Oberheim Matrix 6 – SUNKEN SHIP rescue

I was approached by a client if i could fix Matrix 6 with missing transformer. He said he bought in on Ebay and some other tech he knows failed to get it running. I agreed to have a look and see. The closer I looked the more obvious it became the reason for missing transformer is that the instrument was in a flood or sunk with a ship.

There was massive corrosion everywhere, keybed pcb was really bad and you still could smell the mud combined with cigarette smoke.

I informed the client this is beyond economical repair, but I might adopt it as a pet projet. I was curious if I could get it going.

I started by attaching it to lab power supplies +-12 and +-5 and it looked like the CPU is going and that it has some chance. The snag came with powering up the display. Service manual says there is 9V AC, I connect it to 9V AC and nothing happened. I was kind of suspicious because these VFD displays usually need much higher voltage to operate but the publicly available service manual was so blurry i couldnt really properly see whats there. Luckily there came one friend to the rescue and scanned his paper copy of the manual in appropriate resolution.

I was surprised there really was clearly written 9V AC C.T. but following the schematic it was apparent you can’t make -42V DC from 9V AC C.T. There was clearly error and it meant to be 90V AC C.T. which is 45V effective and some 63V DC after rectification. This is then dropped by regulator to 42V DC stabilised. I only had 2×18 transformer at hand so I tried to connect 36VAC and suddenly it all started up but display was showing garbled data.

However when i booted up to service mode (press store button while powering up) synth sprung to life and I was able to play some sounds

Keybed was in really poor shape, about 30% of the keys wouldnt work. I wanted to test it over midi but I was unable to list through the menus, buttons were all rusted solid and I had to replace them all.

In the mentime I have ordered custom wound toroidal transformer and replacement 3D printed key.

Another issue was when i plugged in the headphones, synth would stop making sound including the main outputs. This was caused by totally corroded contacts in the headphones jack shorting against ground. I replaced all the jacks front and back with new ones becase others were in the similar shape.

When I had it wired up over midi, I focussed on why it wont boot on it’s own but only in the service mode and why even when i loaded the patches it would forget them right away. I suspected the battery but that sill had healthy 3.2V, ram was also good but at the end the culprit was eventually diagnosed to be faulty IC socket under the CPU, one of the legs wasnt making proper contact. I decided to swap the CPU and ROM sockets and viola problem solved.

Last but not least came the keybed, there was severe corrosion on so many traces it was frustrating but eventually one after one I managed to get it going reliably.

Now I had fully working keybed, new buttons and new jack sockets, new transformer, reliable memory keeping the sounds as it should.

There was last thing to deal with and that was the volume slider pot. The corrosion there was so extensive I havent seen anything like it before, there was no way to repair it and it had to be replaced. To my surprise such part was completely unobtainable. I ended up ordering a custom made batch of 100 and sold some of them to my synth tech colleagues throughout the world.

After calibration I noticed cutoff on one of the voices is way out and again this was caused by bad IC socket so I decided all the remaining sockets need to go despite having to desolder so many IC legs

Last thing I did is I installed Tauntek firmware made by amazing Bob Grieb and I gave the panel proper wipe with paint restorer and touched up the rust spots to stop it rusting further. When the owner seen it he couldnt believe it’s his machine, it looked really well.

Later on few more issues surfaced with the leaver pots being too crackly to function properly (again the flood damage) and the aftertouch sensor wasnt really working all that well, I couldnt revive it fully because part of the function requires the felt to be intact and apply pressure and that felt was completely ruined by water so I was only able to gain partial results.