

So when I restored this unit, and it behaved inconsistently when trying to find the LP setdown point after pressing start, I naturally assumed it was the IR LED, which this later 5513 Type featured.

I replaced the LED with an orange LED and adjusted the brightness to get the prescribed 5V at the photo sensor cathode. Then I ran the unit again, and it found the LP setdown point! Case closed, I thought and I played a few more records.

Then out of a sudden it would not find the setdown point again and the cell voltage was off by a couple volts. I thought maybe the brightness adjustment trimmer had an issue due to oxidation, and so I replaced it with a modern 25-turn 25 kOhm encapsulated unit. Then I adjusted to get 5V and tried again. This time it immediately failed and so I started thinking, 'what is left to replace'. The photocell came to mind, but I never had to replace it so far, and I was able to adjust the 5V, so it should be o.k., I thought...that led me to replace TR17, which translates the photocell response to standardized ~20V pulses that can be interpreted by the control system of the deck. I put in a new 547B and tried again. To no avail.

At this point I finally believed it must be the photocell. I disconnected the carriage PCB and connected the cell to the multimeter set to resistance. Then I shone a strong LED flashlight on the sensor, and indeed the sensor responded intermittently. The resistance dropped, and then it went open contact, and then the process repeated itself. Whatever the reason, this was not o.k.

Since this diode-based photocell simply acts as a photoresistor in this circuit, I decided to try replacing it with a standard photoresistor. But first I had to re-design my photocell housing to be able holding such a resistor securely in place. This shows the redesigned set-up:

I basically gave the insert some additional features that matched the resistor shape. This shows the resistor inserted:

And while pushing the insert into the main housing:

This shows the new sensor in place opposite the orange LED that I installed earlier:

A shot from the back:

I had to try out a few different photoresistors before I was able to make this work properly. It turns out that the widely available GL5539 type works reliably under various ambient light scenarios. Note, that it may be necessary to adjust the LED intensity with R88 in order to get a good contrast on the photoresistor that TR17 is reliably switched off and on depending on whether a black stripe is in front of the sensor or not. This is best done with a voltmeter connected to the collector of TR17.