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Monday, January 7, 2019

Beogram 8000 Type 5613: Testing the power supplies and sensor signals

The initial restoration tasks are all completed so it is time to assemble and see what happens when the Beogram is plugged in to the AC socket.

Pressed "Play"
That's a good sign when the turntable starts to work. The question now is what do the supply voltages and sensor signals look like?

I measured the +5VDC supply voltage at the microcomputer board across the C28 capacitor I replaced.

Then I checked the ±15VDC supply voltages.

The power supplies all measure good.
Now for the LDR scanning sensor signals. Those are the forward (<<) and reverse (>>) scanning operations. The service manual calls for around 620mVDC on those signals when they are not pressed.

Here is the related circuit.

I removed the LDR sensor assembly cover to check if the sensor lamp was on. It was.

Now I can make use of my test connector to see what the voltages look like.

These needed some adjusting.
Here are the signals after I made the necessary adjustments (turning the screws on the LDR assembly cover). I always try to set the two sensors at values close to each other and at a non-pressed value of between 620mVDC and 700mVDC.

The next sensor I checked was the platter speed sensor.
Here is the circuit

Here is the actual measurement point for my oscilloscope probe.

Here are the signals for the two platter speeds coming from the tachodisc sensor.

Now for the sensor in the fixed arm that detects the presence of a record on the platter.

Here is the actual measurement point I used.

I checked the sensor at  33⅓ RPM

The signal looks like it is supposed to. It matches up with the service manual description.

The last sensor I checked on this power test run was the tangential arm position sensor.
Here is that circuit.

There is actually one source device and two sensors for the arm position. Both sensors generate pulses when the spindle turns and the Beogram microcomputer counts them to keep track of where the tangential arm carriage is. That allows the microcomputer to know the set down points and the end point of a record.

Here is where I placed my oscilloscope probes.

Initially and through the other sensor tests the arm position detection was working correctly. I even measured the output signals of the two sensors (as they come of their OpAmps).

Then the sensor signals died.

The Beogram cannot function without the arm position known and without the sensor signal the microcomputer counter is lost.

I rechecked the supply voltages and all of the other sensors again. They are all still working. It is just this position sensor circuit that is failed.

I have spare devices for the position sensor assembly so I will dig into that next. The problem could just be a connection so I will check that out first.

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