Beogram 8000: Preparing for Record Play (UPDATED)

This Beogram 8000 is almost ready for the record listening phase of the testing. There are still a few test milestones to check.

One is to calibrate the tonearm/cartridge tracking force. I like to set the tonearm tracking force to 1 gram and calibrate the arm counter-weight for that force with a typical B&O cartridge attached.  This picture shows the components involved with calibrating the tracking force.

I started out with the B&O tracking force scale but I end up finalizing the adjustment with a digital scale.

The tracking force is set so it is on to the Beogram forward and reverse scanning voltage checks. The controls for the left (forward) and right (reverse) tonearm scanning operate an aperture that passes light onto an LDR device. The amount of light changes the voltage on the device which affects the motor control circuitry responsible for moving the tonearm.

The normal voltage level for the forward LDR and reverse LDR devices should be between 0.6 and 0.7 VDC. Adjustments to that voltage are made by adjusting separate screws for the forward and reverse LDR light apertures.

I like the option to check these LDR voltages after the Beogram is re-assembled in its cabinet. It is easy to pop open the control panel but probing the LDR devices to measure them is difficult at that point. For that reason I attach a small test connector that I can use later for re-adjusting the scanning voltages.

Here is the control panel with the test connector installed. Note that the color coding I use on the test connector matches the corresponding signals on the panel's small ribbon cable. The test connector is a female connector so it can't short out when not being used for testing. Just insert a three-prong test jack and the measurements can be made with a DMM.

Now for some initial measurements.

CORRECTION: The following scanning LDR measurement results have been corrected from the original post.

The reverse direction scanning LDR adjusted nicely within the 0.6 to 0.7 VDC range.

So did the forward scanning adjustment.

No problems with the forward and reverse scanning functions.

The third thing I checked was the Beogram phono muting relay. I have started to see more signs of the original muting relays starting to wear. Sometimes these relays fail completely so it is easy to diagnose. Typically though the relay just starts acting up slowly. It may occasionally fail to switch in and out of muting or it could start exhibiting some noise after closing.

On this Beogram I noticed that the muting relay isn't always completely closing when the phono signals are supposed to be muted.

Here are some pictures that show the problem.

The first picture shows the muting relay open where both channels are playing a music source.
This result looks good and shows the relay is working.

When the tonearm lifts the muting relay closes and the phono signals get shorted. The result should be  both left and right channels muted.  This picture shows that the right channel isn't completely muted.

I tapped on the relay a few times and the muting relay started working again.

The phono muting relay issue will have to be fixed but swapping out the relay is something I can easily do later. It doesn't need to hold up testing the Beogram record play functions.

The last check before record play is to check the tangential arm tracking. To perform this check power is removed from the platter so that it can only rotate manually by hand. The P4 connector is disconnected from the main board to do this.

Note: Unplug the Beogram before unplugging and plugging in any board connections.

The linear tangential arm tracking test calls for setting the Beogram arm onto a Bang & Olufsen Test Record 3621001 track 5. The platter is manually rotated and the rotations counted while observing the tracking motor for movement. When the stylus sets down the tracking motor should begin first moving the arm transport after 2 ± 1 rotations. After that the tracking motor should move on every platter rotation.

I have never been able to find a B&O Test Record 3621001 so I use a couple of records I have found worked well for this adjustment.

Surprisingly, this Beogram 8000 did not require any adjustment to the tracking sensor. It tracked my test records perfectly.

Tomorrow I will re-assemble the Beogram and test play a record.

Beogram 8000: Reworking The Phono Muting Relay...Again

Since the replacement of the Beogram 8000 Control Panel board with a spare I had this Beogram has not had any mishaps with unscheduled pauses during record play.  During retesting of this however, the spare phono muting relay I installed is acting up like the original one did. The problem is intermittent but when it occurs the phono signal is not muted when the tonearm is lifted.

This still sounds like a relay problem and I didn't really like the fact that I had to use a replacement for it from another Beogram 8000 turntable instead of with a brand new relay. The problem is the Beogram 8000 output board is designed for a National type relay that is no longer available.

The Beogram 8002 output board it turns out is interchangeable with the Beogram 8000 and the Beogram 8002 muting relay is still available new from my favorite suppliers.

Before swapping output boards and relays I want to do diligence and measure the Beogram Lift signal.

The Beogram 8000 Muting and Lift circuits are related. A Lift command from the processor causes the tonearm to lower or lift followed by the muting circuit engaging or disengaging the phono muting relay.

I checked out the three test points shown on the diagram on my oscilloscope to see what is happening.

The above picture shows the Lift command in red from the Beogram processor. About 2.5 seconds after the Lift signal (to lower the tonearm), the muting relay engages to allow the phono signals to function.

When the tonearm is raised the Lift signal fires again and the muting relay disengages, shorting the outputs together.

I observed this switching function for a while and never saw any problem with it. However, I did see a problem with the muting relay. While the oscilloscope was measuring the Lift signals I connected my DMM to the phono left and right signals to measure the resistance.

Initially, before turning on the Beogram, the resistance measurement between the left and right channels was a short circuit. I measured about 0.4 ohms. When the tonearm lowers the phono signals are opened up and I measure an open circuit. When the tonearm is lifted the left and right channels should return to a short. There were a few times that I observed around 250 ohms to 280 ohms across the channels instead of a short. So the relay contacts are still the problem.

It was time to go ahead and use a Beogram 8002 output board in place of the original Beogram 8000 output board.

Here is the Beogram 8002 phono output board. The following pictures show changing out the muting relay with a new, sealed Omron relay. I also installed the ground switch option on this replacement output board assembly.

Now it was ready to swap the output boards.

The following picture shows the difference between the two output boards when the phono muting is on.

The new relay does a much better job and I feel much better having a brand new relay installed.

The other test point I wanted to measure is the Beogram 8000 tonearm lowering circuit. The damping of the tonearm when it is lowered to play a record is often a trouble spot. Sometimes owners observe the tonearm just dropping instead of slowing lowering. The control of the arm lowering is via an OpAmp circuit that is used to generate a ramp voltage to engage the tonearm lowering solenoid.

This Beogram has been lowering and raising exactly as it should so I captured some measurements to use as a reference. I measured the ramp signal as it reacts to the processor Lift command.

Whew! The Beogram 8000 is connected once again to my audio system where I will exercise it again before sending it home.