Beogram 4002 (5501): Random Stop Issue Fixed - Defective Photo Resistor in the Spindle RPM Sensor

A Beogram 4002 (5501) that I restored last year developed a strange new problem: When playing a record the OFF (>>) function  would be triggered randomly during play as if someone pressed the >> key on the keypad.

Before I will discuss the fix, let's have a look at the circuit diagram (click on the diagram to get it in full resolution):

TR17 in the 'Electronic Switch' is responsible for triggering the >> function of the turntable. Whenever its base is pulled to GND its collector goes high to ~18V and then the arm is lifted and the carriage is driven home. There are three ways this can occur: Via two mechanical switches, one being the "End Switch" (ES) under the carriage and the other the ">>" key on the keypad. The End Switch is the one that sends the carriage back whenever it is driven all the way to the left (i.e. manually via the << key or automatically if there is no record on the platter and the arms go all the way searching for one).

The third way to trigger >> is via the end groove detection mechanism ("RUN-OFF STOP" on the diagram). This works via TR20 whose base is pulled up when the end groove is detected. That connects its collector to ground, and with that the >> function is activated.

The end groove detection mechanism works via an 'analog rotary encoder' that detects light flashes impinging on a photo resistor (OR2) from a bulb (OIL1) that shines its light though four holes in the carriage pulley as they pass by (creating light fluctuations on OR2). 

These light pulses short OR2 to GND causing TR21 to shut down, which increases the voltage at the collector of TR21 to about 19V. These voltage pulses (see oscilloscope trace schematic on the circuit diagram) charge C33, which, via the voltage divider formed by R88/89, pulls up the base of TR20, triggering the >> function.

It is interesting to to note that the bulb OIL0 only comes on when its switch to GND is closed. This switch is activated by the carriage when the tone arm gets close to the record label. This switch is activated by the same tab on the carriage assembly that activates the 17 cm (singles) set-down point switch ("B" in the service manual). This is the reason that the tab that activates B has a long flat shape. Anyway, this switch aims to prevent triggering the the RUN-OFF STOP mechanism when the carriage briefly moves faster between two tracks of a record when the carriage is still far away from the label.

Why does the mechanism not trigger RUN-OFF STOP when the carriage advances normally while playing the last track when OIL1 is already on? C34, R91 and D30 form a network that discharges C33 when there is no light on the sensor. This means there is a competition between charging during illumination episodes, and discharging when there is no light on the sensor. So if the pulley rotates only slowly and occasionally during playback of a track, discharging 'wins' and the base of TR20 is not pulled up high enough. But when the light flashes happen quickly like when the carriage moves fast pulled along by the end groove, then the charging mechanism wins and TR20 is turned on. The beauty of analog control systems!!

Ok, back to the 'Random Stop Issue': After verifying that the ES and OFF switches were working properly, I traced the signals from the base of TR17 into the RUN-OFF STOP circuit. This shows the oscilloscope traces that I measured:

The traces are assigned as follows:

• Yellow: TR17 collector (when this signal goes high the measurement was triggered - the graph shows ±5 sec around the >> event)
• Blue: TR20 base
• Green: TR21 collector
• Red: TR21 base

These traces show that before the >> event happens there is a random statistical fluctuation of the voltage at the collector of TR21 (instead of being close to GND if there is no end groove event). These voltage flashes obviously correlate with the signal at the base of TR21 (suggesting that TR21 is working properly). This leaves only one conclusion: The sensor OR2 randomly short circuits to GND causing these voltage spikes. These spikes can be enough to charge C33 and when the voltage one C33 gets high enough due to these random events, the >> function is triggered. This is seen in the blue trace, which shows that the voltage exceeds the ~0.6-7V threshold needed to turn on a standard silicon based transistor when TR17 is activated.

All this told me that OR2 was faulty. This meant it needed to be replaced. Since this is a special B&O part, which is not available anymore, I decided to design a 3D printed assembly to replace the entire bulb/sensor assembly on the pulley. This shows the setup after I implanted an LED to replace the light bulb last year:

The orange part contains the LED (Newark 78R6602) and its 2k current limiting resistor (Newark 26R3983). It was designed to stick onto the OR2 sensor housing. The current design replaces the entire encoder setup. These are the structural parts of the assembly:

Assembled they look like this:

And with LED, resistor and photo resistor (type "5516", ~10k resistance when dark, ~500 Ohm when illuminated) installed:

And after installation and in action:

Once the set-up was installed I measured the signal at the collector of TR21 when the LED came on:

This signal looks pretty much like what the manual demands. I tested the Beogram by playing some records, and it seems everything is working again! So I am hopeful that this fix took care of the issue.

Beogram 4000: An Entertaining Evening Exploring the Differential Stop Circuit

Before we send a restored unit back to a customer, we always give them a good 'real world' test, where we simply play a bunch of lovely music over a couple weeks and observe if everything works consistently. The listening part is of course part of the fun of Beoloving! There is always music in the work shop! The case of a recently restored Beogram 4000 is a proof in point of the usefulness of such testing phases: While playing a number of records, I noticed that occasionally the arm would not return to the home position once the end groove had been reached. Normally, the Beogram 4000 detects the end groove spiral due to the higher speed of the carriage and then lifts the arm and returns the carriage. Not in this case, though, where the needle simply went all the way in and then kept playing the final turn over and over again...tock...tock...tock...not very Beolovely!

The problem was that this would only happen occasionally, but most times everything would be fine. Such problems are hard to troubleshoot. In the beginning I thought it was simply a MV switch issue (most 4000 issues come from not working switches), but since I had coated the switch terminals with gold, tests demonstrated that the switch was working very reliably. So I concluded the problem must be in the differential stop circuit, which produces the logic DS level that tells the control system that it is time to lift the arm and return the carriage home. This shows the relevant part of the Beogram 4000 circuit:

This is how it works: The detection of the end groove is done via the carriage motor voltage. Since the end groove typically makes the carriage move faster, the motor voltage is larger than during normal play back of a track. That is why the circuit is called Differential Stop (DS), the end groove is not determined by the position of the arm, but by its velocity towards the center of the record. One of the issues is that the arms do not only move faster in the end groove, but also between tracks and in the run in groove at the outer rim of the record. This problem is solved by introducing a carriage activated switch ("MV") that enables the DS circuit only close to the end groove region of the record. This switch is on the second circuit diagram of the 4000 and it connects MV and MV' in the above circuit snippet.

So when the carriage reaches a point about 65mm away from the center of the record MV and MV' are connected. This causes the voltage that is applied across the motor to be applied across base and emitter of TR24 via 1R54 and trimmer 1VR4. The trimmer is used to adjust the carriage speed threshold at which TR24 turns on (this adjustment turned out to be the critical item during this exploration...read on...;-). When TR24 turns on due to high carriage speed, it then turns on TR23 which serves as an additional input that allows other functions of the turntable to disable the DS circuit (mainly system requested and manual FF function via TR25). When TR23 turns on TR22 becomes conductive and grounds its collector. This causes the logic signal DS to go to 0V. DS then activates carriage return via the logic ICs in the control panel. This immediately turns off the DS circuit since the motor voltage reverses (the MV switch remains closed until the carriage clears the switch on the way back to home position). The result is an about 5-10ms pulse at DS as is shown in this oscilloscope snap shot:

The blue trace is the DS signal. Green and yellow are the (motor) voltage measured on both ends of the MV switch against GND (since it is closed at this moment both traces are the same).

We see that when the motor voltage changes due to the reversal of the carriage direction the pulse terminates.

Ok after this circuit discussion...back to the 'issue' that I was trying to solve, the occasional not working of the DS mechanism:

After I verified that the MV switch worked reliably, I thought the issue must be an intermittent problem with the DS circuit. Since the issue was very intermittent, I used an Arduino as event recorder:

I used one of the pin change interrupts to detect the state of DS and print it out on the serial port whenever it would change from 6V to 0V, i.e. when the circuit activated DS. Then I played some records and waited for the issue to happen. Luckily the second record I played triggered the issue. And, guess what, DS indeed did not change. This told me that the issue is in the circuit. I played another record, and it worked again. Then I put on the previous record and it happened again! This finally turned on some light bulbs in my head. I put the other record on and it worked again! The light became very bright in my head and I concluded: It is the record, dummy! The circuit was basically working, but the records are different! This shows the end grooves of the two records in comparison: First the one that made DS work properly:

and the one that caused DS to fail:

What is the difference? The number of turns the end groove takes until it hits the innermost groove. This means that the first record makes the carriage move faster than the second does. The consequence is that the motor voltage is lower when the arm reaches the end groove of the second record. This shows an oscilloscope trace of the motor voltage (measured by connecting the oscilloscope trace to one motor pin, and the oscilloscope GND to the other) as the needle goes from track to end groove:

The waves on the left are the normal voltage fluctuations on the motor as a track is played. Then as the end groove begins the voltage goes down and then the shoots up as the voltage across the motor is large enough to trigger TR24. So it appears that the non-triggering record simply did not cause the voltage to cross the threshold. I ended up adjusting the VR4 trimmer a bit clockwise and then the mechanism worked also for the second record, yet still played the last track to the end (a good test for that is playing a single, since their single track usually extends into 'MV switch closed territory'). So in the end, nothing was wrong, just the adjustment needed to be changed a bit towards more sensitivity. 

An interesting thing to note is that the service manual prescribes to adjust the sensitivity that DS gets triggered at 1.8V motor voltage. This is obviously too sensitive since the amplitude of the waves in the oscilloscope trace above is about 2.2V. If the threshold were adjusted to 1.8V DS would be triggered during normal playback and not only in the end groove. The joys of analog control systems!

Beogram 4002 (5513): End Groove Detection Not Working

When I thought I was close to be done with this Beogram 4002 (5513), I set it up to play a first record on it. All went well until the side was through and the needle approached the end groove. The needle went all the way to the record label and then remained there playing an eternal "tk....tk.....tk....". Like in the old days with fully manual turntables...not very beolovely, though!

I removed the platter and had a look. I connected my oscilloscope to the collector of TR17, which translates the optosensor (4IC1) response from the position sensor into a 0-21V 'digital' signal that can be used to tell the control system where the carriage is located. The signal was all good for the major black bars on the position indicator, but was very weak for the run out groove area. This is usually a sign of a poor alignment of the IR diode relative to the light entry slit on 4IC1. The reason that the end groove marks are more critical in this regard is their small width. This creates a situation where a slight misalignment of the diode sends light into 4IC1 from the side, thereby 'muddying' the bright-dark sequence, which lessens the response of TR17. Another reason for end groove detection malfunction is an incorrect distance between position indicator bar and 4IC1. It needs to be pretty close to ensure that the narrow end groove bars really cover the light entry slit.

Anyway, I tried to reposition the IR diode a bit. This shows PCB 4 removed from the enclosure:

The IR diode is right in the center facing the black front of the sensor (this picture is actually from a different Beogram 4002 than the one that was fixed here...I forgot to take a picture before I repaired things).

When I tried to align the diode a bit better, it promptly broke in two:

I guess the old plastic got a bit brittle over the years...what now??

I tried replacing it with modern IR diodes that I had in stock, but to no avail. The circuit that drives the IR diode did not have a low enough impedance to drive the modern high output IR diodes that I had available. Even with 1R88 fully turned the voltage did not reach the prescribed 1.7V across the diode. I elected to not alter the circuit with a smaller resistor, but rather chose to use a classic red 5 mm LED, which had an appropriate low output and power consumption. This worked very well. Here you can see it implanted:

And in action:

Looks pretty, doesn't it? After this repair the end groove was detected nicely. It is interesting to note that the end groove issue probably was there for some time and omitted on the ebay description for this Beogram: It appears the previous owner tried to fix it by adding a thicker line to the end groove markings with a Sharpie pen:

The end groove markers are the narrow lines to the right. The Sharpie line is the wiggly hand drawn thicker line to the left of the narrow lines. The other lines on the left are the singles set-down marker (narrow) and the 45 RPM change marker (wide band).

It is interesting how they used different widths and the end groove lines-array to be able to elicit different and appropriate responses from the control system depending on the carriage position. Ah, the joys of analog control systems. Microcontrollers are so much more effective, but also much more boring...;-). This is Beolove!

Beogram 4002 (5513): Non-Working End Groove Detection - Repair of a Detached Carriage Position Sensor

I am almost done with the Beogram 4002 (5513) that I am restoring right now. One unusual issue with the unit was the detached carriage position sensor:

The sensor is in the black light shield that is seen 'hanging' at an off-angle. It needs to be vertical that the black sections on the plexiglass 'ruler' that is attached to the carriage can properly block the light coming from the IR diode that is behind the ruler. If the sensor is not close to the ruler stray light can enter causing instabilities in the carriage operation. In this Beogram this manifested itself by a non-working end groove detection. This particular mechanism is especially dependent on a proper positioning of the sensor since the mechanism depends on the step-wise charging of a capacitor until the 'STOP' function can be triggered. If the light is only partially blocked by the narrow end groove detection bars on the ruler the charging mechanism only works partially which can cause the voltage build-up on the capacitor to be too low.

I removed the carriage position PCB:

The sensor is seen detached from the double sided tape underneath. The IR diode sits right in front of the sensor, which is behind the narrow slit in the black housing.

I glued the sensor housing back on with a bit of epoxy and held everything together with a carpenter clamp until the glue bonded:

This shows the sensor after removing the clamp:

Then it was time to reinstall the board:

Now the end groove detection worked again. It is interesting to note that the other functions were working properly even with the detached sensor...

Beogram 4000: End Groove Detection Issue

After polishing the hood I thought I was done with this Beogram 4000, but not so. A last full performance check before boxing it up revealed a strange fault when ON was pressed with no record on the platter. The carriage would properly travel inwards in search of a record, but then before hitting the end switch ES the carriage would suddenly stop. Only pressing FF would release it from this state. However, then after letting the FF key go the carriage would immediately travel back without even touching the ES. In a healthy state the carriage should travel until the ES is activated and then it should return to the home position.

A closer look revealed that the carriage stopped right when it closed the MV switch that enables the end groove detection mechanism. Here is the relevant section of the circuit diagram:

Here is how it works: Whenever the MV switch is activated by the slide, MV' and MV are connected in the diagram. This puts part of the voltage on the motor across BE of TR24. So whenever the motor revs a bit higher, such as when the needle enters the end groove, the transistors 24/23/22 are activated and DS finally goes LOW. This tells the control system that it is time to lift the arm and drive the carriage home. Now this mechanism is of course only useful when the needle is on the record and entering the end groove. When the arm is up and the carriage is driven forward or backward this mechanism needs to be disabled. This is achieved via D4 which feeds the collector of TR25 into the base of TR23. 

The collector can only be high if FF is activated either by pressing the key or via Q0' which is LOW whenever the carriage is driven towards the center of the record after pressing ON. The tracking mechanism does not use TR25 since it directly goes into TR27/28, i.e. end groove detection is not disabled if the carriage tracks normally.

Further tests of the matter revealed that Q0' remained LOW even as the carriage stopped. This told me that the control system was still of the opinion that the carriage should move, hence the fault needed to be on the executive side of the system, i.e. right here in the MV circuit. I measured D4 from the back of the installed main PCB and promptly saw that it was open circuit, which killed the deactivation mechanism, and the end groove detection mechanism was active even with the arm up. After turing the board around I saw that the diode was cracked in half:

The original OA90 diode is a Germanium diode, but can easily be replaced with a modern 1N4148 Si signal diode since its only function is to block any voltage from the MV circuit to apply to the collector of TR25. Here is a picture of the 4148 installed:

After this I replaced the board and gave the deck a test drive. All good again. Listening to Walt Dickerson's "To My Queen" while I write this, which is definitely one of my all time jazz favorites. O.k. maybe now it is time to finally send this 4000 back to its owner.