Beogram 4000: Microsurgery on the Tonearm Wiring

In recent days I made nice progress with the Beogram 4000 from Australia that I have on the bench right now. When I thought I was mostly done I wanted to give it a quick spin to see if any sound would come out of the unit. Unfortunately, I quickly noticed that only one of the channels would produce sound. The other was completely dead.

First I did some measurements between DIN5 pins and cartridge mount connection pads (after preventing the grounding switches from closing with a strip of paper). This yielded that indeed the red (right) channel did not seem to be connected. But I also noticed that blue/green (the signal return wiring) seemed to connect to the center tabs and not the outer ones on the cartridge mount, as would be normal. This suggested that some major 'human creativity' had occurred at some point in the past that changed the tonearm wiring fundamentally. Probably in an attempt to fix this issue.

I did a visual inspection of the wiring, and soon found the root cause for the absent continuity of the right channel: One of the very fine wires that go though the bearing of the tonearm assembly was broken off:

If you look very closely you can actually see the other end of the wire peeking though the thin plastic tube a couple mm from the end of the tube. This meant I needed to find a way to restore the wiring. Luckily, this very thin wiring stretch terminates at the bottom end of the bearing where the wires connect to more solid wires across a small PCB acting as a wire terminal:

I decided to try reconnecting the two wire ends. For this to happen I needed to get the ripped off end inside the tube out in the open, so I could solder the two ends back together. I unsoldered the wires from the terminal PCB and then liberated the plastic tube from the clamp that secures it to the bottom of the carriage assembly:

It turned out that the wires sit fairly loosely inside this tube and I was able to push the broken off wire a bit further into the tube, which happily caused the broken off end to emerge. With fine tweezers I pulled the strand a bit further into the open. These wires are so called 'magnet wire', which means they are insulated by a thin coat of polyurethane. This coat can be burned off in a drop of hot solder at the end of the solder iron tip. A puff of smoke and one has uninsulated wire that can then be soldered. I did this to both ends and then tacked them back together after applying a little bit of flux: 

I then insulated the naked wire stretch with some glue:

I verified that this wire was continuous again and then proceeded identifying which of the wires was connected to what wiring color on the MMC mount. This shows how the colors are connected to the Beolover MMC cartridge mounting tab that I had already installed in this Beogram:

One by one I measured where the thin wires were connected at the tab and then I soldered them to the proper terminal to match the colored wiring on the other side:

This shows the final result on the other end. I had pulled the plastic tube out a bit further than before to give the wires some clearance to the metal bearing shaft, hoping to prevent future damage to them due to reduced stress and no mechanical contact:

I installed the carriage back on its rails and then tried listening to a record again. Happily, the music now came from both speakers, so we can conclude that this issue is fixed. Stay tuned for more details about this restoration effort!