Korg M1 buttons not working

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Mooger5
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Re: Korg M1 buttons not working

Post by Mooger5 » Mon Mar 11, 2019 11:13 pm

Forgot to tell, to test for continuity the unit must be turned off. Select the beeper in the DMM and probe away. If it beeps, there´s continuity between the traces or the wires.
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Re: Korg M1 buttons not working

Post by mikepctvman » Sun Mar 17, 2019 3:32 am

Ok guys one last try here before I give up. I just replaced the 74hc138 (ic25) chip, same buttons are still dead. I got a continuity reading from the buttons through the connectors, i would even watch the multimeter jump when the unresponsive buttons were pushed. Is there anything to test beyond the multiplexer in the main board?

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Re: Korg M1 buttons not working

Post by Rasputin » Sun Mar 17, 2019 4:42 pm

mikepctvman wrote:Ok guys one last try here before I give up.
Pretty much the only thing which would warrant giving up is if there's a problem with the CXD1095Q I/O Expander (IC29), but that's unlikely and we might even be able to prove that it can't be the I/O Expander, if we look at how the circuit works.

M1 KEY MATRIX

The panel buttons are read in a grid or "matrix", so each button is the intersection of a row and a column. Each button lies somewhere within a column group, and also somewhere within a row group. In other words, each button belongs to two different groups with each group containing a different set of keys. Pick any one column and any one row, and there will only be one button in common between those two groups.

If you have a group of dead buttons which all share the same column or row, it is *extremely* likely there is a problem with the functioning of that particular column or row, in an electronic sense. It would be very, very coincidental for buttons to fail mechanically and have all of them just happen to be part of the same and complete column or row group.

If all of the dead buttons share the same column or row, *but* there are still one or more buttons working within that same group then trace damage localized around the dead buttons is very likely, unless they are also physically grouped in the same area -- in which case liquid spill damage, etc. could be suspected.

Columns

The I/O Expander continuously cycles through the columns, activating a particular column when it wants to check a certain group of buttons. The I/O Expander sets the column by turning on different combinations of three of its pins (pins 54, 55, and 56). The different column combinations set by those three pins are decoded into one individual column through the 3-to-8 chip which we've already discussed.

Since the I/O Expander has only three pins to set the column it wants, if the I/O Expander had a bad column pin (dead pin 54, 55 or 56) then more than one column would be affected because the columns are set by the I/O Expander in *combination* and it is the job of the 3-to-8 chip (IC25) to make those combinations a separate column.

So... if one of the I/O Expander column pins dies then you automatically must have more than one column dead, and that means there would be a bunch of buttons dead (a bunch being 7 or more). Therefore, if the failure corresponds to a group of buttons sharing the same column, but no other columns are affected then the problem almost definitely lies at the column decoder, or with the continuity of a trace, header pin, ribbon cable, etc. -- something which affects the circuit on an *individual* column basis. Remember, the I/O Expander only works with *combinations* of columns and the 3-to-8 chip is required to split up the combinations.

Rows

The buttons are also grouped into rows. The I/O Expander reads in the rows via Port B (pins 3, 4, 5, 6, 7, and 64). IC24 is a 4050 hex buffer, and RA7 is a resistor pack. Essentially, IC24 and RA7 help prepare the row signals so the I/O Expander can successfully read them.

Similar to the column functioning, if something was wrong with any of the I/O Expander's Port B pins, one of the 4050 pins, or one of the connectors/traces in the row circuit then a certain group of buttons would stop working -- the exact group would be dependent upon which of the pins/connectors/traces was not working.

Here is where failure of the I/O Expander could be a bit less obvious because it *is* possible for one I/O Expander pin to fail and it would only affect one row group, although all the other potential row failure problems (4050 failure, connector failure, etc.) are all more likely.

Diagnosis

So how do we know where to start? By examining which buttons have failed and trying to group them logically.

Row Groups (destination is ultimately Port B on the I/O Expander, via 4050 buffer):

PB0 = [SW9, SW15, SW25, SW30, SW31]
PB1 = [SW10, SW16, SW26, SW27, SW32]
PB2 = [SW1, SW3, SW11, SW17, SW21, SW28, SW33]
PB3 = [SW5, SW7, SW12, SW18, SW22, SW29, SW34]
PB4 = [SW2, SW4, SW13, SW19, SW23, SW24]
PB5 = [SW6, SW8, SW14, SW20]

Column Groups (source is PA0, PA1, and PA2 from I/O Expander, via 74HC138):

Y0 = [SW1, SW2, SW5, SW6]
Y1 = [SW3, SW4, SW7, SW8]
Y2 = [SW9, SW10, SW11, SW12, SW13, SW14]
Y3 = [SW30, SW32, SW33, SW34]
Y4 = [SW24, SW27, SW28, SW29, SW31]
Y5 = [SW21, SW22, SW23, SW25, SW26]
Y6 = [SW15, SW16, SW17, SW18, SW19, SW20]

In your case, the buttons which failed are designated on the panel board as SW9, SW10, SW11, SW12, SW13, SW14. As you've noted, they are all tied together conspicuously as part of the same circuit area. If we follow the traces and/or look at the schematics (or refer to the above chart), those particular switches are all tied together on the Y2 column line (which should be the 3rd wire on the connector which carries the signal from mainboard to display board).

Since all the buttons correspond to one specific column line and are a conspicuous group with no other button group failures present, the problem almost definitely cannot lie with the I/O Expander. As IC25 is the only component between the I/O Expander and the problem buttons, the problem has to be either IC25, or a trace/connector/pin common to the Y2 line.

To check the continuity of the Y2 line:

With the M1 off, hold one probe to the top of pin 13 on IC25. Touch the other probe to the column side contact of SW14. No buttons need to be depressed for this test.

Conclusions

- If you've replaced IC25 then a stuck column pin is (now) unlikely. But we could test with a 'scope or logic probe and eliminate any element of doubt.

- If there is continuity from IC25 pin 13 all the way to the switches in question then the Y2 line (and any connectors in between) must be good.

- If each switch has continuity between each side of itself when depressed and no continuity between each side of itself when no longer depressed then each switch is physically good.

- If all three of those things are true but those exact switches still do not work then something simple is being missed or there's something *really* unusual going on.

edit: fixed to account for Mooger's superior wisdom and visualization skills...
Last edited by Rasputin on Sun Mar 17, 2019 8:00 pm, edited 1 time in total.

Mooger5
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Re: Korg M1 buttons not working

Post by Mooger5 » Sun Mar 17, 2019 7:35 pm

Rasputin wrote:In your case, the buttons which failed are designated on the panel board as SW15, SW16, SW17, SW18, SW19, SW20.
Pretty sure it´s sw9, 10, 11, 12, 13, 14 - pin 3 of cn4 - y2 line - pin 13 of the chip. According to the photo provided by the OP, it´s the buttons that are closer to the connector.
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Re: Korg M1 buttons not working

Post by Mooger5 » Sun Mar 17, 2019 7:40 pm

mikepctvman wrote:Image
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Re: Korg M1 buttons not working

Post by Mooger5 » Sun Mar 17, 2019 7:46 pm

Image
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Rasputin
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Re: Korg M1 buttons not working

Post by Rasputin » Sun Mar 17, 2019 7:55 pm

Mooger5 wrote:
Rasputin wrote:In your case, the buttons which failed are designated on the panel board as SW15, SW16, SW17, SW18, SW19, SW20.
Pretty sure it´s sw9, 10, 11, 12, 13, 14 - pin 3 of cn4 - y2 line - pin 13 of the chip. According to the photo provided by the OP, it´s the buttons that are closer to the connector.
I knew as soon as I contradicted you that I would be wrong, yet I couldn't figure out how :lol:

I was initially visualizing it in terms of strictly rotation and not as a top-side versus under-side mirroring. I'm dumb.

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Re: Korg M1 buttons not working

Post by Mooger5 » Sun Mar 17, 2019 8:18 pm

Stop that, it´s not a big deal, and all for the sake of Truth.
You got great communication skills. You should write a sticky Primary on synth troubleshooting. Really!
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Re: Korg M1 buttons not working

Post by mikepctvman » Mon Mar 18, 2019 1:18 am

Ah those drawings are very helpful too. I will check all of this, I spent months searching around google and bing for this kind of technical data :shock: you have all been very helpful here

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Re: Korg M1 buttons not working

Post by mikepctvman » Mon Mar 18, 2019 10:49 pm

Conclusions

- If you've replaced IC25 then a stuck column pin is (now) unlikely. But we could test with a 'scope or logic probe and eliminate any element of doubt.

- If there is continuity from IC25 pin 13 all the way to the switches in question then the Y2 line (and any connectors in between) must be good.

- If each switch has continuity between each side of itself when depressed and no continuity between each side of itself when no longer depressed then each switch is physically good.

- If all three of those things are true but those exact switches still do not work then something simple is being missed or there's something *really* unusual going on.

edit: fixed to account for Mooger's superior wisdom and visualization skills...
Checked continuity on all three of those, buttons are still dead. Something weird is indeed going on. Maybe something faulty in the mainboard? The mainboard seems to be the most difficult to find replacement part for an M1 also...

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Re: Korg M1 buttons not working

Post by Mooger5 » Tue Mar 19, 2019 5:12 pm

Would it be possible to post pictures of the actual unit in need of repair?
Please post close ups of the newly installed ic25 on both sides of the pcb. Also post pics of both sides of the actual 1262 board, to inspect the switches and the joints as well...
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Re: Korg M1 buttons not working

Post by mikepctvman » Tue Mar 19, 2019 6:31 pm

Yes I can get photos. I also noticed a few resistors along the way in the traces for those buttons? can those play any part in the problem too?

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Re: Korg M1 buttons not working

Post by Mooger5 » Tue Mar 19, 2019 9:10 pm

They´re diodes and there´s probably nothing wrong with them.
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Re: Korg M1 buttons not working

Post by mikepctvman » Thu Apr 25, 2019 7:39 pm

Sorry it has taken so long to respond I haven't had time to do a teardown of the Korg again to get photos. However I was also wondering, if possible that there is some way to directly "Bypass" things with some wires to check if those buttons will work the way they're supposed to?

Side note - not sure if all M1's do this but I am able to use the "Value" slider as the "up/down" button's effect on certain screen settings...

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Re: Korg M1 buttons not working

Post by mikepctvman » Thu May 02, 2019 3:42 pm

How can I go about jumping the trace to the broken matrix with a wire? what points would I attach the wire(s) to?

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