Ok, that's a G series machine, and would be one of the very first models of the G series 773. I haven't worked on many of those, more familiar with the later runs, 2004 and up, not sure what changes they made from that model to later models, but I know they made changes as they progressed. Alright, like I said, we got a bad connection somewhere, and it could be a battery positive (power) connection and could be a bad ground as well. If your machine is like the later G series machines, you should have a bundle of ground wires connected to a stud on the negative battery cable lug, take that stud nut loose and make sure those ground terminals are clean and making a good connection. Also, but I don't think this is your problem given what you've described, but make sure both positive and negative battery terminals are clean and making good connection. Given the last symptom you described, that is, you can connect jumper cables directly to the battery terminals and it will power up ignition, but as soon as you hit start everything on the dash panels goes dead, it sounds like you have a problem farther upstream from the battery. On later G series, theres a red cable that comes off of the positive battery cable and goes to a terminal block mounted just above the battery and contains a 50 amp master fuse, make sure the connections inside that terminal block is good. If it's a power problem and not a ground problem, that big red wire that leaves the master fuse box splits off into various directions as it goes up the harness, I once ran across a T190 that had a bad crimp connection inside that harness, that one was a PITA to both find and fix, let's hope we don't have to go there with yours. A very common problem with the G series, one that will do exactly as you describe, is a problem with the connector that connects the lower frame harness to the cab harness. Raise the cab and make sure it locks with the safety prop. As you're standing at the front of the machine looking toward the rear, you'll notice the hydraulic tank to your right in the belly of the machine, directly behind the drive lever for the left drive. At the rear of the hyd tank, and about 8 inches above it, you'll find the connector that connects the frame harness to the cab harness. Take that connector apart and check the pins/sockets inside the connector, look for signs of corrosion or electrical arcing. If it's like the later model machines, the connector is held together by a single machine screw in the center of the connector, could be Allen head or it could be a metric hex head screw. If you don't see evidence of a bad connection at that connector, that still don't mean it's good, I once ran across a T300 that did the same thing your machine is doing, and there was no problem with the pins/sockets inside that connector, but rather where the cab side power wire entered the connector had a bad crimp connection to the terminal inside, I had to install a jumper wire around the connector to fix it. Get a probe type test light with a pointed probe end and a cable that connects to a ground. Check the power wires, red wire or red/white wire, on the frame side of the connector, they should be hot. Now probe the cab side wires at the connectors, same wires, red or red/white, they should be hot as well. Now power up ignition, I know, that's kind of a PITA with the cab in the air, but that's what we have to do. Ok, we have ignition powered up, all is lit up on the dash panels, now hit the start button, and let's hope it goes dark, it would be a buzz kill if it fixed its damn self at this point while we have yet to find the problem, I've had it happen before. But assume everything does go dark, right then check those red or red/white wires at the harness connector, see if one of those wires at the cab side of the connector is dead. Check those things out and let me know what you find.