Power supply problem, or GND? (whiney synth) - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: December 01, 2024, 08:36:44 pm
content: I'm working on repairing a Korg synthesizer that has an atrocious whine in the outputs. I'm looking at just the power supply board at this point because the DC rails and grounds seemed noisy: 700mV of noise on 15V (almost 5%).Interestingly, I'm finding the same 700mV of noise on the DC grounds. (the schematics, 1st attachment with no icon, seem to differentiate AGND from DGND, but they are directly connected on the power supply board, and connected to the chassis and AC GND).Also, if I disconnect the other boards from the power board (no load), the 700mV of noise remains, and the -15V rail gets a 1.5V 45ms ramp up sawtooth wave around -15V, and the +15V gets a similar wave but it's a ramp down sawtooth and its top is squared off. See scope grabs.Probing the rectifier (BD1) output shows the "DC" is less rectified than what I would expect. See attached scope grab of funky 160V chopped sine wave. This looks the same with load or no load.- I've pulled and tested all the electrolytic capacitors form the power board, they all tested fine with very low ESR.- I replaced C5 anyway to be sure. The schematics say it should be 150uF but the original was a 220uF, so I replaced it with a good 220uF.- I pulled the rectifier and it also tests fine.My next step is to replace the other electrolytic caps, just in case...but I don't understand:- Why the DC comes out of the rectifier like this.- If it's normal or indicative of the problem.- If the ramp up / ramp down on the 15V rails are normal when there's no load, or if it's indicative of the problem.Am I even safe to assume that the problem is on the power board given that the 700mV of noise is still present when the other boards are not connected?Help a confused beginner. Any insights greatly appreciated! Thanks!

author: cincin
author_rank: Regular Contributor
timestamp: December 03, 2024, 12:56:55 am
content: Well, I replaced all the electrolytic capacitors on the power board, and that did not cure the whine. It did clean up the noise when no other boards are connected, but the noise is still there when everything is connected. The ramp-up ramp-down sawtooths are still present on the 15V/-15V rails (and still chopped on one of them) when there is no load.My theory is now that the problem is upstream (another board, likely the main board), and that the ramp-up ramp-down I'm seeing when there's no load is "normal".I still don't understand how that waveform coming out of the rectifier can be normal, but I'm guessing it could be??

author: CaptDon
author_rank: Super Contributor
timestamp: December 03, 2024, 04:38:15 pm
content: Those waveforms would equate more to a low frequency hum or buzz. Since you have a scope do a signal to noise measurement. Set the volume to max and measure the peak to peak output of a sustained note, then with no keys pressed measure the peak to peak residual noise. Obviously you will have to change vertical scaling on the scope. I would expect around 5VPP for the sustained note measured at the Left/Mono output. The Right output should be similar. The residual noise may be in the 10 to 20 millivolt peak to peak range. Let us know what you find. Some synths are so 'dirty' it isn't funny!!! You need to know how to set up your gain structure on your mixer / amplification chain. On some synths the residual noise remains constant no matter where the volume control is set so we end up turning the volume all the way up to get better signal to noise ratio and then feed a 40DB pad to not overdrive the mixer inputs!! I suspect your 700mv of noise on the ground is being caused by a ground loop between the scope and the Korg. There isn't a real 700mv of crap on the Korg internal ground system.

author: cincin
author_rank: Regular Contributor
timestamp: December 03, 2024, 08:20:33 pm
content: Thanks for the help!Agreed, I don't think that sawtooth is what I'm hearing, I just thought it was a symptom of something going wrong on the power supply board. I may have gotten side-tracked...I did a signal to noise measurement. I had to use the piano sound because finding a monotone continuous tone on this beast, while it's open and upside down, is a challenge. Still, I played the keys as hard as necessary to get the full volume and the signal peaked at something like 3.5V. I bet a full volume tone would max out at 5V as you predicted. The noise was about 75mV with the scope looking at the full spectrum, about 25mV when the scope bandwidth is limited at 20Mhz. Interestingly maybe, there's also a +10mV DC bias to it. I also found +9mV between AGND and DGND even though they are directly connected on the power board via IC3's heatsink.Attached is a recording of the whine, handheld cell phone in front of the amp... because involving my workshop PC's sound card just added another horrible buzz for some reason. The recording starts with the amp On and keyboard Off. I then turn On the keyboard and you can hear the various stages of the whine as the unit boots.It sounds like the digital processor booting, or maybe the LCD since the noise changes with the image, but that could just be the boot sequence. The whine is not affected by the volume pot.The fault that's causing the whine seems to involve or contaminate the ground somehow, but since it's not a typical ground loop hum I've been chasing a failing component or connection to ground that could cause that. If I power the keyboard with a battery pack the noise is gone, but other keyboards in the same scenario are perfectly quiet. I also tried plugging it all (then just one, just the other) into a power conditioner and the noise is less loud but still present.The whine doesn't come out of the headphone jack, but the chassis there is not connected to the rest of the chassis ground or signal ground. If I connect the signal ground at the headphones to the main chassis ground, the noise comes out of the headphones too.I looked at the grounding points extensively, and everything checks out when the keyboard is Off, but when it's On my DMM sometimes returns 20 Ohm of resistance, and sometimes even negative resistance between grounding points or different spots on the chassis (AGND and DGND are both connected the chassis ground). So there's some potential on there(?) but how can that be when it's all well grounded (<0.3 Ohm) the rest of the time?Full schematics are here.Any insight is gold!TY