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

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.