Beogram 4002 (5503): Restoration of the Carriage Position Sensor - Development of a New Beolover Part

The Beogram 4002 (5503) that I am currently restoring had a damaged carriage position sensor photo resistor housing, which had broken off mounting tabs:

I removed the photo resistor housing:

This shows it from the back:

After pulling out the bottom part, the photo resistor can be extracted:

I designed a replacement housing that I can print on a 3D printer. It consists of two parts. This shows them together with the extracted photo resistor:

This shows how to integrate the photo sensor with the assembly:

First straighten the leads of the photo sensor and put it on the insert part like this:

Then slide the main housing over the insert with the photo resistor:

This shows the assembled sensor from the bottom:

Now the assembly can be inserted into the original mounting cutouts:

Once the sensor has been clicked into the PCB, the leads can be soldered to the pads:

I decided to also replace the position sensor light bulb with an LED. This shows the original lightbulb lit up in its housing:

Removal of the housing reveals the light bulb soldered to its two pads:

The right pad is the 24V rail, while the left pad connects to GND via 1R91. It is easy to replace the bulb with a LED: Basically any LED will do. I selected an amber LED with a 2.1cd light output at 20mA that I had in my stash. Since it runs from 24V in this setup, a resistor needs to be put in series to limit the current. I selected a 3.3kOhm resistor that limits the current to about 7mA. This current lights the LED up in a low key way, similar to the output of the light bulb:

This shows the entire restored carriage PCB (I also replaced the 47u electrolytic capacitor and the 8.2 Ohm solenoid resistor):

After I installed the carriage again I tested the sensor output by measuring the collector voltage at TR23, which should give clean 24V signals every time a black segment of the 'plexiglass ruler' gets pulled between the LED and the photo resistor. This shows the signal that the 45 RPM pattern caused, while the carriage swept the platter:

This looks like it should, i.e. we can conclude that the restoration of the carriage position sensor was successful.

Beogram 4000: MMC Cartridge Mount Restoration

I recently received the tonearm of a Beogram 4000 located in Western Australia. Like in many Beogram 4000s the MMC cartridge mount had broken off.

This Beogram still had the 'old style' tonearm, that elegantly plugs into the base of the arm, instead of featuring soldered signal wiring. However, this makes it more difficult to replace the MMC mount since the insert that goes into the aluminum profile tube has an adapter at the rear that has spring loaded contacts that 'grab' the circuit board that reaches into the back end of the arm to make contact. This shows the sad condition of the MMC mount as received:

The first step of any MMC mount replacement is removal of the broken one. They are usually glued into the arms. Luckily, the glue softens when the arms are 'cooked' for 30 min or so. I heated this one together with the also broken one from a Beogram 4000 from the UK that I am restoring right now:

Replacement of the MMC mount is part of my standard Beogram 4000 restoration package since all mounts seem to be quite brittle at this point in time, and it is an unpleasant event if the broken off tab sticks deep in a cherished $800 rebuilt MMC20CL...This shows the liberated insert after the cooking process:

This is the 3D printed replacement part with installed flex-PCB based contact traces:

The back part with the plug-in contacts mounts onto the thin tab at the end of the part:

This shows the signal wires soldered to the new mount:

And this picture shows all the component put back together. I was able to re-use the bottom part of the mount with the grounding contact for the cartridge housing:

After checking all leads for continuity (sometimes these thin wires can be broken inside the insulation) the next step was inserting the assembly into the arm tube:

I put a bit of white wood glue on the sides of the insert when I push it in to make sure it stays put when cartridges are mounted/pulled off. The defunct cartridge makes sure that the front end components of the assembly are positioned correctly to receive cartridges without any gap between housing and arm tube:

And this shows the final result of the operation:

This tonearm is ready to travel back to Western Australia!

Beogram 4000: Replacement of Broken Off MMC Cartridge Mount with 3D Printed Replacement Assembly

I recently received the tonearm of a Beogram 4000 from Belgium. It had a broken off MMC cartridge mounting tab. Sadly, this happens to many Beogram 4000s since they used a plastic that seems to go brittle over the years. Luckily this Beogram 4000 has the old-style MMC adapter insert that connects via spring terminals to a narrow circuit board that sticks in to the arm from the back. This allows easy removal of the arm by just removing one screw and pulling it out. This shows the arm as received:

The plastic tab that inserts into the MMC cartridges is missing and only the flex PCB with the contact traces remained:

It turned out that the plastic insert was glued into this particular arm. This meant I had to simmer this arm for an hour in boiling water in order to soften the glue. After I removed the arm from the hot water I was able to push the assembly out from the back with a rod that fits snugly into the arm. This shows the extracted part:

Luckily, the lower part of the MMC mount with the grounding tab was still intact and could be reused:

The next step was to assemble the replacement part. The first step was to glue the new nickel and gold plated contact traces into the 3D printed plastic part replacement, and transfer the wires over:

Then I assembled the insert. I used a damaged cartridge for positioning the lower part with the grounding tab that it was flush with the installed cartridge. Then I glued the back part with the spring contact tabs onto the 3D printed part:

This shows the back contact tabs after assembly:

The next step was to insert this assembly into the arm in a way that the mounting screw would go straight into the back part while ensuring that the cartridge was snug with the aluminum tube. This was the result:

This is how the MMC mount looks like without cartridge:

Pretty close to the original! This arm is ready for duty again!

Beogram 4000: Replacement of an 'Old Style' MMC Cartridge Mount with a New 3D Printed Assembly

The Beogram 4000 that I am currently restoring also had a cracked MMC cartridge mount. Unfortunately, this is a problem common in many Beogram 4000 since the plastic B&O used for the cartridge mount seems to get brittle as it ages. Later Beogram 4002 and 4004 have cartridge mounts made from a different plastic, and it rarely happens that they break.

The first step was to remove the tone arm to be able to push the plastic assembly out of the arm from the back. Later 4000s have a mount that is glued in and one can remove it by 'cooking' the arm for a while in boiling water to soften the glue until one can push the mount out. More details about this procedure is posted in this blog post.

This post is about doing this replacement for an 'old style' arm, where the mount assembly is bolted into the aluminum profile of the arm. This shows the back end of the arms. The tone arm up front can be removed by taking out the screw that holds the aluminum profile to the arm base:

Then the arm can be pulled out:

Do this carefully, since there is a little spring on the excenter that allows adjusting the arm forward and backward (there is a small plate on it, that has already been removed in the above picture, but is shown in the 2nd picture below under the arm). 

Once the arm is off a small PCB is revealed that makes contact to the MMC mount assembly in the front end of the arm:

Turn the arm around, and the screw that holds the cartridge mount assembly in place can be accessed:

I removed the screw, and then carefully pushed the plastic assembly out from the back of the arm using a 8 mm diameter rod with a smooth end (to not damage the back end of the assembly) that fit into the aluminum profile. The extracted parts are shown in the upper part of the picture below:

The assembly consists of three components: (1) A 'carrier' that also has the tab (broken off) onto which the cartridge is stuck. The tab end of the carrier carries the flex circuit board that has the four traces for making contact to the coils of the cartridge (it came off due to the broken of tab). (2) A separate part that carries a grounding tab to make contact to the metal body of the cartridge, and (3) a part in the back that makes the connection to the PCB reaching into the back of the arm.

Below the extracted parts, the new plastic parts that I designed for this repair are shown. They were printed at Shapeways in black PA12 plastic, which is a sturdy engineering plastic that has a bit of flexibility while also being pretty stiff. Perfect for a cartridge mounting tab that is not supposed to break off when it is torqued a bit.

Since in this case also the grounding tab was damaged, 

I needed to design a replacement for this component, too.

The challenge was to find a suitable metal strip for fashioning a suitable grounding tab. After a bit of head scratching I had the idea to use an 'ammo pack' strip from Molex-style connector terminals:

I removed the terminals and plated the ammo strip with nickel and then with gold. Then I cut a small part off and bolted it into the 3D printed plastic part and bent the end into a suitable tab shape:

The next step was to glue a new flex PCB strip with the coil contacts (also Ni/Au plated) into the business end of the carrier part:

Then it was time to transfer the wiring:

After the soldering was done (use a small tip and be quick not to damage the plastic parts), the insert could be assembled together with the original back end part:

For assembling the ground tab part with the carrier, it is a good idea to stick a cartridge on it, which allows aligning the ground tab part with the back plate of the cartridge. I fixed the ground tab part in place with a dab of super glue gel on either side. The back end was glued on with contact cement (like in the original assembly).

And now came the interesting moment! Would this new assembly line up with screw hole in the arm tube when inserted into the tube?

Luckily it did:

While the cartridge sat snug on the end of the aluminum profile without a gap:

Perfect! This spared me another design iteration to make things match. It rarely happens that a part like this really fits on first attempt, but this time I was lucky! This shows the new cartridge mount with removed cartridge:

Beolovely! 

Time to put the arm back into its place:

A few more adjustments and it will be time to give this Beogram 4000 a first spin!