Korg M1 buttons not working

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mikepctvman
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Korg M1 buttons not working

Post by mikepctvman » Thu Mar 07, 2019 4:45 pm

Hey everyone, I got a used Korg M1, all is fine except I notice the Up/Down, A,B,C and D buttons are not working, I find it a little hard to believe six buttons could all go bad at once, I tried pressing them several times but they are dead. upon inspecting the board it looks like those six buttons are tied together somehow. Any ideas on what to look for to troubleshoot besides replacing the tac switches? I don't see any broken solder traces, all wires and harnesses are connected and look undamaged. kinda stumped... :?:
Last edited by mikepctvman on Sat May 04, 2019 12:01 am, edited 1 time in total.

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meatballfulton
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Re: Korg M1 buttons not working

Post by meatballfulton » Thu Mar 07, 2019 8:10 pm

It's not uncommon for connectors of the wiring harnesses to get corroded. Try unconnecting, spray with contact cleaner and then reconnect.
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Re: Korg M1 buttons not working

Post by Rasputin » Fri Mar 08, 2019 6:26 am

mikepctvman wrote:I find it a little hard to believe six buttons could all go bad at once
You're right, at least mechanically. It's almost definitely an electrical issue. Not a Korg M1 expert, but I believe we can easily narrow this one down:

There are connectors between the panel board and the mainboard. In this case, the one you're looking for should be marked as CN4A on one side and CN4B on the other side, I think.

Use a DMM to check for continuity between each board over that connector. I have a hunch that it will be either the 3rd or the 7th wire (or pin/header) that is bad and not actually connecting both boards, but might as well check them all.

If that connector shows continuity between both boards over every single wire then I would put my money on it being a bad 74HC138 3-to-8 multiplexer labelled IC25. It will have one of its pins (Y2 or Y6, very likely) stuck, meaning its voltage never toggles between 0VDC and 5VDC regardless of what else is happening in the circuit.

Both scenarios are about as likely as the other, but should be a very cheap fix, either way.

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Re: Korg M1 buttons not working

Post by mikepctvman » Fri Mar 08, 2019 3:22 pm

Thanks for the quick responses guys, i will check the wire/harnesses & chip

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Re: Korg M1 buttons not working

Post by mikepctvman » Sat Mar 09, 2019 1:46 am

Update: so I disconnected the wire connector, cleaned, checked all pins for continuity & all passed the checks, might very well be the multiplexer as the culprit now. Can I find just any generic multiplexer off eBay long as the numbers match up? some have different letters before the numbers, Should I replace just the IC25 multiplexer? (there are two next to eachother, other one is labled IC24 & The numbers on the top of the two multiplexers are different if that matters) also what do you all think of using a heat gun to remove the bad multiplexer off the board?

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Re: Korg M1 buttons not working

Post by mikepctvman » Sat Mar 09, 2019 4:00 pm

Here are the chips in my synth, do the letters before the numbers on the tops of the chip matter?:
Image

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Re: Korg M1 buttons not working

Post by Rasputin » Sat Mar 09, 2019 5:21 pm

mikepctvman wrote:Should I replace just the IC25 multiplexer? Also what do you all think of using a heat gun to remove the bad multiplexer off the board?
Well, if you want to do things "properly" then you'd use an oscilloscope or logic probe to measure IC25 on its pins (focusing mostly on the ones that I mentioned) just to verify the diagnosis (aka "my hunch"). Although, depending on your meter, you might be able to see the voltage on one of the pins pegged at either 5V or 0V, while the other pins on IC25 which go to the key matrix might average out to 2.5 or 3V or "jitter" and dance around a particular voltage. If one of the pins I mentioned doesn't seem to measure like the other pins in that group then we're on to something.

But... if you just want to get to fixing your damn synth ;) then I would suggest cutting the pins/legs of IC25 with some "flush cutters" and then desoldering the remaining leg stubs one at a time.

I would personally put a DIP socket in there and then put the replacement chip into that. The 74HC138 part should function without having to be ultra-specific, I believe.

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Re: Korg M1 buttons not working

Post by mikepctvman » Sat Mar 09, 2019 6:31 pm

ok, I understand what you are saying for the most part. sorry for all the silly questions but as for checking the multiplexer, what would I set my multimeter to and which pins would I contact to check if I am getting the 0, 5, 2.5 and 3v readings?

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Re: Korg M1 buttons not working

Post by Rasputin » Sun Mar 10, 2019 12:44 am

For best results, set the meter to DC testing at a range above 5V but no higher than 20V.

74HC138 pinout:

Pin 8 = ground (black probe on the meter stays stuck on this like glue -- always keep it there)

Touch red probe to various pins and the result should be different...

Pin 16 should *always* be 5V (within 4.75V to 5.25V)

These other pins can change states between 0V and 5V and may do often. Depending on how often they change, some may appear to the meter to be somewhere between 0V and 5V. If either Pin 9 and 13 behave differently from the other Y labelled pins then that's very suspect.

Pin 1 = A0
Pin 2 = A1
Pin 3 = A2
Pin 4 = E1
Pin 5 = E2
Pin 6 = E3
Pin 7 = Y7
Pin 9 = Y6
Pin 10 = Y5
Pin 11 = Y4
Pin 12 = Y3
Pin 13 = Y2
Pin 14 = Y1
Pin 15 = Y0

If you've never made the plunge into checking out datasheets, now it's time to dip your toes in the water :geek:

https://www.diodes.com/assets/Datasheets/74HC138.pdf

You'll see most of what I'm getting at if you look at the chip diagram.

As for this chip's specific function in regard to the Korg M1 and your key problem:

Pins 4, 5 and 8 will always be ground (0V). Pin 8 is ground, so it's always grounded. Duh... :D Pins 4 & 5--in this case--are also permanently grounded because that tells the chip it should be enabled for use. If they ever went to 5V then the chip would be disabled (so that's why they're stuck at 0V).

Pin 16 and 6 will always be 5V. Pin 16 is what actually powers the chip, so it's *always* being fed 5V unless the M1 is turned off. Pin 6--just like pins 4 & 5--also tells the chip to enable itself, but it works *backwards* to pins 4 & 5 in that it should never be grounded! If Pin 6 is ever at 0V then the chip will disable itself (so that's why it is stuck at 5V).

A0, A1, and A2 are how the chip is controlled by the CPU, and in turn what is affecting the key matrix and your dead keys. The CPU ultimately selects which button "groups" are being checked by changing to various combinations of A0, A1, and A2. That's the "3" part of the 3-to-8 line decoder/demultiplexer in the 74HC138 chip designation (three lines: A0, A1, and A2).

What are the 8 lines in 3-to-8 line, you ask?

Y0 to Y7. In this case, Y7 is "don't care" because it's not connected to anything and doesn't do anything at all -- a spare part, basically.

But the M1 CPU will be changing the values of A0, A1, and A2, and the various combinations of those 3 lines will select one single line out of the choices of Y0 to Y6. Each one of the Y lines is connected to specific panel buttons "groups" of the M1. So if one of the Y lines is not changing between 0V and 5V then part of the key matrix is dead, and you'll have a problem like the one you are experiencing -- a set of non-responsive buttons.

Why am I specifically focused on Y2 and Y6?

Each Y line is tied to a specific group of buttons, so by looking at which combination/grouping of buttons are dead, we can tell which specific Y line is permanently deactivated. I don't have an M1 here to look at, so I can't tell which buttons are assigned to each Y line, but I'm pretty sure it's going to be either Y2 or Y6. And I'm pretty sure the dead one is going to measure 0V while the other Y lines (except Y7 because it doesn't do anything) are going to show a non-zero voltage because they'll be bouncing between 5V and 0V as they are indirectly toggled on/off by the CPU via A0, A1, and A2.

Edit: I said CPU here, but IC29 (an I/O Expander) is what actually directly does this for the NEC 70216 microprocessor.
Last edited by Rasputin on Sun Mar 17, 2019 5:43 pm, edited 1 time in total.

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Re: Korg M1 buttons not working

Post by mikepctvman » Sun Mar 10, 2019 2:07 am

Thank you so much for the detailed info Rasputin. I have a clear idea of what to do next. I will let you know how it goes.

Just for reference, in this photo is what the back of the button board looks like in an M1, I outlined the six buttons that aren't working on mine, you can see they are indeed tied together somehow and they run to the connector (CN4B I believe) that you mention. Image

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Re: Korg M1 buttons not working

Post by Mooger5 » Sun Mar 10, 2019 12:40 pm

Great writeup, Rasputin.
I checked the PCB drawings in the service manual and it´s Y2. Pin 13 of IC25. The first image posted by the OP shows the PCB is a little scratched next to the connector. Am I seeing it well? There are two traces going between pins 3 and 4 :shock: One must lead to pin 13. I´d check for continuity between that pin and the connector...
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Re: Korg M1 buttons not working

Post by mikepctvman » Sun Mar 10, 2019 7:36 pm

Yes i do notice a possible scratch in that area I didn't realize at first. How would I even go about repairing that? Im going to check back on that, Hope the board isnt damaged beyond repair. Like i mentioned i bought this used, it's in overall great shape but you never know what people mangle up on the inside :?

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Re: Korg M1 buttons not working

Post by Mooger5 » Mon Mar 11, 2019 12:06 am

Well, like Rasputin said, there´s the proper way and there´s the quick easy fix. The latter simply jumping the broken trace with a wire. But before attempting the repair, make sure there lies the fault. If there´s continuity between pin 13 and the connector, then the trace is intact. In such case I agree the chip is faulty and have to be replaced.
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Re: Korg M1 buttons not working

Post by mikepctvman » Mon Mar 11, 2019 12:27 am

Update: I think I am leaning toward a faulty chip like everyone here, I tested voltage/continuity across each chip pin and at the connector while pressing the unresponsive buttons and I watched the voltage jump on the tester as the buttons were pushed, so its getting a connection from the buttons there. The scratch on the board was just a blemish and wiped off with cleaner...

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Re: Korg M1 buttons not working

Post by mikepctvman » Mon Mar 11, 2019 10:43 pm

I have a new multiplexer on the way. Would I still see continuity and my meeter jump when i press the buttons regardless if the multiplexer was good or not?

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