Beogram 4002 (5501): Replacing the Sensor Arm Bulb with a SMD LED on a FlexPCB

***************************Sequel to this post see here******************************

I finally was able to design a LED based upgrade to replace the sensor arm incandescent light bulb in Beogram 4002 units. This was the most difficult bulb to replace due to the tight space available in the sensor arm and the requirement to put out an incandescent looking light spot, while being able to properly back-illuminate the lovely orange-red B&O logo at the end of the arm. 

I quickly realized that only a flexPCB would be able to do the trick. My first experiments involved high brightness amber LEDs combined with a red LED to provide some longer wavelength photons to give the B&O logo a more red appearance. Here is an impression of the design:

While this worked technically, the light spot on the record was way too orange, while the red LED could not compete with the amber unit:

Worse, the red LED could be seen reflected in the record surface separated from the amber spot due to the different projection of the red LED through the lens that focuses the light on the record surface. Anyway, this was not Beolovely, and so I did some research and finally found a latest-development warm white LED with a 95% color rendering index (CRI) value, which promised that this LED would be able to correctly illuminate the B&O logo while looking fairly natural when reflected from the record. This is how my board looked like with it on there:

And fired up:

It is a pretty warm white (2600K temperature equivalent). And this shows the board folded into the sensor arm cavity:

And in action:

Pretty authentic look, even though of course the light spot is not a projection of a bulb filament, i.e. it is a bit more homogeneous than the original bulb pattern. But I think without pointing this out to someone, it would be hard to tell that a white LED is at work here. So far so good...but the real test is of course whether this setup can make enough signal at the sensor output for a reliable detection of the absence of a record. For this I measured the sensor signal on the platter without a record present. This is the signal after being amplified by TR8:

Note that the dips go all the way to zero, which is what is required to properly disable arm lowering when there is no record. Bulbs rarely deliver a stronger signal, so this seems adequate!

Another critical factor is the current draw of the LED board. The Beogram 4002 is equipped with a detection circuit that disables arm lowering if the bulb filament breaks. This circuit requires about 50-60mA in the bulb circuit to enable arm lowering. So I added a resistor to the LED board to match 60mA (the LED on its own only draws about 40 mA...). This shows my bench power supply driving the board with 5.5V (same voltage as is applied to the bulb in the sensor arm):

It appears we have a functioning prototype in play! All that needs to be done now is to clean the board design up a bit to arrive at a final version. This is Beolove!

Beogram 4002 (5501): Installation of New Transport Lock Bushings and New Cabinet Guidance Washers

The restoration of the Beogram 4002 (5501) that is on my bench right now is coming slowly to a conclusion. After restoring the worn keypad surfaces, I moved on to the transport locks and the cabinet guidance washers. The transport lock bushings should always be rebuilt before the floating chassis is adjusted that it sits centered in the enclosure.

There is a video about installing new transport lock bushings in my Beogram 4002 playlist. The bushings can be ordered directly from my Shapeways store. This is the link to the part. You will need two per transport lock.

Here are a few impressions: The picture below shows the situation (as found in most Beogram 4002s with the orange bushings). The bushings are completely fragmented and I had to remove the fragments with a vacuum from throughout the enclosure.

This shows one of the orifices in the sub chassis after cleaning up the fragments:

Installation of the new bushing is simple. They come in two parts which are installed from the top and bottom:

Then the lock can be reinstalled:

It is important to put the bolt in in its correct orientation (the longer end points down). This shows one of the locks fully done:

After that I replaced the cabinet guiding washers. The older 4002s all came with plastic washers, which are often cracked. The 5501 even has the same strange front-center washer like the 4000:

I always replace these with my reproduction parts since they do not guide the cabinet, just hold it down. This particular washer is best replaced with a black guiding washer. Otherwise it can be seen through the crack between cabinet and aluminum panel:

There is a video about this procedure in my Beogram 4002 playlist. The guidance washers can also be ordered directly from my Shapeways store. This is the direct link to the part. You can still use the (lower priced) white version, just use a Sharpie pen and 'paint' it black. The selective laser sintered (SSL) Nylon material is a bit porous, i.e. the ink nicely soaks into the bulk of the washer, giving it a durable black color.

Beogram 4002 (5501): Worn Keypad Restoration - My Process Finally Appears Reproducible...

The Beogram 4002 (5501) that I am restoring right now had the usual worn keypad. Here is an impression of how it looked before I restored it:

As usual the start button was worst...no surprise since it sees most of the action during normal play activities. This shows the keypad after my restoration procedure:

Nice! Looks almost like new!!...It took me a few months and many frustrating attempts to perfect my approach to rebuilding these keypads. But it seems my process finally yields a reproducible outcome. Another piece of the 4002 restoration puzzle in place! 

This keypad had one more issue: One of the RPM panel clamps was broken out preventing the RPM panel from staying down:

This shows the broken part in comparison with a good one that I had 'in stock':

I replaced it and now the keypad looks perfect. But I guess it is time to develop a 3D printed replica...Beolove never ends...;-)

Beogram 4002 (5501): Replacement of End Groove Detection Light Bulb with an LED

After restoring the RPM trimmer panel of the Beogram 4002 (5501) that I am rebuilding right now, it was time to look into the not working end groove detection mechanism. This mechanism is an exciting part of this design. It uses a light bulb and a photoresistor in conjunction with an encoder wheel doubling as pulley to detect the moment when the carriage starts moving faster after the last track of a record is over. This allows cutting short the time it takes until the arm returns to the home position on records that have a wide end groove section (such as many 12" singles or some short playtime LPs).

This shows the setup:

The bulb illuminates small holes that are in the pulley that is driven by the carriage motor. The light sensor is located on the other side of the spindle bearing. Unfortunately, the light bulb fixture of this Beogram had a broken off tab requiring some tape to hold it in place. Here is a picture of the bulb assembly pulled from the bearing unit:

The tab goes into the groove on top of the pulley bearing assembly. The back part of the tab is missing. You can also see very nicely the crack in the pulley. Nick is already working on a redesign of his standard aluminum pulley for the 4002 to add the small holes necessary for the end groove detection mechanism to work. Thanks Nick!

Naturally, this was not a Beolover acceptable state of affairs and so I designed a replacement part upgrading the system with an LED.

Here are a few impressions. This is the finalized component holding an amber high intensity LED (Newark 78R6602) and a 2k resistor (Newark 26R3983) reducing the current to below spec level of the LED. The resistor sits in a small holder on the left side to keep things neat and tidy:

This shows it illuminated at 23V:

And installed:

It press-fits right on top of the pulley bearing assembly

This shows it in action after the end groove circuit is activated via a carriage position switch:

I tested it and it seems to work great.

Here is an explanation how the end groove detection system works:

This shows the relevant section of the circuit diagram:

0IL1 is the LED. The switch 8009021 is the second carriage position switch on the B-rail:

Once 0IL1 comes on, the holes on the pulley begin to illuminate OR2. This causes voltage fluctuations across 1C35 which briefly turn off TR21. This applies a brief voltage pulse to C33 via D31 and R90. Hence, every time a hole of the pulley comes in between 0IL1 and OR2 a small amount of charge is put into C33. Therefore, over time the voltage on C33 increases proportionally to that charge. Once the voltage is high enough, TR20 turns on. TR20 is connected in parallel to the end switch ("ES", A), i.e. to the control system a TR20 in "ON" condition looks like ES was triggered and the arm lifts and the carriage returns home.

You may wonder, "what happens when the lamp turns on, but the needle is still on the last track of the record, and a pulley hole comes by and C33 charges up?" This is why D30 and R91 were added to the circuit as a (competing) path to discharge C33. R91 essentially determines the time it takes to discharge C33, and it was adjusted to discharge it fast enough to compensate for the charge build-up when the pulley only moves slowly when a track is played. In the end groove, however, the carriage moves much faster, and so the pulley holes come by more often, which overwhelms R91, and so the voltage can build up on C33, finally triggering the ES process.

If TR20 is not triggered due to a malfunction of the end groove detection system (dead bulb, for example), the arm will go all the way to the end of the end grove and then ES will be triggered by the carriage and save the day. All the end groove detection system does is to spare the user the wait until the arm goes all the way to the end (and to listen to the static from the groove). A typical B&O approach. Why not make it more perfect if you can?? This is Beolove!

Beogram 4002 (5501): Restoration of RPM Trimmer Panel

After rebuilding the PCBs of the Beogram 4002 (5501) that I am currently restoring, I turned my attention to the RPM trimmer panel. This panel had been replaced previously with the aim to repair a burned out trimmer scale illumination bulb. As a first step I re-worked the solder connections in the wires where the original panel had been cut off. This shows the situation when I received the deck:

I re-soldered all connections and installed proper diameter shrink tubing to protect the solder joints:

Then I replaced the bulbs with my LED replacement boards:

These are specially designed for the 550x Types since the bulbs partake in the RPM switching process, i.e. the LED circuits need to present a similar resistance/current draw as the original bulbs.

Like all my parts, these boards are available to other enthusiasts. Just send me an email, or use the contact form to the right. This video shows how to install the LED boards.

This shows the original situation (I already had removed one bulb when I remembered that I needed to take a picture...):

Then I installed the LED boards:

then I briefly confirmed that the LED replacements worked properly, and then I removed the white plastic fixture from the aluminum panel, since my customer wanted to use a different better-conditon panel instead of the one that was installed. This required to cut off the retaining rings on the aluminum posts after which I was able to remove the assembly:

What I found were already wavy white background strips that serve the back illumination of the ed RPM indicator hands. Wavy backgrounds cause inhomogeneous back illumination, i.e. they are better replaced if long term stability is desired. I usually replace them with white electrical tape:

After that I put the assembly on the new aluminum panel attaching it with M3 circlips and nylon washers. This shows the final outcome with the bulb covers installed over the LED panels:

And here an impression of the front side with 33 RPM selected:

Pretty! And it looks warmly incandescent (a result of using red and green LEDs to mimic the emission spectrum of incandescent light bulbs). All good again in the RPM panel department!

Beogram 4002 (5501): Replacing all Electrolytic Capacitors

After fixing the strange control system issue it was time to plow ahead with the general restoration tasks, and so I replaced all electrolytic capacitors. This is always a good idea, since these units suffer from dried out capacitors, and Ta types can actually go up in flames when they expire. So replacing them with modern Aluminum 105C types is a good idea.

This shows the main PCB in its original condition:

And after replacing the capacitors:

After this I went under the keypad and extracted the output board. This shows its original condition:

And after replacing the output relay time delay capacitor and the old Siemens relay with a modern encapsulated unit:

Like all my parts the relay is available to other enthusiasts, just send me a message using the contact form on the left.

I also installed a grounding switch, which allows connecting signal and system grounds in case there is a hum in the output chain. This shows the switch from below:

Beogram 4002 (5501): Repair of a Strange Fault Caused by a Shorted Diode D6

After replacing the reservoir capacitors in the Beogram 4002 (5501) that I am restoring right now, a strange control system malfunction remained. Here is a description:

After pressing START, the carriage would start moving but then after about 20 mm it would stop and the lights would go out. Then after a few seconds, the lights would come back on and the motor would run on 45 RPM. Once on 45 RPM, I was able to drive the carriage with the >> << buttons, but arm lowering would not work anymore. Switching back to 33 RPM killed everything again, but lowering would briefly function before the lights went out. Pretty strange all this!

Luckily the root cause turned out to be an easy fix. Here is the relevant part from my annotated circuit diagram:

After a bit of head scratching and doing some measurements, it occurred to me that a short circuited D6 would kill the 22V rail if TR2 is on, as it is the case when 33 RPM is selected. This was quickly confirmed by a 0V diode tester reading across D6 in both polarities. After replacing D6 with a new 1N4148 signal diode everything was working again properly.

This is how this fault 'worked': When TR2 is on then its collector is on GND level. If D6 is making a short, GND is applied to R2/D1 (dashed red lines). This kills the voltage on the base of TR1 which then kills the voltage on the base of 0TR1. This turns the 22V rail off and the lights go off. The time delay that was inherent in this process is enabled by C1, which preserves the base voltage on 0TR1 for a couple seconds until the capacitor is empty. At this point TR2 turns off. This allows the voltage to recover since now the anode of D6 is no longer grounded. So it does not matter if the diode is shorting. The voltage rail recovers, and that turns TR3 on via L1 and R8, and the deck is now operating on 45 RPM. Ahhh, the mysteries of analog control systems! This is Beolove!