Can Someone Explain These Numbers?

[waynegreen]

The following are AC voltages measured between the shield (ground) of an RCA cable plugged into an EMU 0404 Sound card and the shield (ground) of a second RCA cable plugged into the AUX input of an SX-1250 receiver. The cables are connected ONLY by the test leads from a FLUKE RMS voltmeter. The computer remained on for all measurements. The computer is plugged into a standard 3 prong surge protector using a standard 3 prong power cable. The surge protector is plugged into a standard 3 prong wall outlet. The receiver is plugged into the same surge protector using a non polarized
2 prong plug. Obviously there are two possible orientations of the Receiver plug. Below are the measured voltages as well as the frequency indicated by the Hz button on the meter.


ORENTATION A
Receiver Power OFF: 104V @60Hz
Receiver Power ON: 36V @60Hz


ORENTATION B
Receiver Power OFF: 2V @60Hz
Receiver Power ON: 75V @60Hz


Seems like the preferred orientation of the Receiver plug is B but
why is the Receiver ground shifting when the power is turned on?
(leaky cap?) Is the 75V a danger to the SC?

 

[dshoaf]

Its ground loop(s) for sure. The problem is that you've got lots of places where they can occur in the current setup. I'll bet your measurements will change a lot if you hook the chassis of the computer and the Pioneer together. First, I'd eliminate all the items that don't really make a difference here like the surge protector and plug all units into the same outlet. BTW, are you sure that the outlet itself is wired correctly *and* that the wiring is making solid connections back at the panel?

Once you've eliminated that, then concern yourself with the computer. Those power supplies were not designed with high priority to leakage/grounding either - they're called Personal for a reason. Suspect this part first once you've eliminated all the other stuff. Lastly, suspect the receiver but only after you've confirmed consistently similar measurements with other items out of the picture.

The biggest problem with ground loops is that you have to 1) eliminate as many items to get a consistent basis of measurments, preferably eliminating them all together before adding in other peripherals and 2) add in only one item at a time and confirm all measurements as you go. If they change significantly, then you've got another one of those pesky devils in there.

This assumes, of course, that your measurement device and the source of AC are also good. I was beat up about questioning everything including all measurments in my engineering classes and it applies doubly with ground loop(s).

Also, assume there is more than one ground loop. There may be only one but that's not a good assumption, either. Don't let anyone kid you - these are real gremlins to exterminate. :alien:

Cheers,

David