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 4002 (5513): First Contact and Two Dead Transistors

I recently received a Beogram 4002. The owner already had started working on it, but then decided to send it over for some trouble shooting. This shows the unit:

It is in pretty good condition. The aluminum parts are almost pristine except a few small scratches in the platter. The keypad has the usual wear pattern and some blank spots on the START button. I took out the aluminum panels and had a look:

When I plugged it in and pressed start, the platter motor started running but the carriage would not move at all. Pressing start results in turning on of TR12 whose collector then goes to about 17V. This voltage turns on TR19 in the H-bridge that controls the carriage motor. I checked the presence of the 17V at TR19 when pressing start. The voltage made it there indicating that TR12 was o.k. Then I checked the voltages at TR20 when TR19 was activated. This test showed that TR20 would not turn on. I extracted it from the board and indeed a transistor tester revealed that it was burned out. I replaced the transistor with a new one, put the board back in place and voila, pressing start activated the carriage.

However, after moving about 2-3mm out, it immediately returned to the home position as if STOP was pressed immediately after start. I checked the voltage at the base of TR13, which controls the stop function. The voltage there was consistently 0V, indicating that either something pulled the base down to 0V or that TR13 itself had a problem. First I checked the start and end switchers, which can pull the base down, but both switches worked properly. Then I extracted TR13 and the transistor tester showed it to be damaged. I replaced it with a new transistor and then everything worked. Pressing start launched the carriage and it found the set down point for LPs and the arm lowered. Then it started racing towards the center of the platter, indicating that the tracking feedback needed some adjusting. Here is a picture of the two extracted transistors:

This Beogram is on the way to recovery!

Beomaster 8000: Replacing the Processor Crystals

A Beomaster 8000 that I restored a few years ago for an Australian customer unfortunately had to come back to my bench due to a malfunction of the processor board. It exhibited rapid-fire power on/off events, which are a sign for a dead crystal on IC4 (slave processor). See here for more detail on how I first ran into this issue on the 8000 that is in my living room. Anyway, here are a few pictures of how I fixed this one. This shows the original setup:

As you can see someone in the past soldered jumper wires directly to the pins of the microcontrollers (savages!) to bridge the often failing vias that connect some of the processor pins to other components on the board. I usually re-solder the vias instead of putting in jumper wires. The reason that these vias fail is poor initial soldering. In the 80s electronics manufacturing just started becoming fully automated and some outcomes were not as high quality as one would desire...

Anyway, here I had to deal with these wires since it is good practice to take the microcontrollers out before replacing the crystals. They can be charged with high voltages when you take them out of the packaging, and this can fry the ICs...and that essentially would mean that a donor Beomaster 8000 would need to be procured. I was able to remove IC3 (right) without trouble but IC4 on the left was stuck. When the wires were soldered on a lot of solder was used and some of it penetrated the lower regions of the socket. I unsoldered the affected legs of the socket and then pulled the IC out. It came out including the three soldered-on pins from the socket. Once it was out it was easy to clean the pins up since I was able to get to the solder without having to heat everything for a dangerously long time...much better for the survival of the precious IC.

I implanted a new IC socket

And then exchanged the crystals and oscillator capacitors (the original ones are 12 pF while the modern crystals need 18 pF caps):

I elected to not solder the jumper wires back on to IC4. Instead I fixed the vias. I left the wires on IC3 in place since every time one heats up a pin on an IC there is a slight chance that damage can occur...

Once it was all back together, I fired the Beomaster up for the first time, and it came on normally, no more relay rattling etc...so I hope this did the trick.

Beolovely!

Beogram 4004 (5526): Restoration of the DC Platter Motor and RPM Stability Test

After replacing the RPM trimmers, the RPM relay and the light bulbs in the RPM trimmer panel with LEDs, it was finally time to restore the DC platter motor of this Beogram 4004 (5526). This sequence is important, since one can only test the RPM stability performance of the motor properly after the entire RPM control circuit has been rebuilt. This shows the extracted motor:

The motor needs to be completely disassembled to get to the dried out Oilite bearings:

The bearings are the two small donuts on the black pad. I infused them with motor oil under vacuum:

The raising bubbles indicate that air is drawn from the porous brass bearing material. This creates space for oil to enter the bearing. After about 12-24 hours the process usually stops, at which point the bearings are again full of oil.

This shows the bearings after the infusion process:

After reassembling the motor I put it back into the deck and adjusted the RPM with the BeoloverRPM device:

Then I used the device to measure a RPM stability curve over 18 hours:

This motor showed an exceptionally stable RPM performance and can be considered ready for duty.

Beomaster 8000: Rattling Main Relays and a Broken 2MHz Oscillator Crystal

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ACHTUNG: Please, note that the crystal exchange procedure described here can damage the microprocessor chips on the uProcessor board. It is recommended to remove the chips before replacing the crystals and the capacitors. Due to the inherent capacitance of these devices, high voltages can be present between their terminals, which upon release, can burn out the gate that forms the oscillator together with the crystal. Make sure you short circuit the leads of the parts before installing them.
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Oh well, nothing lasts for ever!...The fully restored (2011) Beomaster 8000 that I am using in our living room recently broke with a rather strange fault. I woke up at 4 am in the morning to the beeping of an uninterruptible power supply (UPS) in distress. First I thought we had a power outage, but it turned out that the UPS turned itself off due to a malfunction in the connected Beomaster 8000 (I run all my B&O equipment on UPSs to reduce the risk of damage from grid voltage spikes and the like). I restarted the UPS and immediately the main transformer power relays of the 8000 started going on/off in rapid sequence. And then the UPS caved in again, turned itself off and started beeping again. This immediately pointed to a severe fault in the 8000 since a UPS only shuts down if the current drawn exceeds the rated 10 or 16 amps. This suggested to not try running the Beomaster again before having a look inside.

I shut everything down and went back to bed. The next morning I swapped the 8000 out with one of the other 8000s I have around the house in less important locations. A few days later I opened the malfunctioning unit up and ran it. First everything was normal but after a while it started to rattle again. I switched it off immediately and started wondering what might cause such behavior. First I thought there is a problem in one of the outputs triggering the protection circuit. The reason was that I initially detected 15V at the output of the protection circuit (collector of 6TR15) during relay rattling phases. The 15V pull up the base of 6TR11 via 6D12, which then causes the relays to go off, cutting power to the main transformer. This is usually caused by malfunctioning output transistors or overheating of the outputs. However, in this case the 15V were rather a consequence of an entirely different root cause.

After some playing around with the unit and enjoying a few more UPS shutdowns and relay rattling events, I finally figured out that the crystal oscillator of 9IC4 on the main processor board had some issues. I figured this out due to two phenomena:

1) After one of the rattling episodes I was not able to turn the Beomaster back on. The standby LED was lit, but no more reaction to the keypad. Panic ensued since I thought I had accidentally fried the processor or some other disastrous event occurred. So I did a processor self-test by pressing the Monitor key first and then additionally the on/off bar on the keypad. This initiates a self-test sequence. And I finally saw my first error code on a 8000: TE8!

The manual yields a rather cryptic cause for this error:

"Error TE8:    Defect IC:9IC4(RAM)      Or short pin to chassis: 10-11"

Initially I thought I had really fried the slave processor that is responsible for the relays...but then ,after my initial panic subsided and normal brain functions kicked back in, I thought that my maxim "silicon usually dies last" should apply here too, since even a complete main power supply failure and dramatic short circuit etc...would have a hard time killing anything on the processor board. There is simply no direct connection. Furthermore, when I tried to turn the unit on again after an hour or so, it worked normally again indicating a healthy state of affairs on the processor end of things (silicon either is alive or dead...rarely there is an intermittent state in my experience). Also, once it worked again, the error code went away and I got this from the self-test:

A happy Test Passed (TP).

2) The second indication toward the oscillator issue came from an oscilloscope measurement I made after the error code episode:

The probe was hooked up to the base of 6TR11, which is connected to Pin 16 of 9IC4 via some resistors. At this point the unit was working and there was no rattling from the relays at all. But you see some type of digital random oscillation of the signal that normally should be just near-0V when the unit is on. The digital character of the signal already suggested to me that there might be a processor issue...I watched this for some time and then it changed to a more aggressive behavior:

A much stronger deviation from 0V and still pretty random and close to switching 6TR11! And then it went completely crazy and the relay clicking started again as 6TR11 did its job. I did not save the measurement at that point since I was busy turning off the unit as fast as I could...;-). This finally put me on the right track, however, and I started looking into 9IC4. The same random signals were also directly visible at Pin 16 of 9IC4, just on a 0-5V scale since at Pin 16 we are directly at the source of the signal and there are no resistors over which the voltage drops. This told me that indeed the processor was going 'crazy' occasionally. This combined with the error information above that alternatively to a processor malfunction also Pins 10-11 could be 'grounded' finally made me see the light since the oscillator crystal is connected to Pins 10 and 11!

I wiggled the crystal a bit and indeed, I was able to cause the Pin 16 signal to change from completely quiet 0V to the above shown random signals and to 'crazy' causing relay rattling. So I guess what happened was that the clock of the processor started to have random hiccups causing timing issues while processing its firmware, which then caused the normally constant output at Pin 16 to become random triggering the relay circuitry.

I ordered a few 2MHz crystals from Newark (21M6819) along with matching 18pF resonator capacitors (46P6436) - the original crystals run on 12 pF capacitors, i.e. they need to be exchanged along with the crystal to get a proper oscillator signal. After a few days I received the parts and put them in. This shows the original crystal with the two capacitors (brown, right below):

This shows the board with the parts removed:

This is the new smaller crystal in comparison to the original one:

Like most components modern ones are considerably smaller than the original ones...and this finally shows the new units implanted (I also exchanged the 9IC3 components assuming both the original crystals were from the same batch...;-):

The exchange is straight forward, but the right capacitors are difficult to remove since one of their legs is fed through a hollow via, which is a bit of a pain to unsolder due to the very small space between lead and via insert. One has to hold the solder tip to the via to liquefy the solder and then pull the cap out...three hands would be great for this process...;-)

One more thing: It is crucial to cut the leads on the solder side of the board very short to prevent short circuits to the EMI can that encloses the processors:

I had them too long initially, and I got some really spectacular readings on the displays when I turned the unit on. Essentially, it became completely unresponsive showing some random zeros on all of the displays. Another near-heart attack...;-). Cutting the leads solved that problem...live and learn...;-) 

I am running the unit now for a couple days back in the living room and it seems the issue has gone away. Also wiggling the crystal did not cause the issue anymore. So I am assuming we are back in business with this lovely Beomaster 8000!

Remarkably, a few days after this happened to my 8000 I received an email from an Australian customer whose 8000 I restored a couple years ago telling me that his Beomaster developed the exact same symptoms (I am sending him a couple crystals and the capacitors plus some instructions...this is Beolove!).

So I am thinking that exchanging the crystals should become a standard part of any Beomaster 8000 restoration...two at the same time is a rather strange coincidence, which points to a systematic issue with these crystals. Probably the leads are delaminating from the crystal material after 30-35 years...

Time to get back to my other restoration projects!

Beogram 4002 (5514): RPM Performance Measurement after Restoration of DC Motor and Control System

After rebuilding the DC motor and the control system of the Beogram 4002 (5514) that I am restoring right now, it was time to do a RPM performance test. I set up my BeoloverRPM device that allows the long-term logging of the platter RPM, and then did a 24 hrs test:

After the data was logged on my computer, I graphed it relative to a measurement that I took when I initially received the unit for restoration. This shows the two curves in comparison:

The red curve shows the performance before the restoration. The typical telltale sudden negative RPM spikes are visible that indicate failing brass sleeve bearings in the DC motor. This is very common for DC motor Beograms. In this case the spikes are still fairly moderate, but would have been audible since they are in the 1% range. The black curve shows the performance after rebuilding the motor and the installation of new RPM trimmers and RPM relay. The large spikes are gone and the variations are much smaller. There are still some variations, which are related to the intrinsically slightly erratic performance of the analog motor control system, which is affected by temperature changes and probably the moon phase (that was a joke...;-). Some Beograms show them others do not. At this point I do not have an answer why they sometimes occur. The good news is that they are fairly slow over time and also small enough that one cannot hear them. The short term 'wow and flutter' RPM changes (the high frequency 'noise' on these curves) are within the spec given in the service manual (0.05%).  So this Beogram is back in business.

Beogram 4002 (5513): More RPM Measurements

This is a follow up on yesterday's post about this Beogram 4002 (5513). I did an overnight RPM performance measurement with my BeoloverRPM device. This is the curve that I measured:

This result is pretty much as good as it gets for the DC motor versions. Minimal <0.05% variations over time satisfying the specifications stated in the service manual. The initial slope during the first couple hours is usually observed in the 551x and 552x models. I currently think it is a result of D13 (the Zener diode that stabilizes the DC motor supply) and R12 with which it forms a pretty stiff (i.e. high current) voltage divider to set the base voltage of TR2 heating up. After a while the temperature stabilizes at a certain value in equilibrium with the environment. The band gap change in the semiconductor of the Zener changes its Zener voltage and with that the motor slowly gets a bit more power until a sable condition is achieved. This 'ancient' voltage regulator design is a bit of a drawback of the DC motor versions of the 4002. But one should not get to worked up about it since the change is pretty slow, and only represents a total ~0.3% change, hardly noticeable by most listeners. I am thinking this phenomenon could possibly be alleviated via the adaption of the circuit to use a modern voltage regulator. 

Beogram 4002 (5513): Initial RPM Performance Characterization

This is a follow up to my initial post about this Beogram 4002 (5513). In the meantime I did a RPM performance evaluation of the unit using my recently developed BeoloverRPM device.

This measurement clearly demonstrated the suspected sudden speed variations. This graph shows the RPM vs. time over an approximate 24 hrs period:

The sudden negative spikes suggest that the DC motor control system and the motor itself needs an overhaul.