Computer Control & Ground Loops

From the Ten-Tec Reflector May 2, 2007

I got audio distortion whenever I operated the Omni VI+ with my amp, but only if the Omni was connected to my laptop. I found a solution that might help someone else.

I tried most everything. The only real connections from the computer to the radio were the audio out to computer sound card cable and the CAT serial cable. I solved the audio out problem with a Radio Shack audio isolator.

Now, the only other connection between the radio and the computer was the DB25 cable. Mine was DB25 to DB9 to USB/Serial converter to computer. I rebuilt the DB25 by removing everything but pins 2, 3, and 7 for TX, RX, and ground. Didn't help. I put ferrites on the wires (connector 72) that take the serial input to the logic board in the Omni VI. Didn't help. I put .01uf caps from pins 2 and 3 to pin 7 (ground). Didn't help.

Then I got to thinking. The solution to the audio cable had been an audio isolator, called a "Ground Loop Isolator" by Radio Shack. Hmmm, ground loop.

I cut the ground wire on the DB9, removing a ground connection between the computer and the radio. I doubted the CAT interface would work with just pins 2 and 3 between the computer and the radio. But, it did. And, my audio distortion when using the amp was gone. Not sure why it still works, but I'm not asking any questions.

So, the problem is solved…

Thanks to those who provided suggestions.

73 de K1ESE

May 2, 2007

Indeed, John, you have a sneak ground loop path that was duplicated by pin 7 ground. You eliminated that 7 ground wire, and you now have only one ground, likely the chassis ground. In some equipment, 7 was isolated from chassis which was pin 1 in original 25 pin RS 232 implementation. Later, computer makers got lazy and often tied 7 and 1 together and used 7 only between devices. Wonder if you had tried ferrites on 7 wire before you cut it?

You do ground the computer, but just do not know it. Its ground is chassis, which goes to AC connector third pin. (The green or green yellow wire).

RF has a way of getting into AC grounds and causing havoc.

Document what you did in case you disconnect things to take them to Field Day or portable some time. You may need a way to replicate the pin 7 wire ground someday. I keep stuff like that in the instruction manual for the rig or computer device, or on back of logsheet pages. (I keep all logs forever).