Beogram 4000: An Entertaining Afternoon Exploring the Record Detection Circuit

While exploring an issue with the differential stop circuit of the Beogram 4000 that I recently finished restoring, I noticed another strange issue: While record detection worked at the 12" (LP) set down point, it often failed at the 7" (singles) point, i.e. when starting the deck without a record on the platter, it would pass by the LP point, but then set down at the singles point.

But let's first have a look at the circuit diagram: This shows the relevant section covering the detection system:

The sensor signal from BP100 on the left is fed into the base of TR14 via C9. TR14 provides an initial amplification of the weak sensor signal that then is processed further into a digital output signal for the control system. The signal is filtered in a RC network and fed through TR15,16,17,18 which essentially provide current and signal inversion resulting in a digital output signal from the collector of TR19, which is the "DR" input to the control logic in the keypad. DR is 6V if a record is present and 0V if there is no record on the platter. 

So the first step of my investigation was to measure IN and OUT signals of the circuit to see if the issue is in the circuit (and not in the control logic). This is what I got:

The green trace is the amplified sensor signal at TR14. We see the usual oscillation as the ribs on the platter pass underneath the sensor. These oscillations are as specified (~0.2-2.2V), i.e. my LED replacement of the light bulb was still working fine (as expected!...;-). The yellow graph is the DR signal. We see that in the beginning as the sensor arm passes the small gap between home position and platter the signal is 6V as it should be (the gap has no ribs, i.e. it looks like a record to the detection system). Then, when the oscillations start on the green curve (sensor is now over the platter), there are a few strange oscillations in the DR signal before it settles into the prescribed 0V indicating that there indeed is no record. However, as the carriage travels further in, these oscillations start again and then even temporarily give way to a constant 6V stretch before the oscillation starts again. Absolutely not like it should be! When everything works properly the yellow signal stays solidly at 0V until the carriage returns home. 

After having a bit of a think I concluded that TR19 was not providing enough current to the three logic gates that are fed with the DR signal. From the FHC131 datasheet I learned that these ancient gates take an impressive 2 mA per input when they are pulled low. So it seemed that TR19 was not fully on, causing its collector to drift towards 6V. The oscillation is probably a result of the fact that when CE goes high resistance, the BE current is concurrently reduced allowing the voltage from the (feeding) transistor TR18 to recover until the process starts anew.

After this illuminating moment I wondered about the root cause of this issue. Often one can find circuit problems by simply looking at the circuit to see anomalies like heat traces or the like. In this case my eye fell on the strange trimmer that one finds soldered to the main board on the solder side in most Beogram 4000s:

It is not on the circuit diagram. It is a 50k trimmer that connects R37/C13 to GND (I added it to the above diagram in blue). I checked the four 4000s that I have currently around, and their trimmers were all set to the maximum 50k value...somehow this suggested that whoever adjusted them seemed not to think much of them, trying to reduce their current draw as much as possible. At this point I am a bit mystified why the trimmer was added as an afterthought by the designers...there must have been some issue with the circuit under some circumstances, maybe a parts tolerance issue.

Anyway, experimentation and hunches often yield a fix and so I hooked up the oscilloscope to the collector of TR15 and measured with and without the trimmer connected. Disappointingly the curves looked very similar like this:

About what the diagram above suggested. This frustrated me a bit, but there was no stopping at this point and I marched on, connecting the oscilloscope to the collector of TR18, which feeds the base of TR19, i.e. if TR19 has trouble staying on while delivering 6 mA, there needs to be a difference in the TR18 output. And bingo, the trimmer made a big difference there. This is the signal with trimmer:

We see big spikes towards 6V, while the diagram suggests spikes no larger than 1.4V. I took the trimmer out and measured this:

Very small spikes, meaning that in this case TR19 was basically permanently on since TR18 stays fully on, i.e. TR19 should have no trouble providing 6 mA into the gates of the control system. And indeed, with the trimmer gone, the 4000 record detection mechanism started working very reliably. So the lesson learned here is to not only check the signal at TR14, but also at TR18/19 to make sure that the DR signal is stable. Alright, time to enjoy another record one this lovely Beogram 4000! Let's see if that was the last act of this eventful restoration.

**************************Note added in proof***************************************

The great Dillen of Beoworld.org sent me a message after I posted this. He indicated that this trimmer was added to the 4000 circuit as a way to adjust the sensitivity of the circuit due to the advent of transparent vinyl records. If there is a transparent record, the ribs of the platter are visible to the sensor at varying 'contrast' depending on the color and widely varying translucency of such records.

So I went ahead and experimented a bit more using a translucent red and completely clear record. This particular Beogram 4000 was able to detect both without any ambiguity with trimmer absent. 

However, this does not mean that this is the case for all Beogram 4000s, since the sensor circuit is designed with analog amplification stages that are biased using a single BC resistor. This makes the gain dependent on the individual current gain value (Hfa) of each transistor, which can vary significantly, even if the transistors are from the same production run. So this issue is something to keep in mind when working on the 4000 and a verification of proper detection of clear records should be part of the 'standard restoration repertoire'. The joyful exploration of analog control systems continues!..;-)

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 4000: Hood Restoration

The last step of my current Beogram 4000 project was to exchange the badly shipping damaged hood with one from a donor 4000 and restore it. The hood that I had available had the usual scratches from careless storage but otherwise was in good condition:

I sanded the surface down below the scratch depth with 320 grit sand paper. Then it was time to polish it back to shiny and translucent. This shows the hood after the 800 grit step:

almost there. After several more polishing steps it finally looked pretty good again:

Good news for this Beogram 4000! Now it looks almost like new again! A few more days of testing and then it should be ready for its big trip to Hong Kong to its new owner!

Beogram 4000: Cosmetic Items, Final Adjustments, and a Test Drive with George Duke on MPS!

The restoration of the Beogram 4000 from Germany is nearing its end. I adjusted the sub-chassis and arm height and then the arm lowering limit:

The correct adjustment of the arm lowering limit is an important safeguard against stylus damage should the record detection mechanism ever fail. The next step was the calibration of the tracking weight. I always install a M3 nut and washer to bolt the counterweight position safely into place, which is much more stable than the original setup with the locking washer. This shows the original setup:

and the M3 nut in place:

Once that was done I calibrated the arm weight with a digital scale:

The next step was to adjust the tracking feedback:

The blue pot that allows the adjustment of the tracking sensor LED comes in handy for fine tuning of the feedback. 

The final adjustment was the motor voltage. This shows the oscilloscope traces for 33

and 45 RPM:

Nice smooth sine curves as they should be!

The final steps were to replace the scratched and dented main aluminum platter and the arm cover. Conveniently, someone recently parted out a 4002 5501 Beogram and sold the arm cover on ebay, which I snapped up for this project. The early 4002s have the same arm cover like the 4000, i.e. it states "tracking weight adjusting" (instead of "tracking force adjustment" like one the later 4002s) on it. The aluminum platter was exchanged with a pristine one from a donor 4004 that I recently acquired.

And finally, it was time for a test spin. I selected a recently bought record by George Duke: "Liberated Fantasies" on the awesome German MPS label (MPS stands for Music Produziert im Schwarzwald - music produced in the Black Forest...;-). I really like the bunch of records that George Duke recorded in Villingen...in my opinion the most creative part of his career. Here is an impression:

Considering the initial state of this shipping-damaged Beogram 4000, it was an especially happy moment to see and hear it executing flawlessly. Mr. Duke is smiling, too!..;-)

Beogram 4000: Glueing a Delaminated Platter Back Together and Installation of New Cabined Guidance Washers

The Beogram 4000 that I am restoring right now unfortunately had suffered some transport damage due to the inept packaging job of the ebay seller. Since he did not take out the platter, it bounced around and in the process delaminated into two parts. This shows the bottom and top parts of the platter:

It is interesting to have a peek inside the platter assembly. It appears that the lower part of each platter, which provides the main weight of the assembly, was individually balanced, similar to what is done with car wheels after putting on tires. They did that by drilling out small portions of the materials on the too heavy side of the platter (you can see the red pen mark from the balancing process next to the drill pattern):

It is also clear from the markings on each of the ribs of the lower platter that both parts were glued together. Inspired by this finding I used a slow curing epoxy glue, which I applied next to the original glue spots:

Then I placed the top part off the platter and aligned it as good as I could. A 4002 platter served as weight to secure both halves in place while the epoxy cursed for 24 hrs:

In the meantime, I replaced the old cracked cabinet guidance washers with new 3D printed ones. This shows the original center washer at the front of the enclosure:

This one is a strange design aspect of the 4000...it seems to be a flat Teflon washer that has no guiding feature. This one is best replaced with a 3D guidance washer printed in black, since white ones are visible from the front through the crack between the cabinet and the aluminum panels:

This shows the complete set of five 3D printed guidance washers:

They are available through the Beolover Shapeways store (see link under 'pages' on the right).

Beogram 4000: Upgrade of Signal Path with New DIN5 Plug and a Grounding Switch

As usual the original DIN5 plug on the Beogram 4000 that I am currently restoring was badly oxidized. This shows the original plug:

I replaced it with a new all-metal plug that has gold coated terminals:

Another useful upgrade of the signal output of any Beogram is the installation of a switch that allows connecting signal and system grounds. This is a useful feature if the Beogram is ever to be used with a DIN5 to RCA adapter. Since RCA cables do not carry the system ground, connecting it to the signal ground on the Beogram often eliminates humming. This shows the output terminal assembly of the Beogram 4000 in its original condition:

This open layout makes it very convenient to add a switch:

The shown switch position connects the two ground. Flipping the switch would open the connection.

Beogram 4000: Replacement of Sensor Arm Light Bulb with an LED

The final incandescent bulb to be upgraded with an LED in the Beogram 4000 that I am restoring right now was the one in the sensor arm. This LED is the most difficult to replace due to the compact dimensions of the compartment in which it is located. This required the design of a flex PCB based solution since this LED not only needs a current limiting resistor, but also a current bypass to simulate the presence of a light bulb drawing a much more significant current than a modern LED. This current is used by the control system to detect if the light bulb is working, i.e. the LED setup needs to draw a similar current. This shows the bulb compartment pulled out of the sensor arm:

I removed the bulb. This shows the bulb together with the already folded LED assembly:

Then I installed the flex PCB into the bulb compartment. This shows it powered up:

The warm-white LED emits enough red photons to properly light up the B&O logo like the original bulb.

Whenever the sensor arm light source is replaced the record detection circuit needs to be tested to make sure the absence of a record is properly detected. This shows the signal measured at the collector of TR14, which amplifies the sensor response for the control system:

It is as specified in the manual, having an amplitude of about 2V spanning from 0.2 to 2.2V. All good in the sensor department!

Beogram 4000: Restoring the Carriage Position and Solenoid Driven Switches

After rebuilding the keypad cluster of this Beogram 4000 it was time to see to the remaining mechanical switches that control the Beogram's performance. There are two clusters, one beneath the carriage where the switches determine the position of the tonearm, and the other next to the solenoid. The solenoid activated switches are responsible for the activating the tracking mechanism, opening the outputs when the needle is in the groove and to regulate the solenoid power down once the arm is lowered.

This shows the carriage position switches. For getting to them it is best to drive the carriage inwards and remove the red position indicator assembly. Then the board is fully accessible:

After unsoldering the two leads that power the carriage motor and removal of two screws the board can be pulled up and turned around:

Some oxidation is visible on the back of the terminal tabs. For some reason the carriage position switch terminals are usually less oxidized than the other switches under the keypad and next to the solenoid. I removed the terminals

and coated them with gold:

Then it was time to solder them back in and clean the plastic plungers that activate the switches from 40 year old hardened grease. Then I turned my attention to the switches next to the solenoid. The single switch assembly that controls the solenoid power can be directly accessed after removing the two screws that hold it in place:

I extracted the terminal and then removed the assembly that holds the output and tracking activation switches:

This assembly needs to be taken apart while not ripping the thin wires off that are connecting the switches to the control system:

Once this was done I extracted the three terminals. This shows all four terminals as extracted:

'Beautifully' oxidized, I'd say! I brushed the oxide off with 2000 grit paper and a fiber glass brush and then coated them with gold:

That looked much better! I put everything back together and gave the system a first test...Good news: so far everything seems to work...the carriage is now setting down the arm at the 12 inch point and the solenoid actuated properly. A significant milestone has been reached in this restoration!

Beogram 4000: Restoration of the Keypad Cluster

After replacing the electrolytic capacitors and restoring the AC motor of the Beogram 4000 that I am currently restoring, it was time to rebuild the keypad cluster. The PCBs below the keypad house the control center of the Beogram, which is a unique early digital control system based on TTL logic chips. This logic system comes to its operational conclusions largely based on inputs caused by the many mechanical switches throughout the turntable. The keypad contains 8 of them enabling user interaction with the Beogram.

The keypad is held in place by a single screw, which was missing in this Beogram:

An indication that 'human interaction' had taken place earlier (which was confirmed once I looked at the switches on the PCBs - see below). I took the pad out and opened it up:

The upper board contains some of the logic chips and the light bulbs that illuminate the position indicator and the RPM trimmers. The lower PCB is populated with the eight switches for the keypad and one more logic IC. As usual, the switch terminals were heavily corroded:

I removed the board from the keypad, which is necessary for extracting the switch terminals:

This shows the side of the board that houses the switch actuators. The small green, white and grey 'plungers' are actuated by the keys on the keypad, which pushes the switch terminals on the other side of the board making or breaking the associated contact. Note the location of the single grey plunger top left. This is the wrong location for this particular one. It needs to be in the center of the bottom row, since this particular switch is a break switch. This incorrect installation immediately explained the malfunction of the arm lowering circuit that I noticed after the restoration of the main PCB. The green plungers are longer, and therefore this switch, which is responsible for lowering the arm, was permanently open (actuated). This shows the three switch types in comparison:

The grey one is the shortest. It is used for break switches. The green one is used for make switches and the white one is for two-pole make/break switches (the << and >> keys, which have slow and fast functionality, depending on how hard the keys are pressed).

I removed the switch terminals:

After removal of the oxide layer with 2000 grit sand paper, I coated the terminals with a gold layer:

and then soldered them back into place:

The final step was to replace the light bulbs with LEDs. The position indicator scale lights were replaced with custom designed LED boards (available to other B&O enthusiasts), each containing two red-green LEDs tuned to yield an incandescent-like sheen. The RPM trimmer back light bulbs were replaced with standard red LEDs and current limiting resistors:

This shows the LEDs in action after installation:

After that it was time to put the keypad back together. Unfortunately, the center key was not attached to the keypad, i.e. I needed to reinsert it. This can be difficult and there is a danger to scratch the other keys while doing it. For this reason I used 3D printed tools that I developed earlier, which make this process much easier. They allow pre-bending of the spring that holds the pad in place and that allows it to bounce back after pressing it:

Once the spring is bent up, it is fairly easy to get the pad on it. Careful removal of the printed taps releases the spring holding the key in place. And this shows the pad installed:

A test revealed that all keys are now working properly. On to gold coating the remaining switches below the carriage and in the arm lowering mechanism.