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.