Author Topic: Korg MS-20 build - no sound when engaged  (Read 465 times)

bird

Korg MS-20 build - no sound when engaged
« on: March 13, 2021, 01:32:03 PM »
I just built the Korg MS-20 pedal from this Effects Layouts project: http://effectslayouts.blogspot.com/2016/08/korg-ms-20-filter.html, but there's no sound when the circuit is engaged, there's only sound in bypass mode. Here's my answers to the debugging guide.

1. Korg MS-20 pedal from Effects Layouts
2. http://effectslayouts.blogspot.com/2016/08/korg-ms-20-filter.html
3. I followed the layout exactly. This is my sixth pedal, but the first with a PCB I made.
4. Substituted LM13700N for LM13600, but people on Effects Layouts said LM13700 worked.
    Also wired the 3PDT according to Build Your Own Clone methods, not how Effects Layouts suggests. But I made sure all the connections are going to where the should on jacks and board. I did this with another pedal and it worked.
5. Negative ground (right? normal Boss style 9V plug)
6. Sound only in bypass, no sound when engaged. I can't hear any sound when engaged, and I actually noticed that some hum from my guitar/amp went away when it was engaged (hummed in bypass). The indicator LED and the two LEDs on the PCB light up. Some people mentioned the LEDs dimmed with the DPDT switch, but mine are the same either way.

I've tested voltage at a bunch of points in the circuit (not totally sure what I'm doing). The DPDT switch only has current on lug 5 and 6. Since lug 2 connects to the PCB, it seems problematic that there's no voltage there. There's also no voltage going across the 100nf cap connected to the volume pot (but has 7.65 V on side closer to IC), and there's no voltage on any of the volume pot lugs. I'm guessing that's not good: no voltage, no sound?

Test voltages:
9V jack - 9.37 on positive
IC - all pins around 7 to 9, except the one that goes to ground
Diodes:
        5817 = 9.39 positive side and 9.11 negative side (banded side)
        D1 LED = 4.02 + and 1.99 -
        D2 LED = 1.99 + and 0.00 -
Electrolytic caps:
        47uf - positive side = 4, negative = 0
        10uf - positive side = 9.1, negative = 0

Any ideas what could be wrong? I appreciate any help or suggestions for more things to test. Thank you!

antonis

Re: Korg MS-20 build - no sound when engaged
« Reply #1 on: March 13, 2021, 02:11:48 PM »
Hi & Welcome..

Just for a schematic to reference on..

https://www.diystompboxes.com/smfforum/index.php?topic=112676.0

"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

John Lyons

Re: Korg MS-20 build - no sound when engaged
« Reply #2 on: March 13, 2021, 02:35:07 PM »
Check for etching or soldering bridges.
That board is pretty dense with parts per small size.
The Ground plane is very close to the pads for parts and
off board wiring. Use Lupe or strong magnifiers to inspect
the soldering and etching to make sure you do not have a short.
Basic Audio Pedals
www.basicaudio.net/

garcho

Re: Korg MS-20 build - no sound when engaged
« Reply #3 on: March 13, 2021, 03:13:31 PM »
Great circuit to build. And nice job on the troubleshooting post, it's amazing how little effort some people are willing to put into getting free help for themselves.

Quote
IC - all pins around 7 to 9, except the one that goes to ground

Double check your voltages for the IC and post it here, that isn't right, or good. Start there.

Are you familiar with what an "audio probe" is?
More or less, it's a wire that you hold in your hand to touch different locations of the circuit and listen to what's going on with the audio (or whether or not there is any!). That wire needs to go to some kind of amplifier, so you'll have to solder it to a plug of some kind. You'll also need a ground wire, connected to the circuit you're testing and the amplifier. Basically, you cut the end off a guitar cable, attach the loose ground from that cut end to the ground of the circuit you're testing, and the "hot" loose wire is what you use to test points where you know there should be audio. Plug it in to an amp (volume low) and start at the beginning - the input jack. Move from point to point, following where the audio "goes". It can really help you pin point a cold solder joint, a short, or any number of other common mistakes.

The LM13700 will work just fine. All OTAs (what that IC is - operational transconductance amplifier) can be fried by too much current on their IABC pin (1 and 16 on this IC). Yours could be fried if the voltage is 7-9V there, we'll figure that out later.

Take a picture of your 3PDT wiring and post it here.

"Positive" ground is only a thing in Germanium fuzz pedals, all others are going to be "negative" ground.

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"...and weird on top!"

bird

Re: Korg MS-20 build - no sound when engaged
« Reply #4 on: March 13, 2021, 03:44:05 PM »
Thanks for the help everyone.

I just re-checked the voltages on the IC, and they are all in the 7-9 V range. Is there any easy way to test a chip like that?

I can't find any solder or etching bridges, no short circuits from what I can tell.

I tried the audio probe, and besides getting sound from the input jack, just got extremely loud popping sounds when I touched anything with the probe. I had to turn the amp down so low to mitigate I wouldn't have heard any sound from the guitar. Not sure if I didn't make the probe right or if that means something is horribly wrong with the circuit.
« Last Edit: March 13, 2021, 04:02:59 PM by bird »

antonis

Re: Korg MS-20 build - no sound when engaged
« Reply #5 on: March 13, 2021, 04:00:16 PM »
Is there any easy way to test a chip like that?

Take it off the socket and recheck voltages..

e.g. voltage on pins 3, 4, 13 & 14 should be the one you get on D1 Anode (Vr)..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

bird

Re: Korg MS-20 build - no sound when engaged
« Reply #6 on: March 13, 2021, 04:11:49 PM »
Tried testing to voltages on the chip outside of the board, and they were all zero. Not sure if I'm doing it right, but I used pin 6 as ground and put positive to the others -- does that sound right?

antonis

Re: Korg MS-20 build - no sound when engaged
« Reply #7 on: March 13, 2021, 04:29:47 PM »
Tried testing to voltages on the chip outside of the board, and they were all zero. Not sure if I'm doing it right, but I used pin 6 as ground and put positive to the others -- does that sound right?

Quite right but it doesn't tell anything.. :icon_wink:

P.S.
Take chip out and measure voltages on chips socket respective pins..
(let's pretend chip is its place..)
« Last Edit: March 13, 2021, 04:31:23 PM by antonis »
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

StephenGiles

Re: Korg MS-20 build - no sound when engaged
« Reply #8 on: March 13, 2021, 06:14:20 PM »
I remember writing a letter to Korg for the circuit diagram of the MS20, and a month later it arrived in the post!!
"I want my meat burned, like St Joan. Bring me pickles and vicious mustards to pierce the tongue like Cardigan's Lancers.".

garcho

Re: Korg MS-20 build - no sound when engaged
« Reply #9 on: March 14, 2021, 03:10:18 AM »
Quote
Tried testing to voltages on the chip outside of the board, and they were all zero. Not sure if I'm doing it right, but I used pin 6 as ground and put positive to the others -- does that sound right?

Just in case you've misunderstood, what you want to do is measure the voltages on the PCB where the pins would be if you had the IC in the socket. Another way to say it is to take the IC out of the socket, and measure the socket pins. We want to see what the voltages are regardless of an IC in the circuit. In case you know that already and think I'm insane for mentioning it: I don't mean to be condescending or insulting; electronics and electricity can be incredibly confusing when you first start building devices, and this forum is full of green beginners.
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"...and weird on top!"

anotherjim

Re: Korg MS-20 build - no sound when engaged
« Reply #10 on: March 14, 2021, 05:29:42 AM »
Check the LEDs. Now that you fitted them, the cathode side of both LEDs (with the slight flat on the bottom rim) should face toward the Ground trace. The trace labelled Vr should measure about 3v.
Croeso i Diystompboxes.

If they didn't hear you then you didn't say it.

bird

Re: Korg MS-20 build - no sound when engaged
« Reply #11 on: March 15, 2021, 08:57:30 PM »
Thanks for the pointers and explaining checking the socket - don't know why I thought testing the disconnected chip would be anything but zero. I appreciate the help and explanations from you all, electronics are quite new to me.

For some reason the internal LEDs (D1 and D2) don't light up anymore. I don't have any more to replace them, but the engaged pedal didn't work even when they did light up.

Here are the voltages for the IC socket without the chip in it:
1 - 8.38       9 - 0.31
2 - 0.00     10 - 0.00
3 - 2.44     11 - 0.00
4 - 2.45     12 - 0.00
5 - 0.00     13 - 2.45
6 - 9.22     14 - 2.56
7 - 0.00     15 - 0.00
8 - 0.33     16 - 8.38

LEDs: D1 - 2.45 and 0; D2 - 0 and 0 (see original post for values when lit). Double-checked the cathodes, and they're in the correct holes.

I was really excited about this effect, so I'm thinking of re-building this in perfboard. I have more of almost all the parts, so at least that way I can rule out the PCB being the cause. Does that seem reasonable?

Also, if someone could confirm this: I have the input jack's input signal wired to the 3DPT, then going to the DPDT toggle lugs 3 and 4. Then the DPDT is wired to the circuit as shown in the EffectsLayouts diagram. Is that correct?

Here's a couple gut shots in case that's helpful. It was hard to get a clear shot of the DPDT toggle, but I've re-checked both switches several times.