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Grounding problem on a PAiA 9700
MUFF WIGGLER Forum Index -> Fractional Rack Modules  
Author Grounding problem on a PAiA 9700
Oxylum
Hello guys,
I am facing a strange problem with my PAiA 9700 synth (fitted with a 9710, a 9720 and a 9730 boards), installed in the PAiA rack.

Up to now, my Frac and Euro racks were sharing a common power supply, built around two 12V switching power supply modules (made by Ohmeron if I remember well). The PSU modules were in the Euro rack and three wires were going from the Euro to the Frack to carry the +12V and -12V (forgot to say that I modified the boards following PAiA instructions to make them work properly with 12V)

Last week, I decided to split my modular setup in two, one part being around the Euro, the other part being around the Frac. I decided to move the PSU from the Euro rack to the Frac (and buy some new ones for the Euro)

Cabling remaines exactly the same, it was just a physical move from one rack to another. But the PAiA started to react very strangely: first, a lot of hiss noise could be heard on the VCF output, over the signal. eek!
And when I played a first note, the waveforms from the VCO were incredibly distorted. I finally discovered that the PSU were the cause of these problems, because everything became OK again when I dismounted the backpanel.

It seems that there is a kind of ground loops somewhere, because problems occur only when the metal plate on which the PSU are installed comes in contact with the rack itself.

What I do not understand is that I use *exactly* the same schematics and the same power supply when they were shared between my Euro and my Frac racks.
And both racks were installed in the same flightcase, with the mounting brackets being earthed in the same way.

So, right now, my Frac is running with the power supply board just a few centimeters away, but if somebody has any idea of what could explain that phenomenon... and how to solve this... help
fracmonkey
"It seems that there is a kind of ground loops somewhere, because problems occur only when the metal plate on which the PSU are installed comes in contact with the rack itself. "

This may be a clue...

In general, you do not want to ground metal parts of the cabinet, chassis or faceplates, either to the DC com or the AC "safety" ground.

Because of "iffy" connections in the metal chassis, etc due to anodizing for instance, ground loops can occur. These iffy grounds will "fight" the good grounds created by running commons from the jacks etc to the DC commons.
Oxylum
Hello Fracmonkey,

thank you for your suggestions. Maybe I found something yesterday following your idea of "safety ground" related problems.
I discovered that the shell around the power supplies modules are connected inside the module to the PCB. I was assuming that this was just a safety measure to connect the "earth" screw on the connector to the shell, but I saw that some components (the input filter from what I can see) are also connected to this.
I made a test yesterday by mounting the power supply on a isolating plastic sheet. And then, no more problems we're not worthy
I think that some currents are circulating between the rack itself, the jacks on front panel, the AC filter, etc... which are seriously disturbing the modules.
Thanks again for the idea
e-grad
fracmonkey wrote:
In general, you do not want to ground metal parts of the cabinet, chassis or faceplates, either to the DC com or the AC "safety" ground.


Oakley projects have a split panel ground / earth ground scheme. That's why I used to think that would be desirable since Tony's designs are top notch.

BTW I've got a similar ground issue as the original poster. May be it due to my mixture of an Oakley split gnd scheme and a Blacet common gnd scheme?
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