!! Schematics wanted!!
 

On the last three weekends tried to fire up the LTN72 using my 1500W inverter and a separate supply for the accumulator 28V input. The power supply alone if supplied with 115VAC starts drawing 20W above a certain voltage (100VAC approx.) and it also creates a "switch mode typical" sound, but no voltage shows up at its outputs. You may see and hear it here: and I may be missing some other input to the supply to actually create its outputs.
Back within the unit itself and with input of even 125VAC, the LTN72 unit does not start up, too - (0:12 Inverter on, 0:20 switching from Off to StBy, 0:30 back to Off). Again only around 20W of power drawn on the INU 115VAC input, no power drawn on the 115VAC heater input and no power drawn on the battery input. Checking the battery input I can measure that there is a cap inside the LTN72 that is getting charged upon applying power.
Last weekend I checked the supply's internals (everything looks OK, no blown up caps etc) and the power rails do not seem to have shorted caps on them (checked the 5V digital and the supply for the OpAmps within the LTN72RH). So for the moment I will set this project to the pause state and reach out to some people - maybe someone out there has got SCHEMATICS at least of the powersupply or input section of that unit. That would save lot of time over reverse-engineering that beast without knowing whether there is more trouble ahead with broken gyros/accelerometers etc.
Stay tuned...

Well it's really dead. In that I don't remember seeing a unit that dead before. Usually there's some's signs of life even if for just a millisecond. But there's nothing. Sometimes if there is a power supply issue, then some parts of the power supply would work and it would throw up faults for the non working parts - but here we have nothing. This is leading me to think maybe there is some issue in the wiring between the rear plugs and power supply.

Going back 30+ years, if I had received this unit in the workshop. I would have used the following basic workshop fault finding principles.

Unit fails to turn on: 1 - Check power pins not bent / damaged at rear connectors. 2 - Replace power supply and recheck turn on. 3 - if unit still fails, strip rear end and check flexible wiring to rear connectors for burns / cold solder joints. That's it really - not much else to check. Now because it looks like the power supply may be ok - the problem could lie in the wiring between the power connectors and the power supply. So an internal wiring diagram for the rear connectors would be the go here.

On further thought. Since you have applied power to the unit and can hear the whine of the 115, then that means that the power supply is receiving that power. So maybe I would direct my efforts at looking at the wiring between the battery (+28VDC) input pins on the rear plug to the power supply connector. We sometimes found dry solder joints at the rear of the rear plugs as these surfaces were not sealed like the circuit boards are. I would just check with a multimeter where those +28VDC pins goto on the power supply connector.

Something just isn't right here, and I think (hope) it's a simple fix. But as you said, wiring diagrams would help a lot here.