Beogram 8000: An Exploration of the Platter Speed Sensor (Includes Oscilloscope Traces for the Most Important Signals)

I recently received a Beogram 8000 as a parts unit for repairing another Beogram 8000. I wanted to exchange the circuit boards in the hope to alleviate a strange issue that I sadly was not able to figure out. So I rebuilt the boards of the donor 8000 and then plugged them into the other unit.

And then my fun exploration of the platter speed sensor began...;-). Here is what happened: I plugged the Beogram in, and pressed Play. Immediately the platter ramped up madly to about 100 RPM and the "33.33" never appeared (it shows that a stable speed has been reached by the system). The display stayed at "33". 

Head scratching etc...ensued. The first thing I did was plug the board into the other Beogram to see if its platter would also go out of control. It did not. So what gave?

Well, it turned out that some of the Beogram 8000 with old style boards (i.e. with the two piggybacked small boards on the upper end of the main board) did already come with the speed sensor used in the later Beogram 8002. This are the two circuit snippets in comparison:

First the older version:

Note R46 and R47. They are on the main board in this circuit. Now look at the corresponding Beogram 8002 circuit:

Here, the resistors are on a small board attached to the sensor located under the sub platter. We also see that the value of R46/R2 changed and that R47/R3 carries an "X" now, indicative for the somewhat "semi-exploratory beta release approach" applied for these boards (I do not know of any consumer product that has so many different versions and slight undocumented changes than these B&O circuits). I think the X is a sign that they used different IR LEDs and photosensors during the production run of these turntables. 

Anyway, lets have a look at the two sensors as they are mounted in the Beogram:

Again, first the old style setup: Front

 and back:

And now the newer type with the added circuit board:

You can see the two resistors on the front of the board. The upper one is for the IR LED and the lower one for the sensor:

This is how it looks from the back:

Due to this difference the main board needed to be modified for the new sensor version. This shows the relevant square inch of the board configured for the old style sensor:

Note that R46 (big resistor that is mounted horizontally) is present and also R47 (red red orange 22k on the left, vertical). Now let's have a look at the board configured for the new sensor style:

R47 is simply missing, and R46 is replaced with a jumper wire!

And there lies the danger when replacing boards without checking what sensor type is installed:

When an old style sensor setup is plugged into a board configured for the new sensor like it happened to me, the LED is directly connected to the +15V rail via the jumper wire, which will probably cause it to be super bright for about 1 microsecond and then dark forever. Well, some people say it is better to shine brightly for a brief moment than to be a dull boring light for a long time, but in this case I get to figure out what modern LED can replace the original LEDs...;-).

I suspect the OP240 may be a good starting point since it can be used to replace IR diodes in other spots in this vintage of B&O. We will see...probably another post coming up sometime in the near future...;-).

Anyway, I ended up transplanting the new style sensor along with the newer boards and that yielded a working setup (again). For the sake of documentation I measured the relevant signals in the speed sensor setup:

This is the signal right at the sensor (P4-pin7):

This signal is cleaned up to a square by the opamp IC1 which is configured as a 5V comparator:

The above trace was measured at pin 14 of the opamp. This signal is subsequently divided down (R51/R50) that the micro controller can digest it, yielding this signal at P6-pin 2:

And that is it about the speed sensor of the Beogram 8000!