Korg Triton Extreme synth with insane ground loop noise, not ground loop? - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: November 12, 2024, 04:51:36 pm
content: I'm troubleshooting a Korg Triton Extreme 61 synthesizer / keyboard that's very noisy.You can hear the digital section in all the analog outs very clearly WHEN it's plugged into the same AC circuit as the amp. If I power the keyboard with an inverter / battery pack the noise is gone, which would indicate a ground loop problem.But it doesn't sound like a typical ground loop, it's not a 60Hz hum. What can be heard is the digital part of the synth's circuit. I've sniffed around the keyboard's main board with a Mr Carlson Super-Probe type signal tracer, and the noise emanating from the digital ICs is what comes out of the analog outs.Schematics are hereIs this likely a part failure, or a ground path problem?The fact that the noise is not a typical 60Hz hum but the digital section leads me to think that the analog ground and digital ground are no longer properly isolated. Maybe a decoupling cap failed somewhere?But since the noise can't be heard when the keyboard is battery powered, I'm also thinking it could be an oxidized or disconnected ground point somewhere, sending the noise down the patch cable ground causing a noisy ground loop?I've tried:- Plugging the keyboard into the same outlet, different outlets, etc. Same.- Testing the ground resistance from the main board's ground planes to the chassis at the AC connection, 0.3 Ohm or under.- Touching various grounding points on the main board with a grounded wire trying to short gnd to gnd, the noise did not change at all.- Adding a ferrite bead to the chassis ground at the AC in, no change.I haven't tried checking the control board or replacing any caps yet, and I'm trying to decide which I should try first.Any words from the wize?Any smart tests I should perform to get more clues on the nature of the issue?Thanks!

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2024, 06:16:23 pm
content: Try adding a DC choke filter (a big coil in the mH range) between the plus of the power supply and the Korg power input.Choke filters are sometimes find ready made as filters for car radios (to be used for removing the spark plug noises produced by a running engine, noises that might enter into the radio through the power wires). Any other big enough coil should do it, for example the primary of a small transformer. Something like seen here, the last drawing, the one with the D2 diode. In your case, Rload will be the power jack from the Korg.Please ignore the high voltage in the charts on the left side of the schematics, those are for a discussion from another topic:https://www.eevblog.com/forum/beginners/why-usually-there-is-no-diode-across-a-dc-choke-filter/

author: cincin
author_rank: Regular Contributor
timestamp: November 12, 2024, 06:48:33 pm
content: Thanks! now learning about DC choke filters...Do you mean that I should place it between the +15V connector of the power-supply board and the main board? The -15V rail matters less? The keyboard has an internal power supply (page 12 of the schematics above)I'm assuming this noise is a "problem" and not by design. If possible I'd rather fix the problem than patch it. Is adding this choke filter a test to figure out the nature of the problem, or a patch to remove the noise?Cheers!

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2024, 07:16:05 pm
content: Quote from: cincin on November 12, 2024, 04:51:36 pmI might have misunderstood that phrase. I thought you are powering the Korg from the same DC power supply as the power amplifier, thinking it's a DIY audio amplifier, sorry.If your Korg has built in power supply, then it should just work. Adding a choke inside its own power supply won't help. Your Korg already has chokes (L3 and L4 in the schematic). Do not add more.In this case, I would check for dried out and/or bulged electrolytic capacitors, particularly in the SMPS area. If no visible leaked or bulged, I would check for their ESR (if you have an ESR meter that can be used to measure in-circuit capacitors). If not, desolder the electrolytic capacitors one by one (the ones looking like a can) and measure them out of the circuit. Start with the ones that were exposed to the most heated areas, the ones located physically near big radiators, or near other power components that heat the most during a normal use.

author: cincin
author_rank: Regular Contributor
timestamp: November 12, 2024, 07:24:43 pm
content: I did do a visual inspection of the electrolytic caps and they all look fine, but it doesn't always show as you pointed out.I don't think I have any tools that will measure ESR in-circuit, sadly. What tool would you recommend that can do that?95% of it is SMD too, which is less fun to solder/de-solder manually.I will go do a closer inspection of and replace what I can depending on my stash.Thanks for the help!

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2024, 07:25:47 pm
content: To give some examples, I would check C52, C53, then C59, maybe C58, too, from that area. Same for C61, C64, C62, C65.As for the high voltage side, check C5, maybe C14 and C16, too.Some are simply skipping the check entirely, and just replace the old electrolytic capacitors with new ones of the same capacity, voltage and mechanical size (talking about filter capacitors from the power supply, or across the power lines somewhere else, not the electrolytic C used in the audio signal path).

author: cincin
author_rank: Regular Contributor
timestamp: November 12, 2024, 07:27:34 pm
content: Oh! you mean the electrolytic capacitors on the power supply board?I was thinking on the main board, but power supply first might make more sense!! hhaha

author: cincin
author_rank: Regular Contributor
timestamp: November 12, 2024, 07:29:45 pm
content: Thank you. I will go take a closer look at the power supply board.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2024, 08:03:39 pm
content: Forgot to say, does it still make noise if you plug everything normally from AC mains, then you unplug the mains plug from the mains socket, rotate it 180 degrees, then plug it again (if the type of plug you have there permits to plug either way naturally, without altering the mains installation)?Sometimes this simple swapping of the null and phase AC wires by unplugging then replugging it rotated might help. If it doesn't, or if the AC plug/socket does not naturally allow plugging both ways, then just let it be. Or maybe replug the amplifier with the mains plug twisted half a turn.Again, this trick only works if the mains AC socket allows to be plugged either way. If not, do not modify the AC wiring, and do not try to swap the null/phase inside the Korg, it won't fix the noise.

author: cincin
author_rank: Regular Contributor
timestamp: November 12, 2024, 09:29:27 pm
content: Unfortunately, the plug is polarized. 3 prongs North American.

author: cincin
author_rank: Regular Contributor
timestamp: November 13, 2024, 02:23:13 am
content: I plucked out and tested all the electrolytic caps on the power supply board and they all tested perfect. And I do mean perfect, as in far better ESR and VLoss than any of my brand new Nichicon Gold in stock... so I put them back in.Onto the main board! There's quite a few on there. I'll start around the power coming in and work outwards, looking at everything in the power rails (not the signal path).Cheers!

author: RoGeorge
author_rank: Super Contributor
timestamp: November 13, 2024, 05:32:12 am
content: Usually it's the electrolytic capacitors in the power supply that dries out first, with time. If those in the power supply are OK, chances are the capacitors on the rest of the board are also OK, if not better. I'm not sure what I would do from this point.I would probably try to find out where from did the noise come. If it has any headset output, try listening with the headset before, then after you wire the Korg to the external devices. Start with just the Korg plugged to mains and the headset. Does it still has those noises? Can the backlight be turned off (sometimes the backlight has it's own local SMPS power supply that might develop dried out capacitors with time, don't know if this is the case for this model), try if the backlight on/off makes any change in noise.Then add one by one the amplif, PC, other MIDI or external devices you may have connected. I see in the pics it has an output that looks like an S/PDIF. If all else fail, maybe use the optical link instead of the line out.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 13, 2024, 05:48:00 am
content: I've looked at the schematics and it has indeed another SMPS inverter for display and backlight. I would check the electrolytic capacitors from that area, too (i.e. C42, C47, C1, C4).Another check, make sure the Y capacitors are OK (not short-circuited, C2 and C15 in the power supply).

author: nfmax
author_rank: Super Contributor
timestamp: November 13, 2024, 10:37:43 am
content: From your description, it sounds like the Korg synth is injecting noise into the AC supply that powers it, which is then affecting the amplifier through the amplifier’s own power supply when both are connected to the same socket. Try using a mains ‘noise filter’ in either of the power cords - preferably the Korg, but if it works better the other way, go with that.

author: cincin
author_rank: Regular Contributor
timestamp: November 13, 2024, 04:20:46 pm
content: Good call, I will take a look at the LCD power supply next!I know SPDIF or a mains noise filter would work as a work around, but I'd rather find and fix the issue than patch it.Quote from: nfmax on November 13, 2024, 10:37:43 amThat was my original conclusion as well. If I understand correctly, this can happen when the easiest path to ground is through the patch cable to the amp instead of through the chassis ground. That's why I was pondering wether the problem was likely to be a part failure or an oxidized / disconnected ground path. I guess both make sense. As stated above, I'd rather fix the issue if possible at all. I'll be checking the grounding points too.Thanks for the help!

author: cincin
author_rank: Regular Contributor
timestamp: November 19, 2024, 01:28:36 am
content: Well, I replaced the electrolytic capacitors on the LCD board, and those on the MIDI board beside it because it was easy and they were Elna caps which I've had problems with before. I also checked the grounding points and everything seems great. Put it back together: same noise.Interestingly, I also randomly just got a Monster Power HDP 2500, which is a glorified surge protector, but it claims to offer "HD Clean Power Stage 2 removes the effects of dirty power by minimizing noise and interference which adds extra wear and tear on your components".So I plugged-in the keyboard and amp into the fancy box and the noise is still there... though, about half as loud. So the box is filtering something, but not enough.QuoteThe noise is not present in the headset output. My assumption is that the 2 big ferrite beads on the headset board are removing it. The chassis ground and signal ground is the same on the main board, but the chassis ground of the headset board (and slider / bender boards) is different than the signal ground for some reason. Maybe that also prevents the noise from making it out the headset. Connecting the headset chassis ground to the signal ground did not make a difference in the noise.So I guess I'll be checking the backlight idea, the Y capacitors on the power board (I think I already did, but I'll verify), then the capacitors on the main board next, unless anyone has a better idea?Cheers!

author: nfmax
author_rank: Super Contributor
timestamp: November 19, 2024, 10:39:07 am
content: Quote from: cincin on November 19, 2024, 01:28:36 am

author: cincin
author_rank: Regular Contributor
timestamp: November 19, 2024, 04:14:46 pm
content: QuoteI did that too, every which way, same effect: it reduces the noise but doesn't get rid of it.---I did some more testing, I think I'm getting closer, because things are getting weirder!Attached is a photo of the underside of the power board for reference.There are 2 grounds throughout the circuit, the analog ground (AGND, pink in the photo) and digital ground (DGND, yellow in the photo), but they aredirectlyconnected on the power supply board. The connection between the two is done via the support legs of IC3's heatsink (in red). One support leg is in the analog ground, the other in the digital ground. IC3 is the 3.3V rail voltage regulator (ka78r33).When the keyboard is Off, there's no resistance between the AGND and DGND, as you would imagine. Butwhen the unit is On, my DMM indicates at least 20 Ohm of resistance between the two(measured at the power board connector). If I measure it with my leads backwards I get -20 Ohm... so my guess right now is that there's a device somewhere, touching both the AGND and DGNG, that's doing a thing. There's 0.009V between the AGND and DGND when the unit is On. I'm not sure I understand how that's possible considering they are connected. The diode test of my DMM did not find a diode between the grounds.- The noise may be related to the LCD. It does change pitch during the boot sequence and it does so in synch with what is displayed on the screen. But that could be a coincidence.- I checked C2, C3, and C15, the Y capacitors on the power board and they all checkout fine.- If I connect the chassis ground (DGND) to the headset ground at the sleeve, I can hear the noise in the headset too. But somehow I can hear the noise clearly from the outputs without having to connect the AGND to the DGND. If there's noise in the chassis then I could see it affecting the analog trace by proximity around the outputs, and it would not cause this around the headset out because the chassis isn't grounded there. But why isn't this noise / stray voltage simply draining out to the AC ground that's connected to the chassis?I'll be busting out the scope and taking a look at the 15V, 5V, 3V, AGND, DGND, etc, for noise. And I'll be looking at any devices that touches both the AGND and DGND.Anything else I should try? Cheers!