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): Replacing the Light Bulbs of the RPM Control Panel with LED Assemblies

A crucial step to make 400x Beograms 'fit for the future' is to replace their incandescent light bulbs with LEDs, which tend to last for a very long time. This helps preventing future breakdowns and trips to Beolover's bench. The RPM control panel of the Beogram 4004 contains two bulbs that provide the scale illumination. This shows the removed panel of the 4004 that I am currently restoring:

The first step is removing the bulb covers:

Then the bulbs can be unsoldered:

And replaced with the LED assemblies. This shows the assemblies pre-installation:

and after install:

They are designed to be 'drop-in', i.e. just connect like the original bulbs. The final steps are putting the covers back on

and trying them out:

The 'natural' incandescent-like glow is a result of using carefully tuned red/green LEDs on the assemblies. On to the DC motor!

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): Replacing the Light Bulbs in the RPM Trimmer Panel with LED Assemblies

The two remaining light bulbs to be replaced in the Beogram 4004 (5526) that I am restoring right now were in the RPM trimmer panel. It is a good idea to replace these bulbs with LED assemblies since they can influence the RPM stability of the platter motor due to their radiated heat that can change the resistance of the RPM trimmers in some cases.

This shows the RPM trimmer panel after taking it out:

I removed the bulb covers

and extracted the bulbs:

This shows the LED assemblies:

I installed them:

and put the covers back on:

This shows the 33 RPM scale illuminated by the LEDs:

The LED assembly combines red and green LEDs to achieve a 'natural incandescent' look, while providing red photons for properly illuminating the scale hand.

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 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 4004 (5526): Replacement of Incandescent Bulbs in RPM Trimmer Panel with LEDs

Before the DC motor in a Beogram 4004 (5526) can be restored and tested it is a good idea to rebuild the DC motor control section of the main PCB and replace the incandescent bulbs in the RPM trimmer panel. The RPM stability of the platter drive system depends on all of these components. This shows the RPM trimmer panel as removed from the Beogram:

I removed the covers

and unsoldered the light bulbs:

Then I installed the LED replacement assemblies:

They are drop-in replacements for the light bulbs and include all necessary parts allowing running the LEDs from the 24V supply of the Beogram:

The covers fit like before:

This shows the 33 RPM LED in action:

The LEDs look very similar to an incandescent light bulb. This is the result of using red and green LEDs that are carefully tuned to resemble the emission spectrum of an incandescent light bulb.

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 4002 (5513): Replacement of Sensor Arm Bulb with an LED Assembly

After rebuilding the DC motor of the Beogram 4002 that I am restoring right now it was time to replace the light bulb in the sensor arm. This bulb is critical for the record detection circuit, and replacing it with an LED makes sense since LEDs last much longer than light bulbs. This shows the bulb compartment extracted from the arm together with the flexPCB based LED assembly:

The LED assembly 'mimics' the light bulb in terms of drawn current to be compatible with the light bulb detection circuit. This circuit monitors the current through the bulb, and disables the record detection circuit in case the bulb dies. LEDs draw much less current than the bulbs, and therefore an additional current path needs to be provided on the LED board in parallel to the LED itself.

The LED board folds into the compartment:

After soldering it in, I started the Beogram to see if everything works:

The warm-white LED has enough red emission to render the B&O logo properly red as if it were illuminated by the original incandescent bulb.

The final step of any light source replacement in the sensor arm is to check if the ribs on the platter are properly detected. TR3 amplifies the sensor signal and its collector needs to be at 4V DC when there is no signal (record present). Unfortunately, the circuit design of the 4002 biases the base of TR3 via a single resistor connected to the collector. This makes the working point of TR3 dependent on its current amplification factor, which can vary widely between transistors of the same type. Hence, many Beograms have a too small voltage at the collector of TR3, reducing the signal generated by the ribs on the platter as they go by underneath the sensor. This can be rectified by replacing R26 (1 MOhm) with a 2 MOhm multi-turn trimmer:

I usually install the trimmer first on the solder side to be able to adjust the collector voltage  with the board installed and the turntable on. Once the 4V are set, the trimmer can be installed from the component side:

The final step is to measure the sensor signal at the collector with the platter spinning underneath:

This Beogram passed the test with flying colors: The signal is very close to the prescribed 6Vpp. If the signal is too small, then TR3 is not biased correctly, and if the signal does not go to zero when a rib goes by, then the LED or the bulb is not in the correct location within the bulb compartment of the sensor arm.