Ground loop gremlins

[fsaviator]

Hey everyone...  need some help here.  I'm at a standstill until I figure this out - I apologize for the lengthy post but you'll see I had little choice.

I've spent two days trying to isolate what appears to be a ground loop that I noticed when I leaned against my TQ and pedestal.  The annunciators in my firepanel would flicker...  took a bit but figured out the TQ side panel had rubbed paint off and was touching the CDU bay in the front, and pedestal in the back

After much searching I determined that the CPFlight five pin cable going to the pedestal was grounding out to the frame somewhere.

More digging and I isolated it to my CPFLight MCP, and finally after more searching to the serial connection on the MCP.  Apparently one of the pins in the RS232 connector grounds out to the frame! 

The serial cable is connected to USB to serial cable connected to my Main PC WHICH IS OFF.  When I remove the USB from the computer, the ground loop is gone.

Now here is where I need help...

I removed all the connections from the PC to the sim... no cables from the PC to any part of the Sim or even visuals..  only the Power cable and the USB to serial adapter and there is still a ground loop, even when the PC is off.  Unplug the PC and the ground loop is gone.

I assume there must be a short elsewhere, and I should now start unplugging power cables connected to the same circuit to isolate the item then trace down the short?

Thoughts?

I'm so close to being up and flying and now I'm bogged down in this, but I know if I don't fix this, I'll have gremlins loose driving me crazy.

[fsaviator]

I traced the issue down to my MIP power supply.  It's an ATX PS that I modified with terminal blocks to provide +5VDC and +12VDC to my fuse blocks.  As soon as I pulled that out, my frame and PC are no longer "connected".  I wonder if it was because I had it mounted in the shell...

Got it connected back up outside the shell and the issue was resolved.  Just to confirm, I ran a jumper from the ATX PS housing to frame, and the problem was recreated....  the frame and PC were once again "connected", through a USB Hub powered by the PS (which was connected to the PC).

Am I making a big deal of nothing?  I guess once I get everything back together we'll find out if the flickering lights in the Fire panel persist.

[Trevor Hale]

Great Find warren.  Sometimes these things arent big deals, until they become one.  Glad you found it.
Bob had something similar once, his landing gear would deploy randomly that was a problem LOL.
Trev