Micro Korg strange issue

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Micro Korg strange issue

Postby msleaveamix » Tue Apr 24, 2018 7:36 am

Hi everybody. I discovered this forum because I'm looking to fix an old MicroKORG that has already been well used.
A few month ago the arpeggiator and the octave button stopped working. I first thought about a bad preset that made the machine buggy. So I did a factory reset as I discovered on a youtube video and in some tech-manuals. But this didn't fixed anything.

Here is exactly the problem :
* I can't switch arpeggiator on or off
* I can't change the octave up or down
* When I change the preset, I can have different settings loaded like arpeggiator on, and then I go back to another preset and it goes off again. Anyway this is blocked

What I tried :
* Factory Reset
* Opened it, and checked the connections, cleaned it

My questions are :
* Is this possible that it is a "fixable" problem, like just changing a component or two, or is this a chip-based problem and thus not fixable.
* If I can't do it by myself, is that model worth the price of sending it to a repair shop, or is it better to buy a new one

Thanks for reading,

Maxime
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Re: Micro Korg strange issue

Postby Rasputin » Tue Apr 24, 2018 8:22 pm

msleaveamix wrote:Here is exactly the problem :
* I can't switch arpeggiator on or off
* I can't change the octave up or down
* When I change the preset, I can have different settings loaded like arpeggiator on, and then I go back to another preset and it goes off again. Anyway this is blocked


Did they all go out exactly the same time? Or did they gradually stop responding after being "flaky" for a little while? Has the Microkorg suffered any physical trauma like liquid spills or drops?

If it was something electrical then it would either be a bad trace or the LV138A which is like $0.50 (not counting labor), but all of them should have died suddenly and all at once. If Shift, Side A/B, Formant Hold, and Write also do not work then it's 100% electrical.

If it's gotten worse, been intermittent or any of the other buttons I listed *do* work then it's physical, so probably the tact switches due to a liquid spill, etc.

The switch buttons in question lead back to one particular LV138A decoder/multiplexer. From pin 9 of that LV138A, follow the trace and look for breaks or corrosion, particularly before it connects to any of the dead switches. A continuity tester or digital multimeter would work well to test the connection between pin 9 and the switches.
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Re: Micro Korg strange issue

Postby msleaveamix » Wed Apr 25, 2018 6:52 am

Hi, and thanks a lot for the quick reply!

So yes it stopped working during usage, and everything got out simultaneously.

For the physical trauma, the only think is that it was already broken (no sound at all) when I received it for the first time. Then I cleaned it's inside and it worked again, I was thinking that "this is the good oldschool hardware, never stops working". But a year after, I played with my friends several times in the week, taking good care of it, and there was no drops or liquid spill, the only thing that could happen is the moisture in the air because it was an old house, not well heated, I dont especially think this comes from this.

So I'm gonna test what you said to me, and hope it comes from this LV138A chip.

Again, a huge thank for your well documented reply, I'm gonna check what how are the connections with my multimeter.
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Re: Micro Korg strange issue

Postby msleaveamix » Wed Apr 25, 2018 7:36 am

I checked the connections at the moment, and it seems that a track is not conducting.

The break is located between a bridge that is located between the 6-7 buttons (after D13), and the D19 diod. I wonder if I can fix this because it very thin. But tonight I'll go to a electronic workshop near my house and they have some good skills that will probably help me :)
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Re: Micro Korg strange issue

Postby Rasputin » Wed Apr 25, 2018 8:42 am

msleaveamix wrote:I checked the connections at the moment, and it seems that a track is not conducting.

The break is located between a bridge that is located between the 6-7 buttons (after D13), and the D19 diod. I wonder if I can fix this because it very thin. But tonight I'll go to a electronic workshop near my house and they have some good skills that will probably help me :)


Glad you found a probable cause! One thing that can help with fixing thin traces is to mask both sides parallel along the trace where it is broken. That way none of your repairs will touch the surrounding area, as the only area exposed would be where you haven't masked it with tape or some other protective (and removable) layer, such as masking tape, etc.

Then you can scratch away with a fiberglass pen and only erode the track you want to work, and then solder a very thin wire to the exposed trace (over the break, of course) or even sometimes just a tiny bridge of solder. It all depends on the damage.

But if you have someone to help you, all the better. Good luck, let us know what happens.
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Re: Micro Korg strange issue

Postby msleaveamix » Wed May 02, 2018 6:06 pm

So I found the exact problem, it is a via that obstructed. I don't know how to fix these very small holes, I already tried to go through it with spike but it doesn't work, it's like if it's totally blocked. I tried to put some tin on it but it also doesn't work. Do you have a method, or a tool I can use to fix this?
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Re: Micro Korg strange issue

Postby Rasputin » Wed May 02, 2018 6:21 pm

msleaveamix wrote:So I found the exact problem, it is a via that obstructed. I don't know how to fix these very small holes, I already tried to go through it with spike but it doesn't work, it's like if it's totally blocked. I tried to put some tin on it but it also doesn't work. Do you have a method, or a tool I can use to fix this?


Can you take a picture?

Traditional methods would be to use solder braid (multi-standed copper wire spread out into a "fan" shape, basically), or a solder sucker.

Other methods would be to use a toothpick to jam through the hole while heating the solder. It may help to actually add more solder before using any of these methods!

I have to say that a via with solder in it sounds like an odd reason the buttons wouldn't work. Where did such a glob of solder suddenly come from and why did that break continuity?
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Re: Micro Korg strange issue

Postby msleaveamix » Wed May 02, 2018 6:45 pm

It's not the solder the obstruct the via, I tried to add solder expecting that it will help to go through it!

When I watch it under a lamp, I can clearly see that it is not normal. I think maybe it has overheated and then the link has been broken. The problem is placed on the first via just on top of the D19.

I have no very small pick, but I just bought some 0.1mm drill on ebay, maybe it will help unobstructing the via. What I fear is that I already broke the track by heating with my solder, but this is the game sadly. I will switch it back on, only after having fixed this via problem and controlled every connection around it.

I don't think removing the solder will help so much. I have the solder sucker and the braid, but do you really think it can be unobstructed like this? This would imply to heat the hole very precisely, and I don't have a so precise iron.

I tried to make pictures of the problem but it's too crappy quality for this size and I don't have a macro mode.
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Re: Micro Korg strange issue

Postby Rasputin » Wed May 02, 2018 7:10 pm

msleaveamix wrote:I don't think removing the solder will help so much. I have the solder sucker and the braid, but do you really think it can be unobstructed like this? This would imply to heat the hole very precisely, and I don't have a so precise iron.


It's hard to say anything about the via, especially without seeing it in person. However...

D19 is a double-diode so it (somewhat oddly) has 3 pins. I believe pin 1 goes to SW9 (octave down switch) and pin 2 goes to SW10 (octave up switch). Pin 3 is the common pin, so if there is a break in the trace before D19 pin 3 but after D13 then I believe that would break the Octave Up, Octave Down and Arpeggiator buttons.

D19 pin 3 could probably be jumped directly to D13 pin 3 with very thin single strand wire such as kynar**. If for some reason D19 is mangled or broken beyond repair, replace it an MC2838 (or NTE595, probably).

It should be possible to identify exactly where the break is using a multimeter, but I'm not sure exactly what you have done to this via, so we are dealing with unknown circuits now.

EDIT: This probably would have been the best repair attempt initially.
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Re: Micro Korg strange issue

Postby msleaveamix » Wed May 02, 2018 7:34 pm

Thanks for this accurate answer Rasputin! The problem is exactly what you said.

I checked and the break is exactly in the via. The conductivity between the last via located between buttons 6 and 7 and the via that is broken is ok, and from the other side of the via to the D19 is ok too. I tested with my multimeter.

I haven't done anything dangerous and have only done some checks, and also trying to resolder the VIA.

I think I'll wait for the next electronic repair session at my local fablab to have the right tools to make the D13/D19 jump. Thanks you again for your help!
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Re: Micro Korg strange issue

Postby Rasputin » Wed May 02, 2018 7:47 pm

msleaveamix wrote:Thanks for this accurate answer Rasputin! The problem is exactly what you said.

I checked and the break is exactly in the via. The conductivity between the last via located between buttons 6 and 7 and the via that is broken is ok, and from the other side of the via to the D19 is ok too. I tested with my multimeter.

I haven't done anything dangerous and have only done some checks, and also trying to resolder the VIA.

I think I'll wait for the next electronic repair session at my local fablab to have the right tools to make the D13/D19 jump. Thanks you again for your help!


I just opened up my Microkorg to see what you may have done and to check a couple things. The extra solder in the via is probably not even an issue at all. It might even be ideal because you should be able to jump pin 1 of D13 directly to that via (right next to pin 1 of D19) and not even have to try to clear the solder out of it. You could even add more because that via connects directly (electrically, more than physically) to pin 1 of D13 anyway and from there it goes straight to D16.

Really, the via is just there to bring the line which goes from D13 to the other side of the board with the Write switch back to the side of the board with D19/D16. Jumping a thin wire just keeps it on the same side of the board. I don't think that should be any problem at all.

In other words, D13/D19/D16 are all on the same side of the board and the via is just to get the signal over to the side of the board with the physical switches. The problem seems to be that after the Write switch, the signal can't cross back over to D19 because of the broken via. We could also solder a wire from that via on the front side of the board to the same via on the back side of the board. Just a small 1.5" loop should do it.

That might be the ideal fix because the wire would be very short and it wouldn't heat up either of the diode joints.

I'm pretty sure this fix will be very easy with the right wire, solder tip, and a steady hand. Use a little flux and you should be golden.
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