Beogram 4000: Installation of New Beolover Sensor Arm Photocell

The Beogram 4000 from Australia that I am restoring right now kept on giving! After I fixed the tonearm wiring to get both stereo channels working further tests revealed that it would not recognize the absence of a record and eagerly put down the arm onto an empty platter. Luckily I had already adjusted the arm lowering limit and so nothing happened to my MMC20 EN cartridge.

Ignoring the absence of a record is a telltale sign for a dead photocell in the sensor arm. The circuitry recognizes a dead sensor arm bulb and shuts down arm lowering if this is detected. However, a damaged photocell is not recognized. It simply simulates a black surface on the platter, and so the arm lowers.

I pulled out the sensor compartment and closer inspection revealed that one of the two electrodes on the photocell had disconnected. It looked like this:

This photo is from an earlier post about this type of issue, where the photocell of a Beogram 4002 had come apart. This earlier post reports about my initial development efforts aiming for a suitable replacement of the original photocell. 

I removed the photocell fragments:

And then it was time to install the newly designed Beolover Sensor Arm Photocell for Beogram 4000, 4002, and 4004. It is available via the Beolover Store. This shows the business side of the part:

At the front end (left) there are three photodiodes in parallel (to ensure sufficient current generation). On the back end (right) a 3D printed alignment piece is installed that helps keeping the cell properly oriented and in its proper spot relative to the lens in the sensor arm compartment.

This shows the backside with the two contact leads:

This is a real photocell like the original. It puts out ~0.45V photovoltage when exposed to sufficient light:

This means that unlike with other photocell replacement schemes no circuit modification is necessary. This is a direct drop-in part that replaces the original photocell (B&O part 8760002)

Installation is simple: Just stick the leads into the holes in the small circuit board attached to the sensor arm compartment (it is a good idea to use a solder sucker to remove the solder from the solder pads on the backside of this board to ensure the holes are open after unsoldering the original cell):

Then insert the photocell into the small compartment where the original cell was located and align the small protrusion on the alignment piece with the hollow rivet that holds the PCB to the plastic part: 

Then press the back end down until the frontend with the photocells hits the roof of the compartment:

Then the leads can be soldered to the pads on the underside of the PCB:

This shows the underside with the red and blue photo cell leads attached:

I tested the new cell with my oscilloscope, which was connected to the collector of the sensor transistor 1TR14 whose collector DC bias was adjusted to be 1.8V:

 

The photocell yielded a strong 2.5V amplitude signal over the rotating platter. More than spec. So all is good again in the record detection department.

Beogram 4000/2/4: Update of Sensor Arm LED Assembly. No More Crumbly Flex-PCB Mess!

I recently re-designed the sensor arm LED assembly for Beogram 4000, 4002, and 4004. The previous version using a flex-PCB proved too difficult to install for some customers. My updated design is based on a small 'hard' PCB that fits precisely into the bulb compartment and uses an insert for proper positioning (it is important that the LED is approximately in the spot the lightbulb filament occupied).

This shows the new components:

And here a still from the video below that shows the LED 'in action':

The LED board produces a nice focused spot on the platter and also the B&O logo is lit up properly in the correct hue.

I updated my original sensor arm video and it shows now how to install these parts in Beogram 4002 and 4004. The video also shows how to update the record detection circuit. It usually benefits from installing a new transistor for amplifying the sensor signal. The procedure for Beogram 4000 is similar, but naturally the board looks a bit different, i.e. you will need to find the transistor for the sensor signal amplification via the service manual. Enjoy!:

Beogram 4004 (5526): (New Beolover Video!) Exchanging the Sensor Arm Light Bulb with an LED Assembly

After getting the DC platter motor finally to run properly, it was time to exchange the last light bulb in the Beogram 4004 (5526) that I am restoring right now, the sensor (detector) arm bulb with a LED. This process has still been a bit 'experimental', but I think we finally have come to a stable process that should be relatively easy to replicate by other B&O enthusiasts around the world. So I decided it was finally time to make a video that outlines this process for those who would like to implement our LED assembly (send an email or use the contact form on the right if you are interested in getting the part). This shows the LED assembly next to the bulb compartment in the sensor arm:

The video discusses the detector circuit, how to upgrade it for reliable performance, and how to install the LED assembly. Enjoy!

Beogram 4004 (5526): Replacement of Sensor Arm Bulb with a LED

After restoring the record detection circuit and the PCBs it was decided that the sensor arm light bulb should also be replaced with a LED to ensure operability of the deck down the road. These old light bulb like to fail, especially during shipping, i.e. it is a good idea to replace the bulbs with LEDs when restoring a Beogram. This shows the original light bulb in the sensor compartment together with the replacement flexPCB-based LED assembly:

I removed the bulb and inserted the LED board, which folds snugly into the compartment:

This shows the LED in action:

The LED is a warm-white type, i.e. there are enough red photons to ensure a properly colored B&O logo.

Whenever the light source in the sensor arm is replaced, the sensor response needs to be checked. The signal strength is highly dependent on the location of the light source in the compartment. The signal at the collector of the signal amplifying transistor TR3 needs to be close to 6V or better. This shows the measurement I made after the installation:

This Beogram record detection circuit is now operating better than spec (7.9V amplitude), i.e. we can confidently press the START button. 

Beogram 4004 (5526): Replacement of Sensor Arm Light Bulb with an LED Assembly

When I restore a Beogram I usually exchange the incandescent light bulbs with LED assemblies. LEDs last much longer and yield a constant light output over their lifetime. This post discusses the replacement of the sensor arm light bulb of the Beogram 4004 (5526) that I am currently restoring. The bulb and sensor compartment can easily be pulled out of the aluminum profile (if your fingernails are not cut too short..;-):

The flex-PCB based LED assembly is shown on the right. It uses a warm white LED which produces enough red photons to make the B&O logo at the end of the sensor arm light up red. I unsoldered the bulb and extracted it. The LED assembly folds into the vacated bulb compartment:

This shows the assembly in action:

Whenever the light source is replaced in the sensor compartment, the record detection circuit needs to be checked for proper functioning. A stylus crashing onto the platter without a record present can get expensive...

The first step is ensuring that the collector of the sensor amplification transistor TR3 has the prescribed 4V DC without a sensor signal present. Usually, it shows a lower voltage due to the design of the circuit that relies on biasing TR3 via a single 1MOhm resistor connected to the collector. Due to variations in the transistor gain Hfe between individual transistors of the same type, this biasing scheme is not reliable since the base bias will vary with each transistor. This can be remedied by replacing the biasing resistor with a 2 MOhm multi-turn trimmer and then adjusting it for 4V at the collector. This is shown here:

I usually install the trimmer on the solder side of the PCB so one can adjust it while the board is powered up, and then I unsolder it and install it on the component side.

Once 4V are established, it is time to measure the sensor signal at the TR3 collector when the arm is over the rotating platter. For that a jumper wire needs to be soldered to the collector and the platter installed. This shows the oscilloscope trace measured after the LED was installed:

The amplitude should be between 5 and 6V, and the valleys should go all the way to 0V. This Beogram passed with flying colors. If the trace does not hit 0V, then usually the LED (or light bulb) does not sit in the right spot. This can sometimes be a bit tedious, but after a few trials one usually can achieve a trace like the one shown here.

Beogram 4004 (5525): Replacement of Sensor Arm Bulb with LED Assembly and Calibration of Sensor Amplification Circuit

A Beogram 4004 that I restored a few years back returned to my bench for some TLC/adjustments. Along with the adjustments it was decided to also update the restoration to the latest Beolover standard, i.e. the sensor arm bulb needed replacement with an LED assembly, which was not yet available during this Beograms's first visit.

This shows the sensor arm bulb cabinet pulled out together with the flexPCB based LED replacement circuit:

After extracting the bulb I installed the LED assembly and fired it up:

The LED that is used on the board is a warm white LED, i.e. it has enough red emission to make the B&O logo light up nicely.

The next step of this procedure is to adjust the biasing of TR3 properly. TR3 is responsible for amplifying the weak signal of the photo diode that is in the sensor arm. By design the base of this transistor is biased with a 1MOhm resistor (R26) pulled up by the collector. Unfortunately, this biasing scheme depends on the transistor gain Hf, which can vary quite drastically across several transistors of the same production run.

As a consequence one rarely finds a Beogram 4002 or 4004 that has the proper manual-specified 4V at the collector. This issue can be corrected by adjusting R26 until 4V are present. This can be done by replacing R26 with a 2MOhm multi-turn trimmer, and adjusting for 4V. This shows the trimmer installed from the solder side of the board that it can be adjusted while the Beogram is running:

After adjusting the collector voltage to be 4V I unsoldered the trimmer, and installed it from the component side:

After this it was time to measure the sensor signal at the collector of TR3:

The amplitude of the signal is 6.3V, exceeding the manual specified 6V, which is good. It should not be smaller, but a bit larger is o.k. Important here is also that the signal drops (that correspond to the passing of a platter rib) go all the way down to 0V. 

This result indicated that the sensor circuit was working properly, i.e it was time to test with and without a record present. All went well. I will play it a bit more and then it will be time to send it back to its owner!

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 4004 (5526): Replacing the Light Bulb in the Sensor Arm with a LED Assembly

After replacing the transport lock bushings in the Beogram 4004 (5526) that is currently on my bench it was time to replace the light bulb in the sensor arm with a LED-based assembly. This shows the small bulb compartment pulled out of the sensor arm front:

The small flexPCB next to it is the LED circuit. Since modern high output LEDs have a much lower current draw than the original small light bulbs the LED circuit features a current bypass that adjusts the current to a level similar to the light bulb. This is important since there is a circuit on the main PCB that detects bulb malfunctions via current measurement, and the low current of LEDs would trigger this circuit, preventing the arm lowering mechanism from doing its job.

The flexPCB needs to be folded to fit into the bulb compartment. This shows the folded board next to the light bulb,

and installed in the compartment:

After replacing the compartment in the arm I tested the setup:

The B&O logo shows up with its usual red-orange glow. This is a result of using a warm white LED, which has enough red emission to properly illuminate the logo. The final step of any sensor arm light source replacement is measuring the sensor signal when the platter is spinning without a record on it. This shows the signal measured at the collector of TR3:

Each voltage drop corresponds to a black rib passing under the sensor. When everything is o.k. the drops should go close to 0V, while the amplitude should exceed 4V.  This Beogram 4004 passed the test with flying colors!

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: 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!