Michigan Beogram 8002 Restoration: Sensor Measurements

The Beogram 8002 from Michigan is performing very good with its turntable functional tests.

I connected some test leads to the Beogram's position sensors, speed sensor, record detection sensor and the forward servo drive signal (IC2 Pin 7).  Reference Beolover's post on the servo motor check.

Here are my test points on the Beogram 8002 PCB 1 board.

First, the position sensors (1 & 2) along with the IC2 Pin 7 output to the servo motor.

This picture shows the signals when the Beogram 8002 is doing a forward scan over the platter.

The image on the left is of the Fast Forward Scan and the image on the right is the Slow Forward Scan.

I placed a test record on the platter and measured the sensors again while playing a record.

The servo IC2 Pin 7 signal varies as the servo motor advances the tonearm assembly to track the record groove.  It does this once every revolution of the platter.

The tonearm controls worked perfectly and the Beogram 8002 record play was great.

Next I swapped out the test lead on the Position 2 and IC2 Pin 7 test points with the Speed Sensor and Record Detection test points.

The Record Detection on the Beogram 8002 is interesting compared to the Beogram 8000.
The Beogram 8000 platter is very similar to the Beogram 4002/4004 platters so the platter detection in the fixed arm just measures a series of pulses by detecting the platter ribs.

With the Beogram 8002 B&O designed a special laser engraved platter with two different sections of black markings as well as a flat gray section.

Here are the Beogram 8000 and Beogram 8002 platters side by side.

The Beogram 8002 platter design results in a different record detection signal than the Beogram 8000.

Here are photos of the Record Detection sensor measurements and the Platter Speed sensor measurements.  I kept the Position 1 sensor as a reference.

This first set of images is with the platter speed at 33.33 RPM.
I actually set the speed to 33.33 RPM to show the Record Detection sensor at the black marking level and at the gray section level.  The photo on the left shows the Record Detection sensor over the black markings.

Here is he platter speed set to 45 RPM but I only captured the signals over the gray section of the platter.

I am satisfied that this Beogram 8002 is functioning properly now.

I will play a couple of records with the components still removed from the cabinet just to test the sound and make sure the audio mute relay is working.

Then I will do the cabinet repairs and reassemble the Beogram 8002 to complete the project.

Beogram 8000: Wrapping up the Workshop Beogram with speed sensor replacement testing

It is time to wrap up testing with the Workshop Beogram 8000 for now.  
The test platform has helped me restore and repair five sets of Beogram 8000 circuit boards along with using the quick connect/disconnect socket on five uC (2IC1) devices.

The Workshop Beogram also gave me a way to test the speed sensor (OPE2).
I discovered that I had two faulty speed sensor devices in my collection of Beogram 8000 parts so this last Workshop Beogram 8000 blog post will show the result of my search for a speed sensor replacement device.

The original two types of speed sensor (OPE2) devices in the Beogram 8000 are these two Bang & Olufsen part numbers - 8330007 (the early type sensor with resistors R46 & R47 on PCB 1) and 8005067 (the later type sensor with resistors R2 & R3 on the speed sensor assembly).
I don't know the story with those part numbers but the 8005067 assembly is what is called out in the later model Beogram 800x turntables (BG8000 & BG8002).  Maybe B&O had reserved that OPE2 part number but didn't release it until later.

Both sensor devices were similar slot type optical switches.  They consist of an emitter that is an IR LED light source and an NPN photo transistor sensor. They are a single assembly with four leads.

Here are the two types of Beogram 8000 speed sensor assemblies.

Here are the wiring diagrams for both speed sensor assembly types.

I was unable to find data sheets for either type of speed sensor that B&O used for these two types of slot type optical sensors.

However, I did find a few good candidates that are the same size and have the same pin configuration (same layout of the leads).

To check these sensors out I have one more bench test mode for my Workshop Beogram 8000.  That is an empty floating chassis with a platter hub/tachometer and the mount for a speed sensor.
Electrically I run the wires from the speed sensor I am testing to a breadboard setup to measure the slot optical sensor output (with an oscilloscope).  The breadboard contains necessary current limiting resistors (R46/R47 or R2/R3).  I use a bench power supply to provide the +15 VDC power for the circuit.

The first tests are for a B&O 8330007 speed sensor device.

Because this is an 8330007 B&O speed sensor I am using current limiting resistors per the schematic. It also matches what was installed on the PCB 1 board for this type of sensor assembly.

Applying the +15 VDC power to this circuit and manually rotating the platter hub here is the oscilloscope measurement.

This 8330007 device is a working speed sensor that I have also functionally tested with the Workshop Beogram 8000 turntable.
The amount of current this sensor circuit draws with the +15 VDC power applied is 30mA.

The second test setup is with a B&O 8005067 speed sensor assembly. It has built in current limiting resistors (R2 and R3).

Since the 8005067 speed sensor assembly has current limiting resistors built into the assembly it isn't necessary to connect up the test components on the bread board like the 8330007 speed sensor.  I can just connect my scope probe and DC power supply directly to the sensor assembly.

With +15 VDC applied to the 8005067 sensor assembly I manually turned the platter hub again and measured the speed sensor signal at the photo transistor collector.  This sensor drew around 20mA while it was operating.

Just like the first sensor, the 8330007, this 8005067 sensor is also a fully working unit that I tested with the Workshop Beogram 8000 turntable.  

In the third test I tried out one of my replacement candidate slot type optical switches.
The device I chose for this test is the TT Electronics/Optek Technology OPB370N55.
Here is the test setup with that device.  It will require the use of the breadboard to provide the current limiting resistors.

To get a usable signal from this new sensor device I had to adjust the trimmers to around 2.4KΩ for R46 and around 6.8KΩ for R47.
Using those values I get a good, healthy speed sensor signal that uses about 10mA with the +15 VDC power applied.  That is half the amount of current used by the 8005067 devices and a third of the amount of current used by the 8330007 devices.

Now that I can see what values I need for the two current limiting resistors I can try out this replacement sensor device in a failed 8005067 speed sensor assembly and test it with the Workbench Beogram 8000 turntable.

Here is my test Beogram 8000 speed sensor with the new slot type optical switch installed.
I am using the 8005067 assembly for the test and I am using 2.2KΩ for R2 and 6.8KΩ for R3.

Before testing the new sensor device under full control of the Beogram 8000 I used my P4 test harness to do a manual check where I rotate the platter hub and measure the sensor signal coming off the sensor and at the microcomputer, 2IC1 pin 40.

That looks perfect.  The raw signal off the OPB370N55 collector is a nice strong signal that feeds the 1IC1 Op-amp before being routed up to the uC pin 40 input (via the signal conditioning flip-flop).

My final check is to connect up the new speed sensor to the Beogram 8000 P4 harness normally and see how it works under full control of the Beogram 8000.

For this test I utilized the Beogram 8000 "Turn" function button.
The signal at the uC pin 40 still looks great and the Beogram 8000 control circuitry has no problem locking in the platter speed.

It looks to me like the TT Electronics/Optek Technology OPB370N55 Slotted Optical Switch is a good replacement for the Beogram 8000 speed sensor device.

If you try it keep in mind that you must change the resistors (either 1R46 & 1R47 or R2 & R3) according to the version your Beogram uses.

There are two other Slotted Optical Switch devices by TT Electronics/Optek Technology that I will try.

OPB871N55 and OPB825.  They will require testing to determine their current limiting resistor values but I believe they could be used as a replacement device as well as the OPB370N55.

This concludes this round of Workshop Beogram 8000 projects.

I will be retooling my workbenches to begin some restoration work on some Beomaster 4400 units next.

Beogram 8000: Testing speed sensors with the Workbench Beogram 8000

The Workshop Beogram 8000 unit has already aided me in restoring a number of Beogram 8000 PCB 1 and PCB 2 boards.  One set was extra troublesome as it had some broken traces that were not visible.

This photo shows the boards finally working as I finished off the testing by playing a record.

One thing I discovered in my board testing was that a problem with any part of the speed sensor circuit could potentially cause damage to other components of the platter drive system.  I am referring to the speed sensor itself and the path the signal takes to the Beogram microcomputer (2IC1) pin 40.

Having a problem with the speed sensor is similar to attempting to operate the Beogram 8000 with the rotor removed (sometimes referred to as the sub-platter).  In both cases when the Play or Turn buttons are pressed the Beogram uC will start sending commands to turn the platter.  The uC expects a speed signal as feedback to regulate the platter drive.  When the speed sensor signal is missing (i.e. broken or the rotor is not in place to turn the tachodisc) the uC will keep trying to drive the platter.  Typically what happens is the F1 fuse (300mA, slow for USA model Beogram 8000 turntables) in the transformer compartment will blow and shut things down.  However, damage can also occur to the platter drive transistors 0TR2 and 0TR3.  It would probably also be possible to damage some other components in the platter drive circuitry.

A symptom of a broken sensor signal is that the platter will immediately ramp up to a very high speed when either the Play or Turn button are pressed.  If power is not removed quick enough there can be some component damage.

This problem existed in the PCB 2 assembly of this troublesome set of Beogram boards.
The speed sensor signal was good all the way into the PCB 2 assembly but failed to reach pin 40 of the uC IC due to a broken trace. 

The route of the speed sensor signal to the uC pin 40 can vary depending on when the Beogram 8000 was manufactured or whether it received a service modification.

This photo shows the two types of PCB 2 assemblies found in Beogram 8000 turntables with regard to the speed sensor signal and the uC pin 40.

In the early model Beogram 8000 units the speed sensor signal went from the sensor at the platter hub through an OpAmp (1IC1) then on to 2IC1 (uC) pin 40 via pin 2 of the ribbon cable that connects PCB 1 and PCB 2.

Later in production B&O engineers found that tolerance differences between 1IC1 and 2IC1 could cause periodic variations in the platter speed.  So to solve that problem a 4013 flip-flop circuit was introduced to stabilize the signal.

This makes a difference when trouble-shooting a suspect speed sensor issue.

The early model PCB 2 assemblies had a short trace from the ribbon cable pin 2 over to the 2IC1 (uC) pin 40.
The later models routed the incoming ribbon cable pin 2 over to the new 4013 flip-flop by a yellow wire then returned the conditioned signal to a solder point next to 2IC1 (uC) pin 40 by an orange wire.
So there are more places to lose the signal in the modified PCB 2 assemblies.

In the problem PCB 2 I had to repair, the short trace between the conditioned signal (orange wire) and 2IC1-40 was open. The break was not visible to me but it measured open circuit and the board's primary problem was no speed sensor signal.  Adding a jumper from the conditioned signal directly to 2IC1-40 solved the problem.

I found that it is easy to check 2IC1-40 with an oscilloscope and see if there is a good speed sensor signal. 

However, if the signal is not there then there will not be time to check for the signal and deal with the platter drive having no feedback.

The solution I came up with to make the easy measurement and protect the Beogram circuit components is a new speed sensor test harness for the platter drive cable.
That is the cable that connects to PCB 1 via the P4 board connector.
Disconnecting the P4 cable allows operating the Beogram 8000 without the platter drive in operation.

It is a handy mode when doing things like adjusting the record tracking sensor and setting the tracking force.  

I decided to make a P4 harness that only connected up the three wires for the speed sensor assembly.

This allows me to operate the Beogram 8000 in the mode where the platter drive is disabled except I have the speed sensor signals available to test.

Here is that harness ready to use with my Workshop Beogram 8000.

I have two oscilloscope probes in place.
One for the raw speed sensor signal right at the sensor.  The other will be connected to 2IC1 (uC) pin 40.

Now I can press the Play button or the Turn button and manually rotate the platter hub to check the speed sensor.

Here is a closer look at what the oscilloscope sees at those two measurement points for a properly operating speed sensor.

I now pre-test the speed sensor functionality of a Beogram 8000 using the speed sensor only P4 harness before trying out the full P4 harness.  That way I will get an early warning that there is a problem before risking the loss of a component.

Beogram From North Carolina: Speed Control and Braking Working Again

The speed control and braking problem described in the previous post about this Beogram 8002 is fixed now.  It turned out to be an easy repair that was made easier to diagnose since I have the luxury of owning quite a few spare, fully functional boards for the Beogram 8002.  It is one of my favorite Beogram turntables and I have several that I use for my own use. 

So instead of diving into a search for a bad sensor, my first trouble-shooting step was to swap the Beogram 8002 control boards.  This would quickly tell me if the problem was a sensor or a board problem.

It was a board problem.  The swapped board worked perfectly. 
That meant the problem was either a bad connection or a failed component on the circuit board.
Before pulling parts it is always a good idea to carefully inspect the boards first.
Sure enough...I found a wire that came loose.  Looking up the wire's function made sense why it was causing the problem I experienced.  The wire was for the Beogram platter speed control.  It doesn't get more obvious than that.

This picture shows the speed sensor wire (the red dots in the photo) fitted properly again.

This excerpt from the Beogram 8002 schematic shows the wire function.

For a reference, here is speed sensor 0PE2

Hooking the Beogram 8002 components back up and checking the Play test again results in the speed control and braking functions doing their job.

The Beogram 8002 speed indicator now displays the full speed values (down to two digits) that it measures during operation of the platter.

Starting with Standby mode followed by 33.33 RPM and 45.00 RPM here are some photos.

Now I think all of the electrical functions of this Beogram 8002 are working correctly.
I will do some measurements tomorrow to verify that thinking but for now I am going to stop and enjoy the feeling of solving the problem :-).

Beogram 8000: Sensor Checks and Adjustments

The problem with the Beogram 8000 detector arm not detecting an empty platter turned out to be a simple adjustment to the sensor source lamp positioning.

The Beogram 8000 schematic shows the detector arm circuit and what the expected sensor signal should look like.

Measuring at C19 I could see the signal was not as strong as it needed to be.

I used a flashlight to apply more light and saw that the sensor output increased up to a working level.

The easiest thing to adjust and check at this point is the sensor source lamp position. I opened the lamp and sensor housing to move the lamp down a little inside the housing.

That did the trick. The detector arm now functions normally with the correct output voltage on the detector circuit.

While I had my Beogram test setup in place and my scope warmed up I checked the tangential arm position sensors. Here is the circuit diagram.

Both sensor outputs look good and the Beogram has been working as it should with regard to the tangential arm movement.

Another sensor output to check is the platter speed. This Beogram still has the original, plastic printed tacho disk. Let's see how it measures. Here is the circuit and measurement point.

Here is the sensor output for 33⅓ RPM and 45 RPM.

Last are the forward/reverse scanning LDR sensors. This is the adjustment I always add my custom connector for so I can check the LDR voltages at any time without having to completely open up the Beogram again.

Here is the circuit.

Here is a photo of the test connector I always add for the this LDR adjustment. This photo is from an earlier Beogram 8000 restoration project. It is the same on this current project. The wire colors used on the test connector were chosen to match the Beogram ribbon cable for those signals.

Here are the forward and reverse LDR measurements after adjusting them to be between 0.6 VDC and 0.7 VDC.

I believe this Beogram is working correctly electronically. Now I need to check the tonearm balance and the distance to platter surface. After that I will check the cartridge tracking and, if necessary, adjust the linear tracking sensor. I should be playing records on this Beogram real soon.