DA49 Acting Up

I've had intermittent failures on a Audio-Technica DA49 distro. At first it seemed that it could only power 3 of the units before it would go into some sort of failure mode where the power LED on the DA49 would go dim and the receivers would shut down. We were able to limp through a show by using a separate power supply for one of the 3000's and the distro would maintain the other three (I kept all of the antennas patched in).

More recently it wouldn't power on more than two so I started to investigate. We have two of these units and swapping the power supplies made no difference. I measured 12v from the power supply at all times, even when it failed. The weird thing is that the faulty DA49 would power on the other bank of receivers just fine. In that instance I hadn't reconnected the antenna cables from the test receivers so it got me thinking.

I went back to the original receivers and just plugged in only the power. I had already opened the DA49 and realized there's not a whole lot going on inside, particularly with power supply. It looks like the power distro part just has some kind of regulator IC and everything's wired in serial, so it seemed unlikely that a failure here had occurred.

Anyway, once I powered on without antenna cables attached it worked! I then patched in each antenna cable one at a time and it stayed on. Power cycled a few times and no issues.

So I'm wondering what exactly is going on. It is possible that I accidentally mismatched antenna cables when patching it in initially, but no changes were made from the initial failure to the later failure. When I attempted to replicate by mismatching antenna wiring it did not result in a failure. I don't know that mixing up antenna wiring would actually cause a failure like this in any event. I know it has the ability to run powered antennas, but I can't find any literature about troubleshooting here.

Help?

Check to make sure that the antenna power switch inside the unit is off, provided you have passive antennas. The only thing I can guess is some antenna connection that should be a DC open is drawing DC current and loading the power supply down.

I have an AT DA600 that was doing the same thing. It would just shut off in mid performance. Power cycling might turn it back on, or not. Took it apart and of course powered on every time on the bench. Everything inside looked good, nothing burnt or bulging caps. Put it back in service and it started doing same thing. Wound up pulling out of rack and just powering with individual PS's. It now sits on the shelf of "what do I do with this stuff now?" It died around 10 years of service. I am starting to really retire my 3000's now. I used all passive antennas with this setup, cascaded two units together.

This one is less than a year old. I had another failure, prior to buying the distro, where a power supply for an individual unit failed. Replaced it and had no further issues, but I'm wondering if there is an issue with one of the units that would cause a short somwhere? I was going to send the distro to AT for repairs, but I'm not wanting to send it off if the problem is further down the line. And seeing how it's built I'm not convinced the problem is confined to the distro, but may be a symptom of another issue like FMEng said.

Some of these are 600Mhz units anyway so it might make more sense to throw money at new RF units rather than spend $$ on hardware that may soon be outlawed. Thanks for the insight!