# Vox AC30TB ground loop question



## nonreverb (Sep 19, 2006)

Well...I've got a question for all you techs out there...from one tech to another. Has anyone worked on or know the "official" mod to solve the ground loop issue on the UK-made Korg Vox AC30's? I picked up one of these a while ago and although I like the sound, the build is a little shoddy.
There are many discussion groups where the problem is discussed but no one offers up the fix. I could and maybe will find the solution myself but if someone knows the mod, it would make things go a lot faster...
Thanks in advance, Richard:smile:


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## Wild Bill (May 3, 2006)

nonreverb said:


> Well...I've got a question for all you techs out there...from one tech to another. Has anyone worked on or know the "official" mod to solve the ground loop issue on the UK-made Korg Vox AC30's? I picked up one of these a while ago and although I like the sound, the build is a little shoddy.
> There are many discussion groups where the problem is discussed but no one offers up the fix. I could and maybe will find the solution myself but if someone knows the mod, it would make things go a lot faster...
> Thanks in advance, Richard:smile:


Richard, I did some googling myself and it appears the reason you can't find the mod is because there isn't one!

The actual circuit board was poorly designed, with at least two ground loops built in by the way they ran the traces for connections. Later models had a completely different board.

You can look at the board yourself and see if you can find the loops and perhaps cut traces and add jumpers to correct the grounding problem. Or, you can turf the board and re-wire the amp true point-to-point.

Just another PITA thing about printed circuit boards!

:food-smiley-004:


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## WCGill (Mar 27, 2009)

How about regrounding it? I'm not familiar with this model, nor the issues, but it seems like a logical buss or star-ground system would work. I'm sure it won't be easy either.


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## nonreverb (Sep 19, 2006)

WCGill said:


> How about regrounding it? I'm not familiar with this model, nor the issues, but it seems like a logical buss or star-ground system would work. I'm sure it won't be easy either.


Believe it or not, it HAS a star ground. Like Bill mentioned in the previous post, the circuit board design on the early Korg-built AC30 had problems.
I actually like the sound of this amp but there are certain mods I'm doing to it to make it more reliable...ie. replacing the power supply caps with spragues and removing both the caps and cathode resistors from the board to their own termimal strip.The cathode resistors are presently on the board touching a couple of electrolytic caps!!...Crazy.


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## Wild Bill (May 3, 2006)

WCGill said:


> How about regrounding it? I'm not familiar with this model, nor the issues, but it seems like a logical buss or star-ground system would work. I'm sure it won't be easy either.


It's a printed circuit board! There are no wires that you can easily move around, only traces "carved in stone" on the board.

Progress was wonderful, eh? I hate pcb's! Give me old fashioned tie strips any day.

:food-smiley-004:


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## nonreverb (Sep 19, 2006)

Wild Bill said:


> It's a printed circuit board! There are no wires that you can easily move around, only traces "carved in stone" on the board.
> 
> Progress was wonderful, eh? I hate pcb's! Give me old fashioned tie strips any day.
> 
> :food-smiley-004:


I'll keep you posted on my progress.... or lack of it


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## mkaye (Jan 25, 2009)

adding some wires to the ground plane on the circuit board and then tying them all back to 1 point may help (at least it has for me in the past when i used to build crossovers)

mark


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## nonreverb (Sep 19, 2006)

mkaye said:


> adding some wires to the ground plane on the circuit board and then tying them all back to 1 point may help (at least it has for me in the past when i used to build crossovers)
> 
> mark


The amp has a star ground already, the problem is definitely a design flaw as it was corrected shortly into production. My guess is that breaking traces will be required and hardwiring new grounds in. Particularly on the normal channel as that's where most of the problem is.


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## nonreverb (Sep 19, 2006)

Well...here's a first..the normal volume pot was making most of the noise. It appears that the OEM ones are lousy at shielding the signal. I replaced it with a 500K CTS pot and voila! no more hum. I'm planning on replacing all the volume controls based on my finding. What a pain in the ass this process has been.


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## nonreverb (Sep 19, 2006)

nonreverb said:


> Well...here's a first..the normal volume pot was making most of the noise. It appears that the OEM ones are lousy at shielding the signal. I replaced it with a 500K CTS pot and voila! no more hum. I'm planning on replacing all the volume controls based on my finding. What a pain in the ass this process has been.


Amp is back together and working great. I also found a signal trace that was running beside the heater wires. Rewired it with shielded wire and re-routed it. That took care of the remaining noise...


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## greco (Jul 15, 2007)

Congrats...I'm impressed with your determination and skills :bow:

I'll bet you will be happy to see the end of the inside of the chassis of that amp...still, you learned/tried some new things (I assume) that you can apply in future. 

Cheers

Dave


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## shoretyus (Jan 6, 2007)

Can't wait to try it.... sometime.


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## nonreverb (Sep 19, 2006)

greco said:


> Congrats...I'm impressed with your determination and skills :bow:
> 
> I'll bet you will be happy to see the end of the inside of the chassis of that amp...still, you learned/tried some new things (I assume) that you can apply in future.
> 
> ...


Thanks Dave,

I think my next adventure with this amp will be to convert it from PCB to point to point.
But that can wait till another day...:smile:


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