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!

Beogram 4004 Type 5526: Record Detector Testing and Adjustments

This Beogram turntable has a new LED lamp source for the record detector circuit that is in the fixed arm. Part of that installation involved switching out a fixed 1MOhm resistor (1R26) with a 2MOhm trimmer resistor to allow better tuning of that detector circuit voltage. With the turntable parts re-installed I started the procedure of making an initial setting of the new 1R26 trimmer so the collector of 1TR3 is at 4 VDC when the fixed arm lamp is on.

I made that adjustment then moved the 1R26 trimmer resistor to the component side of the board where it belongs (so it is out of the way of the platter).

A quick test of the Beogram Start, without a record on the platter, passed with the sensor circuit correctly recognizing there was no record on the platter.

To complete this test however it is always a good idea to actually see the signal on an oscilloscope. I connected a probe wire to the 1TR3 collector and then to my scope. When I ran the fixed arm and tonearm over the platter I paused it so I could measure what the detector circuit is seeing.

I was surprised and disappointed to see that the peak to peak voltage of the signal was not from 0 V to around 6 V as expected. It only drops to about 2 V and goes up to 4.72 V.

So while the Beogram appears to detect an empty platter as it should, the signal is not ideal and could lead to problems later on. A little bit less peak to peak voltage and this circuit will not detect an empty platter.

I wanted to know if the problem was in the new Beolover light source assembly or with the fixed arm light sensor. The easy thing to try first was to use a powerful LED flashlight to enhance the light on the platter. When I did that I could make the 1TR3 collector signal go to its full range. But look how much light it took for the sensor to register that. Something must be wrong with the sensor.

As a second part of the test I pulled out the fixed arm lamp and sensor assembly a ways so I could move the Beolover LED module and watch the effect on the 1TR3 collector voltage.

Moving the LED light source could not make the signal peak to peak range any better. It could make it worse though and confirmed my fear that a little less voltage would cause the platter detection to fail.

Next step then is to swap out the fixed arm sensor. I decided to swap out the entire assembly first with one of my spares (from another Beogram 4004 turntable).

Here is the removal of the current sensor assembly that has the Beolover LED light source.

Here is the replacement sensor assembly. It still has an original incandescent light source.

Just like that, the 1TR3 collector signal went right to what is expected. The peak to peak voltage is from 0 V to about 6 V.

That voltage is with a dimmer light source as the Beolover LED light source is brighter than the incandescent lamp. The original fixed arm platter sensor is the problem and will have to be replaced.

Beogram 4000: Exchanging the Electrolytic Capacitors, the RPM Relay, and Adjusting the Motor Signal

I marched on with the restoration of the Beogram 4000 that is on my bench right now. After replacing the reservoir and motor capacitors, I went ahead and replaced all electrolytic caps on the main and power supply board with Japanese made 105C grade long lasting units. I also exchanged the RPM relay, since I recently extracted several dead or inconsistent relays of this type from Beogram 4000 and 4002 units. This made me think that these relays should be part of any 'full' restoration to ensure long-term stability. My goal is to get these elegant vintage beauties to a new-like performance and reliability. These pictures show the main PCB in its original state. This is the solder side. There is one Tantalum capacitor in the top left corner that was added post-PCB-design. Even the Danish masters go through a learning curve, I guess...;-):

Here is the component side:

And a detail shot of the RPM relay:

This is a picture of the relay component that I recently developed. It uses a modern encapsulated SMD relay that is broken out with a small PCB that fits precisely to the pinout of the original relay. Hence it can be plugged straight into the original solder pads and no external wiring is necessary. Plug'n Play!

This assembly is available to other enthusiasts. Just send me an email if you are interested.

This shows the PCB after replacing the capacitors and the relay:

This shows the replaced sole cap on the solder side:

On to the power supply board. It has only two Tantalum caps on it (the red dots):

And here with new ones:

After replacing the boards and testing that everything was working properly, I adjusted the motor signal. The AC motor is run by an on-board Wien bridge oscillator whose feedback needs to be adjusted for the right oscillation frequency for 33 and 45 RPM. There is also a potentiometer that controls the amplitude that goes into the power amplifier that finally drives the motor. I recently made  some more detailed posts (link 1, link 2) about the adjustment procedure. Here is a picture that shows how to apply the probes for the measurement:

One needs to be careful to select the right end of the phase capacitor to get the correct amplitude as prescribed in the manual for 33 RPM (6V). Here are the oscilloscope shots after I adjusted the trimmers to spec. This is the 33 RPM trace

and here is the one for 45 (it will usually be a bit lower than 6V and there is no way to independently adjust it - this is analog technology!..;-):

Beautifully clean traces in this unit! Absolutely lovely! This is Beolove!

Beogram 4002: Replacing the Detector Arm Light Bulb, Discussion of the Circuit and Relevant Oscilloscope Traces

The Beogram 4002 that just arrived needed a new light bulb for the detector arm. I replaced it with a new one. While it is a fairly simple operation to exchange the bulb, it can be a bit tricky to get the deck working again if the new bulb is not in the correct location. I made a short video about the progress that shows how to implant the new bulb, and also discusses the adjustment of its position. The video also discusses the relevant parts of the circuit and shows key oscilloscope measurements on a working detector set-up:

Here are high res pics of the oscilloscope traces. This shows the  weak signal at the base of TR3 while the platter spins under the detector:

This shows the signal after amplification through TR3 at its collector:

The signal is now about 6Vp-p. I have seen values between 5 and 7V in different 4002/4 units. This shows a representative trace at this point for a light bulb that is not properly adjusted, i.e. does not put enough light under the sensor lens:

Note that the Vp-p is now less than 1.5V due to the reduced light intensity. This shows the resulting signal at the base of TR4 after it passes capacitor C16:

the signal is now 1Vp-p but at a 21V offset to pull TR4 down and turn it on. When it is turned on capacitor 18 charges up with a sawtooth in sync with the platter ribs as they pass by under the detector:

Finally, the charged capacitor pulls the base of TR6 up via R34 (1M) and with this the tone arm lowering circuit is deprived of voltage and the arm cannot be lowered anymore. What a lovely analog control system!