MMC20 Cartridge Mount Replacement: First Design Iteration

A while back Sonavor posted some shocking pictures on his thread about Beogram 400x repair and restoration on Beoworld.org. He was about to finish his restoration of a beautiful Beogram 4000 and after having had to meet challenges like replacing the line transformer and the strobe lamp, the gods of vintage hifi gave him one last challenge: When he tried the deck out, the cartridge mount broke off, probably due the previous owner having cracked the small plastic tab that inserts the electrical connections into the cartridge body and also holds it in place. A typical B&O design: Very elegant but needs a refined touch to use it properly. Here are two of Sonavor's pictures after he extracted the broken cartridge mount after dipping the extracted arm into hot water for a while to loosen up the glue:

Absolutely nothing one wants to encounter while working on such a beautiful design! He was able to fix it by extracting a mount from a lesser model, but we started wondering if these mounts can be reproduced with modern 3D printing technology. Obviously the contacts are the biggest challenge. It is interesting to see that in the 4000 they were formed by a flex circuit board that was integrated into the plastic part. In more recent models the contacts were actual Au coated Cu-Be tabs held in place with a plastic bar (see below).

Sonavor then sent me a spare tonearm with a loose mount giving me an opportunity to have a closer look (I was reluctant to extract the mount from one of my working turntables, as you can probably understand...;-). So I set out to build a replacement part. The first step was to make a model of the plastic part in CAD software and have it printed. I did that and yesterday I finally received the first iteration of this design from Shapeways. This shows the part (blue) in comparison with the original part that holds the cartridge:

I decided to go the 4000-way for the contacts using the flex PCB approach. This required to glue the board in place to the plastic part, which has a 5 mil recess to accommodate the board thickness (visible in the picture if you look closely). To achieve a good bond I designed a tool matching the shape of the PCB area. This allowed me to apply pressure to all parts of the PCB during the curing process:

I made a flex PCB with traces with the right pitch. The traces are 1 mm wide and spaced by gaps of 0.7 mm, i.e. the pitch is 1.7 mm. Here is a photo of the final board in comparison with the contacts on the mount:

Then I cut one of the strips to shape and coated the traces with solder. In the final version this will hopefully be an Au coating:

The next step was glueing it to the mount. I used standard epoxy. Superglue would probably be another option. My thinking was that epoxy is a bit flexible, giving this PCB a bit of leeway when someone sticks on a cartridge:

After 10 min holding the assembly (I will definitely design a second tool for the backside of the mount for the next iteration that I can use a carpenter clamp for that...;-) and cutting the traces to length the PCB was attached:

And assembled with the lower part of the assembly that features the grounding pin:

And here with cartridge :

For comparison here is a photo of the original part. A quite nice match!:

I evaluated the conductivity across the contacts. For both original and new part I measured a nice 770 Ohm across each of the coils, so that is pretty promising.

What was not promising that much was that the contact was a bit intermittent on one of the coils. This means I need to redesign the shape of the bump a bit that presses the contacts into the contacts on the cartridge. Also my parts was a tad to thick on the back end (3D printing is not an exact science (yet) and achieving a perfect fit is usually a trial and error effort needing a few iterations), and I had trouble sticking it into the arm all the way. I think I could force it in, but then that would not be Beolove!..;-):

After these tests, I replaced the cartridge a few times and after about 20 cycles one of the traces came off...this can probably be addressed by recessing the PCB at the tip of the mount to reduce the forces on the traces at the end of the board during the insertion process. On to the next design cycle.