Beogram 4002 (5513): End Groove Detection Not Working

When I thought I was close to be done with this Beogram 4002 (5513), I set it up to play a first record on it. All went well until the side was through and the needle approached the end groove. The needle went all the way to the record label and then remained there playing an eternal "tk....tk.....tk....". Like in the old days with fully manual turntables...not very beolovely, though!

I removed the platter and had a look. I connected my oscilloscope to the collector of TR17, which translates the optosensor (4IC1) response from the position sensor into a 0-21V 'digital' signal that can be used to tell the control system where the carriage is located. The signal was all good for the major black bars on the position indicator, but was very weak for the run out groove area. This is usually a sign of a poor alignment of the IR diode relative to the light entry slit on 4IC1. The reason that the end groove marks are more critical in this regard is their small width. This creates a situation where a slight misalignment of the diode sends light into 4IC1 from the side, thereby 'muddying' the bright-dark sequence, which lessens the response of TR17. Another reason for end groove detection malfunction is an incorrect distance between position indicator bar and 4IC1. It needs to be pretty close to ensure that the narrow end groove bars really cover the light entry slit.

Anyway, I tried to reposition the IR diode a bit. This shows PCB 4 removed from the enclosure:

The IR diode is right in the center facing the black front of the sensor (this picture is actually from a different Beogram 4002 than the one that was fixed here...I forgot to take a picture before I repaired things).

When I tried to align the diode a bit better, it promptly broke in two:

I guess the old plastic got a bit brittle over the years...what now??

I tried replacing it with modern IR diodes that I had in stock, but to no avail. The circuit that drives the IR diode did not have a low enough impedance to drive the modern high output IR diodes that I had available. Even with 1R88 fully turned the voltage did not reach the prescribed 1.7V across the diode. I elected to not alter the circuit with a smaller resistor, but rather chose to use a classic red 5 mm LED, which had an appropriate low output and power consumption. This worked very well. Here you can see it implanted:

And in action:

Looks pretty, doesn't it? After this repair the end groove was detected nicely. It is interesting to note that the end groove issue probably was there for some time and omitted on the ebay description for this Beogram: It appears the previous owner tried to fix it by adding a thicker line to the end groove markings with a Sharpie pen:

The end groove markers are the narrow lines to the right. The Sharpie line is the wiggly hand drawn thicker line to the left of the narrow lines. The other lines on the left are the singles set-down marker (narrow) and the 45 RPM change marker (wide band).

It is interesting how they used different widths and the end groove lines-array to be able to elicit different and appropriate responses from the control system depending on the carriage position. Ah, the joys of analog control systems. Microcontrollers are so much more effective, but also much more boring...;-). This is Beolove!