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 Forum index » DIY Hardware and Software » The Repair Shop
Ground loop debugging/fixing (Old Synth)
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Regebro



Joined: Sep 12, 2004
Posts: 44

PostPosted: Wed Jan 15, 2020 6:21 am    Post subject: Ground loop debugging/fixing (Old Synth) Reply with quote  Mark this post and the followings unread

I know, I know, I know, I know. You "fix" ground loops by using balanced cables and lifting ground etc, blah blah.

But I want to try to understand why my OSC OSCar causes a ground loop, so I'm digging into this. I'm aware that maybe I can't fix it, and it's just how things are, but no matter. I do not want to apply voodoo magic, so please, no arguments of "try changing the connection from your mixer to your speakers to wires with the shield connected to the ground, or if that doesn't work, try using cables where they aren't connected". I'm trying to find the *source* and see if it's fixable.

To that end, this is what I know so far:

The 100 hz buzz only happens when connecting the OSCar to my mixer, who is also grounded, and my powered speakers, who also have a ground. If I connect the OSCar to an amp that is powered by wall wart, all is fine.

As expected therefore, the output from the OSCar in isolation is fine. Digging into the OSCar, I can clearly see a buzz of about a few tens of mV between the output 0V and the "chassis" earth (ie the ground cable coming in from the power plug). I suspect that this is the source. The 0V of the OSCar seems to have a buzz compared to the ground. That seems to actually mostly happen **inside the connector** from the transformer to the circuit board, and yes I know that sounds like crazy talk, but I'll prove it to you:

https://youtu.be/xT8H3TeKPJ4

Is this normal? Should I just like replace this connector or something? Would that help? I don't want to run this with a ground lift box for all eternity. Smile
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Regebro



Joined: Sep 12, 2004
Posts: 44

PostPosted: Wed Jan 15, 2020 11:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Nope, false alarm: I flipped the circuitboard upside down, and the buzz is introduced into the 0V over the first few cm of the circuitboard. It doesn't seem to be magnetic. I though it could be, since the transformer is there, but then it would change when you move the circuitboard, and it doesn't.

Perchance there is some capacitance in the circuitboard that causes it. I can improve it with additional 0V paths, but only around 50-65% better. Maybe the +5V and -5V has similar issues, and it could be improved again with additional paths for that, but that seems silly.

Replacing the power supply with an unearthed one is also a possibility.
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PHOBoS



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PostPosted: Thu Jan 16, 2020 2:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

I don't think I can be of any help (also read the thread on muffwiggler) but I am curious what the earth connection is connected to
as I think you mentioned the case is made of wood and plastic. Is there an electrical connection to another part of the synth ?

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Grumble



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PostPosted: Thu Jan 16, 2020 2:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

@work I also design amplifiers but more for measuring emg, eog, eeg and the like, so they are quite sensitive.
I noticed that they sometimes pick up 50hz hum, but the weird thing is that it depends where I put them on the workbench Shocked
I also noticed that it was gone if I ground the metal part of the workbench (it consists of a metal frame with synthetic material working area).
Maybe this doesn’t fit to your situation, but it just tells me how strange problems arrise and how they can be avoided or brought to a solution.

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Regebro



Joined: Sep 12, 2004
Posts: 44

PostPosted: Sun Jan 19, 2020 5:53 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
I don't think I can be of any help (also read the thread on muffwiggler) but I am curious what the earth connection is connected to
as I think you mentioned the case is made of wood and plastic. Is there an electrical connection to another part of the synth ?


No, it's just connected to the case of the transformer and the 0V of the circuit board.
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PHOBoS



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PostPosted: Mon Jan 20, 2020 4:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah ok, so it is connected to the 0V and not just to some metal that isn't electrically connected.
Yeah, that can cause all kinds of difficult and hard to solve problems.

I have a book with some audio circuits and there is a section about dealing with hum which shows adding a 100 ohm resistor between
earth and 0V. note that earth is connected directly to the chassis though so if you have any (metal) connectors mounted to it that would
probably make it useless, but if your case isn't made of metal that might work.

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Regebro



Joined: Sep 12, 2004
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PostPosted: Mon Jan 20, 2020 6:20 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
I have a book with some audio circuits and there is a section about dealing with hum which shows adding a 100 ohm resistor between
earth and 0V.


Ah, yeah, I thought something like that could work, but I'm unsure about the implications and safety stuff etc.
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PHOBoS



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PostPosted: Mon Jan 20, 2020 7:00 am    Post subject: Reply with quote  Mark this post and the followings unread

Regebro wrote:
Ah, yeah, I thought something like that could work, but I'm unsure about the implications and safety stuff etc.


I thought about that aswell and I don't think it would cause any problems safety wise. The earth connection protects against any
metal parts coming 'live' at least if connected through an RCD which will trip if the leakage current is too high. If for some reason
the 0V/GND would become live then the current through a 100 ohms resistor is still more than enough to trip the RCD.

Also if you would have a metal case then metal connectors should be insulated from it otherwise the 100 ohm resistor would be useless.
Ideally shielding should not be connected to 0V only to earth and only on 1 side but that requires different/balanced connections.


edit: I am not a safety expert so do it at your on own risk

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Regebro



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PostPosted: Mon Mar 23, 2020 12:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Solved!

https://youtu.be/SoE-R3BJc3I (at 8:30 if you are impatient)
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PHOBoS



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PostPosted: Mon Mar 23, 2020 7:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Good to hear you got rid of the hum Smile

I am not sure if I've seen a capacitor been used before and it seems a bit odd, but hey if it works. And as long as you have the earth
still connected directly to the chassis it should still be safe. Frustrating to see how some people (not you) can do such a horrible repair job.

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Regebro



Joined: Sep 12, 2004
Posts: 44

PostPosted: Mon Mar 23, 2020 8:21 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
Good to hear you got rid of the hum Smile

I am not sure if I've seen a capacitor been used before and it seems a bit odd, but hey if it works. And as long as you have the earth
still connected directly to the chassis it should still be safe. Frustrating to see how some people (not you) can do such a horrible repair job.


I did a horrible repair job as well, to be honest. But shockingly, the professionals did just as badly. As mentioned, the plan was to make it work for 35 years more, and I strongly doubt that I succeeded with that.

Much of that is because the PCB board is brittle. I basically tends to melt when you solder on it. You need a combination of low temperature, and liquid flux, and be quick to not have melting of the PCB.

Maybe, sometime in the far future, when I've learned modern tools for making PCB's, I'll make a batch of new PCBs that fix all the problems, like the daughter board hack, actually connecting the battery to the main PCB where it belongs, etc. It would be a fun challenge!

Edit: Oh, and I used a capacitor, because that's what the paper from the Royal Institute och Technology told me to use. And it was very thorough, so it seemed like the authors knew what they are talking about. It's in Swedish, though.
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PHOBoS



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PostPosted: Mon Mar 23, 2020 9:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Regebro wrote:
But shockingly, the professionals did just as badly.

That's what I am referring too, assuming those are people that actually ask money for repairs.
Honestly they should be ashamed of themselves.

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Regebro



Joined: Sep 12, 2004
Posts: 44

PostPosted: Tue Mar 24, 2020 7:19 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
Regebro wrote:
But shockingly, the professionals did just as badly.

That's what I am referring too, assuming those are people that actually ask money for repairs.
Honestly they should be ashamed of themselves.


Yeah, the first repair was quick and expensive, and as you saw, there was still shoddy soldering involved. The other one cost the same as the first, but the guy certainly spent a lot more time on it, and also bought some components, so that wasn't expensive. But it was just shoddy, like if he was on time pressure.

But, lessons learned:

1. I need to be more careful when desoldering, especially things like sockets and connectors.
2. Never let somebody do repairs on old equipment unless I sit and watch.
3. Don't fix it if it ain't broken. No, not even if it is could need improvements. Wait till it breaks.
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