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Wednesday, October 27, 2021

Beogram 8000: Testing speed sensors with the Workbench Beogram 8000

The Workshop Beogram 8000 unit has already aided me in restoring a number of Beogram 8000 PCB 1 and PCB 2 boards.  One set was extra troublesome as it had some broken traces that were not visible.

This photo shows the boards finally working as I finished off the testing by playing a record.

One thing I discovered in my board testing was that a problem with any part of the speed sensor circuit could potentially cause damage to other components of the platter drive system.  I am referring to the speed sensor itself and the path the signal takes to the Beogram microcomputer (2IC1) pin 40.

Having a problem with the speed sensor is similar to attempting to operate the Beogram 8000 with the rotor removed (sometimes referred to as the sub-platter).  In both cases when the Play or Turn buttons are pressed the Beogram uC will start sending commands to turn the platter.  The uC expects a speed signal as feedback to regulate the platter drive.  When the speed sensor signal is missing (i.e. broken or the rotor is not in place to turn the tachodisc) the uC will keep trying to drive the platter.  Typically what happens is the F1 fuse (300mA, slow for USA model Beogram 8000 turntables) in the transformer compartment will blow and shut things down.  However, damage can also occur to the platter drive transistors 0TR2 and 0TR3.  It would probably also be possible to damage some other components in the platter drive circuitry.

A symptom of a broken sensor signal is that the platter will immediately ramp up to a very high speed when either the Play or Turn button are pressed.  If power is not removed quick enough there can be some component damage.

This problem existed in the PCB 2 assembly of this troublesome set of Beogram boards.
The speed sensor signal was good all the way into the PCB 2 assembly but failed to reach pin 40 of the uC IC due to a broken trace. 

The route of the speed sensor signal to the uC pin 40 can vary depending on when the Beogram 8000 was manufactured or whether it received a service modification.

This photo shows the two types of PCB 2 assemblies found in Beogram 8000 turntables with regard to the speed sensor signal and the uC pin 40.

In the early model Beogram 8000 units the speed sensor signal went from the sensor at the platter hub through an OpAmp (1IC1) then on to 2IC1 (uC) pin 40 via pin 2 of the ribbon cable that connects PCB 1 and PCB 2.

Later in production B&O engineers found that tolerance differences between 1IC1 and 2IC1 could cause periodic variations in the platter speed.  So to solve that problem a 4013 flip-flop circuit was introduced to stabilize the signal.

This makes a difference when trouble-shooting a suspect speed sensor issue.
The early model PCB 2 assemblies had a short trace from the ribbon cable pin 2 over to the 2IC1 (uC) pin 40.
The later models routed the incoming ribbon cable pin 2 over to the new 4013 flip-flop by a yellow wire then returned the conditioned signal to a solder point next to 2IC1 (uC) pin 40 by an orange wire.
So there are more places to lose the signal in the modified PCB 2 assemblies.

In the problem PCB 2 I had to repair, the short trace between the conditioned signal (orange wire) and 2IC1-40 was open. The break was not visible to me but it measured open circuit and the board's primary problem was no speed sensor signal.  Adding a jumper from the conditioned signal directly to 2IC1-40 solved the problem.

I found that it is easy to check 2IC1-40 with an oscilloscope and see if there is a good speed sensor signal. 
However, if the signal is not there then there will not be time to check for the signal and deal with the platter drive having no feedback.

The solution I came up with to make the easy measurement and protect the Beogram circuit components is a new speed sensor test harness for the platter drive cable.
That is the cable that connects to PCB 1 via the P4 board connector.
Disconnecting the P4 cable allows operating the Beogram 8000 without the platter drive in operation.
It is a handy mode when doing things like adjusting the record tracking sensor and setting the tracking force.  

I decided to make a P4 harness that only connected up the three wires for the speed sensor assembly.
This allows me to operate the Beogram 8000 in the mode where the platter drive is disabled except I have the speed sensor signals available to test.

Here is that harness ready to use with my Workshop Beogram 8000.

I have two oscilloscope probes in place.
One for the raw speed sensor signal right at the sensor.  The other will be connected to 2IC1 (uC) pin 40.

Now I can press the Play button or the Turn button and manually rotate the platter hub to check the speed sensor.

Here is a closer look at what the oscilloscope sees at those two measurement points for a properly operating speed sensor.

I now pre-test the speed sensor functionality of a Beogram 8000 using the speed sensor only P4 harness before trying out the full P4 harness.  That way I will get an early warning that there is a problem before risking the loss of a component.

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