IOGear GCS-634 TeardownPosted: September 26, 2010
I’ve been having some RFI problems lately with my computer setup at home. I run my workstation and server through a IOGear GCS-634 USB KVM connected to a Dell 2309W display, a Logitech Marble Mouse and a USB keyboard. This KVM is a four-input model that switches USB, VGA and audio between computers; two of the KVM cables are hardwired to the unit, the other two (which are included with the KVM) are optionally connected to the unit. I use two of the four inputs and use the others when I occasionally fix a computer on the bench.
I wasn’t certain I would find the problem just by opening up the KVM, but I seldom have the opportunity to take apart even my own electronics and I always take pictures when I do.
This is the bottom of the box. The screws are under the outside corners of the rubber pad in the center.
The circuit board:
The component side of the board:
I wonder if this is a JTAG port on the right?
I’m still looking for the RFI. At first I suspected a ground loop between my workstation and server as they are on different outlets and probably different circuits; whenever National Grid drops a phase to my building feed, only my workstation or my server will go dead or reboot, usually not both at once. However I’ve disconnected the server from the KVM and the interference still persists. I’ve measured between the ground frames of the two machines and I see no significant voltage, AC or DC, between the two (A 1V difference would be enough to warrant remediation in a typical installation.) I changed out the power cord on the server since I tend to reuse older cords—no change. I ran without the KVM for a few days and no RFI.
It only happens when the workstation is switched in on the KVM and does not appear on the server’s KVM display. It shows up as vertical waviness; when I play games, the interference is most pronounced on vertical lines and features, especially at the lower resolutions of the old games I tend to play. This points to RFI in the vertical sync line of the VGA output. You can actually see the vertical jitter on the screen at a frequency of around 5 to 30 Hz as it surges up and damps down. I had it happen while I was writing this post but it damped down just as I got the camera.
My VGA card, an eVGA GeForce GT-240, may just be unusually susceptible to RFI. It seems like it’s ringing. I wish Tek let me take their scope home the other day—I would have found the answer tout suite! I have an idea for fixing this based on an RS-232 metal dongle shell, VGA connectors, ribbon cable, caps and beads, that I will try when I get a chance.