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By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...

Wednesday, September 15, 2021

Bay Area Beogram 8002: Oscilloscope Measurements

This post is a wrap up of the functional testing on this Beogram 8002 turntable.
For my own reference between various Beogram 8002 turntables I like to collect oscilloscope photos of key Beogram signals.

Here is a photo of my test leads connected to the main board of the Beogram 8002.

I have test leads for six Beogram 8002 signals -

Position Sensor 1 (the position of the tangential arm assembly)
Position Sensor 2
Detector Arm (the signal from the sensor in the fixed arm)
Speed Sensor (the signal from the platter speed sensor)
Servo Motor Forward
Servo Motor Reverse

My oscilloscope has four channels so I will just monitor what is happening on the servo motor and the position sensors first.

As a reference, here are the signals when the Beogram 8002 is parked in the Standby mode.

Pressing the Play button drives the Beogram tangential arm assembly forward and disengages the SO switch.
Notice there is a designed delay from when the SO switch disengages and the position sensors start to cycle.

The servo motor advances the tangential arm assembly and detects a record on the platter.  The servo slows down the advance as it reaches the set down point and stops to lower the tonearm.

Once the tonearm sets down the servo will advance again as the tonearm moves through the lead-in groove of a record.

There will be periodic activity on the position sensors and servo forward voltage as a record plays and the Beogram keeps the tonearm tangential to the record. The position sensor signal pulses will vary depending on how much compensation the servo motor is told to adjust for by the tracking sensor.

When the end of the record is reached the servo motor moves the tonearm through the run-out groove and stops. The motor reverses and begins the return of the tangential arm assembly back to the Standby mode position.

The reverse to home (or Standby position) can also be initiated during record play by pressing the Stop button.

Another action that results in a return to the Standby position is when the fast scanning button is used to move the tonearm forward until the maximum forward position is reached. 

The Beogram 8002 servo control drives the tangential arm assembly back to home where the SO switch is engaged.

To look at the platter speed sensor and the arm detector sensor I swapped out those test leads with the two oscilloscope leads I had been using to monitor the servo motor.

I will just show one photo with these connected. 
This photo shows the Beogram scanning for a record.  The position sensors show the tangential arm assembly is moving.  The speed sensor shows that the platter is rotating (at 33.3 RPM) and the arm detector sensor is showing pulses that indicate there is no record on the platter.

Next I changed two of the oscilloscope probes to measure the forward servo motor command and the Beogram tracking sensor. 
That meant adding the Tracking Sensor signal probe wire to the main board.

Looking at the basic turntable functions again with the tracking sensor being monitored gives these photos.
Note: Keep in mind that the Tracking Sensor signal is the tonearm movement away from its tangential position with the record play path. When that occurs the Tracking Sensor signal increase results in the forward motor drive to increase.

The following picture is the same scenario as the previous one except this is set to capture the first second of the press Play button event.  The position sensors don't begin counting until about 300 milliseconds after the tangential arm assembly starts moving.

This picture is of typical record play events over a 12 second period.

Finally, this is a picture of the end of record run-out groove event.

I am ready now to do some minor work on the Beogram 8002 cabinet and reassemble this turntable for the listening tests.

Saturday, September 11, 2021

Bay Area Beogram 8002: Checking the Forward/Reverse Scanning LDR Devices

I left off the previous post thinking there was a possible problem with one of the Beogram 8002 forward/reverse scanning LDR devices.

In testing the resistance of the two LDR devices with +15 VDC on the light source I saw one LDR measure a lot different from the other.  I figured one of the devices had become faulty.

When I removed the two LDR devices and checked their resistance values again they measured a lot closer to each other.  

Note: Without a calibrated light source my comparison check was to place an LED work light about ten inches directly above an LDR and measure the resistance.  Then, without moving the light and checking the other LDR in the same place I could make the second device measurement. The measured resistance should hopefully be very close on both devices.

The measurements are not identical but they aren't as far off as I suspected.

I decided to go ahead and replace the two devices along with the light source.
Since this Beogram involves some shipping costs I feel that it is safest to go with new components here.

Here are the new scanning components installed.
A new lamp source (LED with 1KΩ resistor) and two LDR devices.

I repeated the adjustment procedure on the two LDR devices again.  This time the two screws that control the light flow to the LDR devices both have working room for future adjustments. Both LDR devices were able to adjust to the desired, neutral voltage level.

Unfortunately there was still a problem. The activation mechanics of the forward scan button did not feel right to me. It didn't return to its original position quickly.

I took the button panel apart again to investigate the mechanics of the spring. 
I also compared a few other button panels from Beogram 8000 and 8002 units.

This photo shows the button side of the panel and where the scanning buttons strike the spring mechanism.  Both buttons share a single spring mechanism is kind of like a teeter-totter (or seesaw).  Pressing a scanning button causes that side of the teeter-totter spring to go down.  The other side will try to go up but is blocked by the back of the button panel. So each button should perform on its own side of the spring mechanism. 
Pressing a button depresses the spring. Releasing the button results in the spring returning to its neutral position.

What I observed in manually operating the spring mechanism is that the portion of the spring with the aperture would not return all the way to neutral when it was released. 

That is why the screw that controls the light source to the LDR had to be opened up so much for the reverse scanning LDR.

The neutral position should have full exposure of the LDR to the light source and the light source control screw positioned so the neutral position LDR voltage measures between 620mV and 700mV.

When a scanning button is partially pressed down, the opening (aperture) in the spring moves in front of the LDR cutting off some of the light.  That results in the resistance value increasing which in turn, causes the voltage level for that LDR to increase.  When the button is fully depressed the LDR is completely cut off from the light source and the maximum voltage across the LDR occurs...which is the fast scan operation.

The following photo shows that the spring mechanism with the apertures for the LDR devices just sits in place, resting on the center pivot point. It can teeter-totter either direction.  That surface of the spring mates to the underside of the button panel.

I have the spring lifted up and away from the panel in this photo to show the pivot point more clearly.
When fully installed the pivot point rests on the plastic base below it.

I removed the spring mechanism completely to adjust it.

After a few iterations of bending the spring (at the pivot point and at the ends) I achieved a much better result of the scanning button action on the spring mechanism.

Both the forward and reverse LDR devices were able to adjust to a good neutral value and more importantly, they return to that neutral position after being engaged.

All of the Beogram 8002 operating functions still work and I am happier with the scanning buttons now.
I will keep an eye on them when I get to the listening tests

Tuesday, September 7, 2021

Bay Area Beogram 8002: Service Manual Adjustments and Record Play Test

On the remaining service manual adjustments I need the platter motor connector disconnected for a couple of them...The tracking force adjustment and the tracking sensor adjustment.

For the tracking force adjustment an MMC phono cartridge must be installed on the tonearm.
I also like to set the sliding tracking force knob to 1 gram.

The tonearm can then be moved over the platter (that isn't turning because the platter motor connector is disconnected).  With a tracking force scale on the platter I can measure the tracking force of the cartridge when it is lowered onto a record. In this case I should measure 1 gram on the scale.
In cases where a selected 1 gram on the tracking force slider doesn't result in a 1 gram measurement on the scale, the tonearm counterweight must be adjusted.

The Beogram 8002 tonearm counterweight mounts on the back of the tonearm assembly and is secured with a single screw.  To make an adjustment the tonearm counterweight screw is loosened and slid forward or backwards depending on whether increasing or decreasing the tracking force.

It often takes a couple of iterations to make the adjustment but it should end up measuring 1 gram on the scale.

The counterweight is secured and the next adjustment with the platter motor still disconnected is the record tracking sensor.

When the Beogram 8002 is in Play mode and the tangential arm assembly is moved towards the record platter you should see a glow from the tracking sensor lamp as shown in the picture below.

For the adjustment of the tracking sensor there is a small adjustment screw to the side of the sensor housing.

The adjustment procedure for this sensor is to place the tonearm (with a phono cartridge) onto a record.  There was originally a Bang & Olufsen test record 3621001.  Track 5 is what the service manual says to use.  I do have a couple of Bang & Olufsen test records 3621003 but I don't know if they are similar.
So my technique is to use a couple of vintage records I own to test out the record tracking sensor.

The actual adjustment is to manually rotate the platter (with a record) by hand and observe that the Beogram servo motor begins to advance the spindle within 2 ± 1 revolutions.  After that the spindle should advance on every revolution.

Here is a photo of the tonearm lowered onto my test record for the adjustment.

My final adjustment is for the forward and reverse scanning LDR devices.
They function fine but I need to see where they measure in relation to what the service manual recommends.

The service manual calls for 620mVDC on each LDR (after the Beogram has been playing for about 5 minutes).  I like to set the value at around 650mVDC.

The two LDR devices in this Beogram 8002 were just over 700mVDC.  That value still works but I adjusted the two adjustment screws to get 650mVDC on each LDR.

Once those adjustments were completed I ran through a number of Beogram 8002 functional tests.
Everything worked perfectly including the Pause function where the tonearm returns to the stop location and pressing Play returns the tonearm to the spot where Pause was pressed.  

Normally I would continue on and start reinstalling the turntable components back in the Beogram 8002 cabinet to start some listening tests.

However, I am not entirely happy with those scanning LDR devices.  I was able to adjust them to the values I wanted but the two LDR devices are quite different from each other in their settings.

You can see in this photo what one LDR adjustment screw is way higher than the other.

The two LDR devices are not expected to be identical to each other but they should be closer to each other than this.  The higher adjustment screw feels like it is at its limit.

Opening up the scanning LDR and lamp compartment I discovered that I was correct. One of the LDR adjustment screws was at its limit.  It could not be adjusted any more.

I don't like this type of situation.  While the LDR is in a good working position there is no room for any future adjustments as the sensor lamp starts to lose intensity.

Looking inside at the LDR devices and the lamp I can see that the lamp does appear to function okay.

The way the scanning sensors work is the lamp illuminates each LDR through two separate, metal apertures that move with the scanning pushbutton action.  Pressing a scan button on the Beogram 8002 control panel closes the aperture on the respective LDR.  That changes the resistance of the LDR and calls for more voltage on that direction of the servo motor (that drives the tangential arm assembly).

When neither of the scanning buttons are pressed the LDR devices are in their neutral state which should be in that 620mVDC to 700mVDC range.  The two large, black screws adjust how much light from the lamp strikes the LDR in that neutral state.

As a test I opened up the other LDR adjustment screw so that both LDR devices will get the maximum amount of light from the lamp.

I put the LDR and lamp assembly cover back on and applied a +15 VDC test voltage to the lamp.
With both scanning buttons in their neutral position I measured the resistance on each LDR.
One measured around 16kΩ and the other around 43kΩ.  Quite a difference!

As I said, the two LDR devices are not expected to measure identically but in this case one is way different from the other.  The LDR measuring 43kΩ in the test is the one that had to be adjusted to its travel limit during the adjustment procedure.

My next step then is to look through my parts bin for some spare LDR devices that I know I have.
I will have to test a few to find a good pair that are closer to each other.

I think I will also replace the sensor lamp.  I don't believe it is a problem yet but if I replace one or both LDR devices then I think I should also install a new lamp.

Sunday, September 5, 2021

Bay Area Beogram 8002: Trying Out the Restored Components

 After the capacitor replacement in the previous post, I installed my test connector for ease of testing the scanning LDR devices.

Removing the button control panel from its circuit board I found the usual bit of dust that accumulates over the years. 

The dust is always loose type dust and easily wipes clean.
I soldered on the test connector and reassembled the button panel.

Another task I completed was cleaning and lubricating the tangential arm assembly.
The drive spindle and nut were in really good shape and I didn't see anything out of the ordinary.

Here are the before and after shots of the spindle and nut cleaning.

The rubber ends of the rear shaft (for the tangential arm assembly) were starting to dry out so I cleaned them and applied some rubber treatment.

The Beogram 8002 units for markets with a line voltage of 60 Hz have the rear tangential arm assembly shaft with the rubber end dampers. 50 Hz line voltage markets do not. I always wondered if the Bang & Olufsen design engineers knew the 60 Hz damping would be required from the beginning or as a result of testing the product.

Before fully reinstalling the tangential arm components I reassembled the arm assembly without the spindle and nut.

When installed this way I can manually move the arm assembly along its rails which is handy to adjust the fixed arm and tonearm for horizontal parallelism.

Earlier when examining this Beogram I could see that the arms were a little off.

Either the tonearm has to be adjusted up or the fixed arm adjusted down.

To go along with the adjustment there is the service manual adjustment for the height of the arm above the platter.

That is the reason I installed the arm assembly where I can easily slide it by hand.
I installed the platter and slid the arm assembly over it to check the height.

The fixed arm was too high and needed to be lowered.  The top of the fixed arm needs to be 19.5mm from the platter surface.

Using the single screw at the rear of the fixed arm I adjusted its height to the platter. 

After the fixed arm was lowered the tonearm still needed a slight adjustment up to be parallel.
That is another reason to do this with the spindle components out of the way.
The adjustment screw for the tonearm is on the underside of the tangential arm assembly so the assembly has to be flipped over to get to it.

Now with the arm height and horizontal parallelism adjustments made I lubricated the tangential arm components as I reinstalled them.

On the spindle I use a mixture of Rocol MTS 2000 and ESSO NUTO HP32 (1:1 ratio).
On the front and rear shafts the arm assembly slides on I put a thin layer of white D.C. M-kote paste.
For the two mounting points of the spindle onto the floating chassis I used some Tri-Flow synthetic grease.

I also installed a new servo belt as the old one was starting to develop cracks.
This replacement belt is one of Martin Olsen's reproductions.

The floating chassis parts are now ready to hook up to the Beogram 8002 electrical components for a quick functional test.

Here are the main Beogram 8002 components reconnected and ready.

The Standby dot is on so everything is good so far.
Pressing the Play button resulted in the Beogram moving the tangential arm assembly out over the platter to look for a record.  At the 17 cm mark the Beogram controller changed the platter speed from 33 RPM to 45 RPM.

It is always a bit of a thrill to see everything work properly on the first test.

The platter speed adjusts itself quickly and accurately.
The Turn, Stop and speed selection buttons all work correctly.
The forward and reverse scanning buttons work but I will still go through that service manual adjustment.

The next step is to go through the service manual adjustments to finish the functional testing.
I will also hook up some wires to view the sensor signals on the oscilloscope.