The Mighty MKS-20 - What I've learned - diyAudio
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Old 15th August 2014, 11:03 PM   #1
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Join Date: Aug 2014
Default The Mighty MKS-20 - What I've learned

The MKS-20 is a well loved "vintage" rack mount keyboard/piano module.

1. When you get a distorted/overdrive sound, this is caused by a small relay on the output of the the audio side of the unit. It gets dirty and/or the pins get oxidized. This relay is located on the effects side of the unit (light brown pc board & the largest of the 4) that is covered with a square piece of sheet metal - the bottom of the unit.

The other side has the green pc boards which holds the brains (cpu) and the memory.

So, looking at the brown side, the relay is located towards the middle of the unit, closest to the I/O board that houses the midi and analog jacks. The relay itself has a clear plastic cover with the pin configuration. It's gonna be smaller that what you are thinking.

You can pry the clear cover gently using smaller needle nose pliers. If you use larger ones, you may accidentally crack the plastic cover. If so, you can just use clear tape to repair the crack. Now, you are gonna have to a gentle grip, but firm enough to pry it gently back and forth until the cover comes off. I've done this on several occasions and never had a relay come loose on me. The cover will come off first before that happens.

With the cover off of the relay. You will notice the "arm" that swings back and forth. When the unit is off, it'll be in one position. When the unit is on, it'll swing over to the other position to connect the audio signal to the output jacks. It is possible that when the unit turns on, the relay won't click over to connect the pins.

When powering on the unit, you should hear a faint "click." If you don't, the relay is probably really dirty and/or stuck.

With the cover off, spray the living crap out of with caig deoxit, or something similar. i used radio shack's contact cleaner, and i've also used crc contact cleaner...both with decent results. it may also entail cleaning the contacts with some type of object. I use an alcohol prep pad (very difficult btw), and i've also used a flat metal nail file (i know this sounds crazy, but it works as long as you don't over do it). Keep testing the unit once you clean it really well to make sure the relay operates correctly.

Once you've cleaned it really well, turn it on to see if the distortion in the signal is gone. If so, you are good to go.

One extra step I do, is to get an old gift card/walmart card and cut a very small square shaped piece. i then place that within the relay to make sure the relay stays on no matter what. you'll see the gap once you turn on the unit in the relay. you'll want to place that small piece of card in that gap to keep the arm switched over permanently. Place the cover back over the relay (it should just snap back in place). Use some tape if necessary.

Yes this is ghetto, but i've used it several times, and it works!

ALSO - If you don't wanna fool with it that way, you can unhook all cabling, undo the connectors holding the light brown board on, and jumper the output pins on the bottom of the relay to the next components in line. There's another guy that wrote on this in google land, and he gives which pins go to the next component.

The only drawback to what I have described, is a very faint, very short "whine" that goes away after a second or so. Other than that, this fix works beautifully.

2. I've also learned that each fuse on the power supply - i think there's 4, go to a different part of the unit, and just because the fuse looks good, doesn't mean that it is. You can tell of course by measuring the fuse out with an ohm meter after the unit has been unplugged. The type of fuses that came stock in the unit have a ceramic middle piece that stays intact, but the small coil that wraps around it, is what can burn out. You can also usually see some grayish smokey residue within the fuse if you look carefully.

Your unit may power up, but if it gives you no sound with a midi signal present, the likely culprit is a blown fuse. Go ahead and replace the electrolytic caps in the power supply first - which is a 2200uf, 35 volt +, and a 6800uf - 16v +.

Also, one more note - i've noticed that the internal screws of an mks come out very easily due to wear and tear, traveling, etc. you may find a few of these floating around in the chassis when you take it apart. when you get done repairing your mks, put a small dab of super glue or got glue on each one to ensure they wont move.

Later Mashed Potaters -
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