I have only had one power pack fail, and it was old so we just replaced it. Usually the problems have been a burned out bulb, bad connector or broken/chafed wire.
The last problem I had (other than a burned out bulb) was caused by a chafed wire. When you turned it on, the unaffected circuits would flash a few times before the system shut down on its own. You could hear the chafed wire go SNAP! SNAP! where it grounded. It was all charred and nasty.
When you say it is mounted on the back of your Cub, do you mean a self contained anti-collision strobe (like a 90081), or do you have a remote strobe head(s) with a power pack (like an A490) mounted somewhere else?
I guess either way, I'd troubleshoot it using their online manual. If that didn't pan out, I'd ask if they can repair the old one and get an estimate. They may not be willing to do repairs on the the little stuff, but At $500 to $600 for a new unit, I'd ask.