[Shop-talk] Ground loop?

[w]

On 2/13/2014 11:10 PM, Dave Cavanaugh wrote:
> I have a new Yaesu FT-7900 dual band mobile ham radio I use for my base
> station.  I had intended to run it from a Pyramid 12 amp 12 VDC
> conventional power supply.  It (the power supply) is a big, heavy
> brick.  In limited use it seemed to work fine.  Then I tried to hook up
> a programming cable to upload my channel info into the radio via a USB

Sounds like the prob is obviously in the inverter. I did a three month 
apprenticeship type summer job as a computer tech in high school at 15 
y.o. back when we fixed the things vs just board swap, and later flunked 
out of Double-E.

Important concept: "ground" is absolute in concept, but relative in 
practice. You may have 12VDC on the outputs, but it's "floating" and who 
knows what it is relative to the next device.

Back in 1982 we had a customer with a 9-track refrigerator sized tape 
drive that was failing. But worked perfectly when we got on site. 
Eventually figured out there was a bad ground strap in it, but the first 
thing we did was throw the oscilloscope on it effectively fixing the 
ground with the probes and couldn't get it to fail. Arrgh!

Put your meter or silly-scope between the mains, inverter output, PC, 
etc, in both AC and DC coupled and look for voltage offsets. Guessing 
there's a big one between your inverter and the PC.

USB is a more complex bitch than you might think -- it's 5VDC and 
initially allowed to pull 100ma, but allowed to chat with the root hub 
controller and request to use more power in 100ma increments. Would take 
very little to throw that off, as you found out. Just be glad you didn't 
fry something.

Could you just use a PC power supply to run it? They're cheap, and clean 
high amp 12VDC outs these days. Hit NewEgg for a Corsair or Thermaltake 
brand for about $40.

HTH. Sorry I'm kinda floating on this one myself.

-wayne