Quote:
Originally Posted by
oldgearguy
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OK, since I didn't have my MKS-20 open (it's in the rack and all wired in and a hassle to get out and opened up again) I decided to take a couple pictures of the schematics and we can trace through it instead.
Looking at mks20_relay_1 first we see the signal coming in from the panel board on pins 18 and 20 (left side just below the big circled "3"). The relay is the dotted box labeled RL1. The signal leaves the relay and goes to capacitors C6 and C7 and then leaves the board via pins 16 and 17 of the small connector on the far right.
The relay normally routes pins 4 and 13 (the output pins) to ground (pin 6 and pin 9). The coil (curly thing between pins 1 and 16) is what causes the contacts to change over after the audio circuit is warmed up and stable.
To bypass the relay functionality, we need to permanently connect pin 8 to pin 4 and pin 13 to pin 9. Or in the larger view of things, we want to connect the incoming signal from connector pins 18 and 20 to capacitors C6 and C7.
Look at mks20_relay_2 now.
You can see the rectangle labeled RL1 and the larger connector to the right (labeled CN4) and the small connector to the left labeled CN3). The L/R orientation is flipped from the other picture because one is a logical drawing laid out to read left to right and this one is an actual circuit board layout showing the components laid out in the most optimal way electrically.
So, we're looking to connect pins 18 and 20 from CN4 to the capacitors C6 and C7 (located just above the relay). Follow the dark gray trace from pin 18 into the relay. Follow the gray trace from C7 into the relay. So, by connecting a jumper wire between those 2 gray traces in the upper right of RL1, you will connect pin 18 to capacitor C7.
Similarly, trace pin 20 into the relay and trace C6 into the relay. Note that by connecting a jumper between those two points in the lower right part of RL1 you will connect pin 20 to capacitor C6. You should end up with parallel jumpers running left to right from the middle of RL1 to the right side of RL1.
You should not have anything soldered into the 2 holes on the far left side of RL1 (near the diode labeled D7) that is for the coil and should be left unconnected.
Finally - since I did this relay jumpering as part of a larger overhaul (including recapping the unit and cleaning the volume control on the front panel) it will be interesting to see if the distortion you have completely goes away by removing the relay or if additional work is needed.
OMG! This is just great, thank you so much. This method is 100% working.
I had really bad issues with my MKS20:
- loud noise on startup;
- distortion on high velocity;
- no sound on low velocity;
- background hiss when any key is pressed;
- attacks are dull, no brightness;
- uneven pan both in phones and main outputs.
Checked PSU - no problem. I thought there are some problems with DAC or memory or something else. As I also had dead backlight, I wanted to start with fixing this simple problem, so a googled and fortunately got here! Seemed like I had a little chance that this modification can solve my bigger problem. I soldered both jumpers as described, now my MKS20 is back to life! Sounds absolutely fantastic, no hiss, beautiful attacks, tremolo and chorus are marvelous, outputs are evenly panned, no distortion at all!
And I did not change any capacitors, just jumpers.
Thank you so much for your noble work and altruism!
PS. By the way, the LCD is also like new now! But not because of jumpers!
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