Beogram 8002 From North Carolina: Finishing up the service manual adjustments

I was able to get quite a lot done since the previous post.

I performed the power supply tests, adjusted the tracking force by calibrating the tonearm counterweight, adjusted the tracking sensor setting and adjusted the forward/reverse scan LDR voltages.

To begin with here are the supply voltage checks from the supply filtering capacitors for +5V, +15V and -15V.

I also measured the regulated +5V that goes to various places in the Beogram 8002 (including the microcomputer board).

In this earlier post for this Beogram 8002 I had to fix the tonearm counterweight.
Now it was time to calibrate and adjust the tonearm for 1 gram of tracking force.
For the tracking force adjustment and the record tracking sensor adjustment that will follow I don't want the Beogram 8002 platter to rotate when the turntable is on.  To facilitate that I unplugged the P4 connector on the main PCB (with the Beogram AC plug disconnected as well of course).

The other test wires connected (temporarily) to the back of the main board are for monitoring some key Beogram 8002 signals with an oscilloscope.  I will make those measurements later in this post.

For the tracking force calibration and adjustment I installed a cartridge on the tonearm and set the tracking force to 1 gram.  Next, I adjusted the counterweight position using the set screw until my tracking force scale measure 1 gram.

Now that the tracking force is set I ran through the tracking sensor adjustment steps.
For this adjustment a test record is set on the platter.  The platter drive is still not connected so it can only be turned manually.

The procedure says to lower the tonearm with the cartridge stylus onto a record groove.
The platter is turned manually while observing the Beogram 8002 servo motor (tangential arm assembly motor).  The servo motor should advance the tonearm the first time within 2 platter rotations...plus or minus 1 rotation.  So between 1 and 3 rotations.  After that the servo motor will perform advances as necessary. It is a good idea to use the same test record(s) for this adjustment as the record grooves vary between vinyl records. Using the same set of records for the adjustment will allow the setting to be consistent.

The service manual refers to the tracking sensor adjustment screw as "screw B" shown here.
Note the tracking sensor lamp illuminating from inside its housing.  If that lamp is not on then of course replace that first.  I don't believe I have ever come across one that was out on a Beogram 800x turntable.

After a few iterations and the adjustment check performed on two different test records that I like to use, I was satisfied with the result.

I unplugged the Beogram 8002 AC plug and reinserted the P4 platter drive connector.

Then I plugged the Beogram back in and tested its play of a record.

One last adjustment is on the forward and reverse scanning control sensors.  The forward and reverse scanning buttons press a spring loaded aperture between a light source and an LDR. Two LDRs actually.  One for the forward scan and one for the reverse scan.  The change in the light through the aperture determines the scanning operation.
The steady-state (no scanning) voltages on those LDR sensors should be adjusted to around 620mV.
The adjustment for the LDR voltages is via a large screw at the back of the button panel. One for the forward LDR and one for the reverse LDR.

This is an important adjustment on the Beogram 8002.  If the scanning LDR voltage level is far enough off, the Beogram 8002 tonearm movement will not operate properly.

I have always found it very inconvenient to make this adjustment as it requires opening the Beogram 8002 into its service position. 

There are often cases where I want to double-check the scanning LDR voltage levels after the Beogram 8002 is reassembled in its cabinet.  Opening it all back up just for this measurement and adjustment is a pain.

For that reason I started installing a small test connector that I leave in the Beogram (both the 8000 and 8002 turntables).  With the test connector I can later make the measurement and adjustment again by only popping off the Beogram button control panel. It is easy to remove and reinstall.

Here is the Beogram 8002 button control panel removed for installation of the test connector.
The spring loaded aperture controls for the scanning LDRs are shown highlighted in the photo.

Here is the test connector I install.  The connector and its 30 gauge wiring will easily fit into the Beogram 8002 out of sight when the turntable is reassembled into its cabinet.

To use the test connector for the LDR adjustments I just plug in my three pronged test leads and make measurements with a DMM.

This photo shows the adjusted LDR voltage for the Reverse LDR sensor.

Very nice.  It is great to see the Beogram 8002 operating correctly again.

Now...while I have my test wires hooked up to the main board I collected some screen shots of the following signals at various modes of operation for the Beogram 8002.

The signals I wanted to observe are the Record Detection Sensor, the Platter Speed sensor and the two sensors on the tangential arm drive spindle.

Here are the oscilloscope measurements of those signals during various Beogram 8002 operating modes.

As a reference, here are the signals when the Beogram 8002 is off.

Here are the signals when pressing the Play button.

Here are a couple of pictures of the signals when the Beogram 8002 is returning home after searching for a record on the platter.  You can see that different patterns of bands on the Beogram 8002 platter affect the Detector Arm signal. Notice the difference in relative position between the two position sensor signals in these reverse direction photos and the previous, forward direction. They swapped positions on the oscilloscope trace.

Finally, here is a trace of the signals when a record is playing. This trace triggered on position sensor 2 in order to catch the activity by the servo motor advancing the tonearm. Note that the Detector Arm signal is silent as there is a record on the platter.  

It isn't a requirement to measure these signals during a Beogram 8002 restoration but I wanted to start collecting some reference photos to have in comparing the turntable restoration projects I have been working on.  They provide another visual reference of the turntable operation.

Tomorrow I will work on the Beogram's dust cover hinge repair. 

After that it will be ready to reassemble and move on to some record play testing before going home.