Got a free power inverter and it doesn't work. Any ideas?

So I was in town the other day with grandfather for some business with a piece of property. While there we stopped by an old friend of his for a good chat. While there the guy directed us to a trailer full of old stuff. Among the junk was a heavy duty power inverter. The type that converts 12V DC to 115V AC. It's a Xantrex Prowatt 3000 model. Since been discontinued.

Story behind it is this thing was on the railroad being used when it just up and quit. No more AC power. So I started trying to diagnose it. Here's what I've found out so far.

All the fuses inside it are good. Nothing is blown.
Hooking it to a battery and turning it on shows it to be getting voltage to the built in battery volt meter.
When I hook the battery cables up to it hot there is a massive spark like it is drawing a large amount of amps. The built in battery amp draw meter never shows anything.
The cooling fan will try to go for a split second then stop.
The PCB boards all look fine. No cut traces or any damage at all.
I have the battery hooked properly so there is no reverse current on the 12V DC side. Kinda hard to hook it backwards when the cable lugs are color coded...
The circuit breaker is good. In fact, I've been doing my tests without it going straight to the PCB for voltage checks.
And of course, no sign of any voltage whatsoever on the 115V AC side of things.

So since I do get a voltage reading from the built in meter I know it's not completely dead. Maybe a logic chip, transistor, capacitor, or resistor blew out on the DC to AC side of things? System seems like it first steps the voltage up to the 115V it runs at then converts it to AC. I don't know. Didn't want to probe the transformers for fear of damaging my meter, the inverter, or both...

Anyone good at doing mystery diagnosis? If you need a video of its behavior I can make one.

 

My first guess would be that the switching circuit isn't working correctly. This would make the power transistors stay in one phase and blow up internally. Since it seems like a nice inverter it probably has some type of internal protection circuit to prevent the power transistors from getting overloaded. So my guess is that it shuts the whole box down before they actually destroy themselves. This would by why the fan shuts down after a second or two as well.

Thats just a guess though.

Keep in mind that your working in a high voltage / current environment. A good rule of thumb is to make sure you only use one hand inside the device while it has power and to ensure all caps are discharged before you dig into it when not powered on. Also ensure that your body is not acting as a ground in any way, which you probably aren't in this case. These things can and will kill you.

 

It does sound like the switching circuit might be fried or have a fried part. I might look into replacing the resistors, transistors, and logic chips first. The switching transistors seems like they still work from the massive amp draw when I connect the battery with the main switch on. It goes into shut down right away no doubt to protect them.

As for being careful, believe me. I am very careful with it. Been shocked by AC as well as DC too many times to be reckless. It wasn't fun the time I had a welder set to 24VDC at 120 amps and I got the full meal deal when it went through my heart. Wonder I lived through it with no damage...