Something to keep in mind: Some power supplies come with the negative terminal grounded, that is, connected to the metal cabinet which is normally connected to the green wire in the power plug, but some are left floating. Interesting things happen when two or more power supplies are connected to different loads in the same rack, because there may be a DC potential on the grounding system. It is not always a good idea to have the negative lead grounded in more than one location.
73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Bob Sent: Tuesday, July 29, 2008 1:06 PM To: [email protected] Subject: [Repeater-Builder] Re: Power Supply Question Mike: In my only vaguely-informed opinion, the situation with two power supplies as you describe is not unreasonable, but a ground loop may or may not be the problem. What is the exact nature of the symptoms? Hum? Buzz? Controller freaking out? If you're running a DC cooling fan (or fans) temporarily disconnect same and see if the problem goes away. Over the years in the broadcast biz, I've seem situations where lifting a ground provided a miracle cure, and almost exact same situations where adding a ground did likewise. I'm convinced that ground problems fall under the heading of FM--"freakin' magic"! That said, try some experiments: Tie the two power supply grounds together. If none of the gear is floating above ground (or otherwise in a configuration that could result in a difference of potential between cabinets), see what happens if you bond all the various chassis together (but avoid daisy-chain grounding). Try lifting the ground of the AC cord (use one of those 2-to-3 prong adapters); this is only for diagnostic purposes--long term you don't want to leave things this way. Use a voltmeter--one probe on the ground of the P.A., one on the exciter's ground--to see if there's any potential. Then, after you complete all these various science projects, double check the fan situation again. Switch to a DC-relay switched AC powered fan. Good luck. Keep us posted. 73 de K5IQ Bob
