Monday, October 10, 2011

Korg MS10 Repairs

Long, long ago. In an ebay auction far far away, I had thought that I would be parting ways with My Korg MS10, as I had with many other bits of music gear before it.
As you will see, this was not to be.

The person who had won the auction, had a mutual aquantance through a certain synthesizer building gang that we were involved in. OK, not a gang, but a bit of a club for nerds.
Synth Nerds. When he came to pickup the synth I was proud to show him many of my recent builds. This included one Music From Outer space "Sound Lab", WSG and partially built modular synth.
I'd also built a stack of pedals and there was all sorts of electronic stuff littering my work area.

We chatted for a bit about whatever, and he played with the MS10. Everything was fine, he paid. We parted ways. But not for long.
By the end of the week, I received a call that the filter knob had stopped working. This was unexpected, as I'd been careful with the synth.
Sure enough, when he brought it back, the filter pot was all jammed up. I opened it up to have a look see, and discovered that the carbon track inside the potentiometer had actually crumbled.
This was a first for me, but since it was 30 years old, I guess it was bound to happen.

The potentiometers in the synth all have long legs at a right angle. They also double as fasteners for the circuit board to attach to the face plate of the synth.
I jimmied up a temporary fix, but needed to order the part from a specialist overseas. I think it was sourced from Small Bear, who have a great selection of uncommon parts.
While waiting for it to arrive, he asked me to add a ring mod to it, as he likes to make lots of weird noises. I ordered a "Real Ring Mod" from Ken stone and the MS10 just happens to have the right output impedance to enable it to work without any extra electronics. Sweet.

Once that was all done, I handed it back to the owner, who was pleased as punch about his synth again.

Some time down the track, it stopped working and it was back on my bench for repair again. I don't remember what the problem was this time, but a fuse had blown and I installed a new one.
Returned to the owner and it was business as usual.

I may have seen it again in the mean time, but in the last week I received a call from him again.
The synth had been dropped, a key was broken, one end smashed and it wouldn't turn on.
I thought the worst. Smashed keys are no fun and I expected to see a severely damaged synth turn up on my bench.

When he brought it in, it didn't look too bad, a bit of plastic from an end cap had broken off, but it could be glued back on.
The other end of the synth was bound in electrical tape and covered the last F key, which turned out to be the broken one.
Indeed, the unit wouldn't turn on. I checked the fuse and power cord. They were ok. Something else was wrong.
What this means, is that to find the problem, I had to disassemble the entire thing!
That involves around 30 screws and a similar amount of nuts. It takes a little while to do, but otherwise no big deal.

With the chassis open, the keyboard needs to be disconnected, which is only connected by a little socket on the side of the board. Easy.
The ground also needs to be disconnected from the base plate. This done, the keybed can be set aside.

With my multimeter set to the appropriate range, I tested the mains side of the transformer for the right voltage coming from the socket (240v in Oz).
This also ensured that the on switch/volume control was working. The next thing to do is to check the output transformer. The voltage range should be between 10 - 20v depending on the tap.
I'd determined that was fine and proceeded to the main board. No damage was evident on the back (solder side) and there was nothing indicating a burned out component on the front.
So first things first, poke around with the voltage probe. Starting at the voltage regulator.

Right away it was obvious that there was no voltage on one of the regulators. Then I poked the middle pin and the LFO lamp flashed to life.
Aha! There was the problem, right in front. First go. Upon closer inspection (I needed my spectacles) there was clear evidence that the middle pin for the regulator had cracked.
It was simple fix. Pull out the cracked pin from the board, put in a single header pin, which has a little plastic collar on it to stop the pin falling through the board.
I soldered the middle pin of the regulator back in place and the electronics were back in business.

The next fix was a simple mechanical repair. The key which had broken, could not have been broken in a better place. All that had happened, was that one of the plastic tabs to keep the key balanced on it's spring was snapped off. The piece that should have been floating around inside, was nowhere to be seen. so I rummaged in my parts bin for some suitable material and I found a piece of perf

A couple of other bits needed to be glued back on. I used some epoxy and it's holding together nicely.

Finally I reassembled the synth and informed the owner that it was good to go.
This message was accompanied with a warning.

Should I ever see the inside of this synthesizer ever again, I would grind it up and eat it sprinkled on my cereal!

The voltage regulator with a crack on the middle pin

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