DPX plus 3b 2 cabinets. System has one Digital station card the rest are all IST. When I arrived on site the IST ports in the main cabinet were ringing but the upper were not. I assumed the Ring Generator in the upper cabinet was bad. So I moved all the IST cards into the uper cabinet and swapped out the ring generator. At this point none of the IST ports would ring. I tried moving all the IST cards to the main and moving everything else to the upper cabinet, tried different combination's of power supplies and ring generators and put everything back the way it was originally and none of the IST ports ring anymore.

First question is do both cabinets have to have a ring generator even if all the IST cards are all in one, the only reason I ask is because that's the way it sounds in the manual. I wouldn't assume that you need 2 ring generators if all the IST boards were in one cabinet.
I'm kinda thinking that something in the upper cabinet caused the first ring gen to go bad and when I moved the known good one up it blew it out and now I have two defective.

I'm looking for suggestions on where to go next. What to replace and where to start.

I know this post is old but I am having exactly the same problem. Any ideas?

you need to look at your ring gen that had a problem going out