No fuses in the UXxx12 series of inverters but seem to remember a small
circuit breaker you might check.
Going a bit deeper, there should be an 18 pin IC near the charger
end of the PCB. Maybe in an IC socket. If it's in an IC socket, make sure it's
pushed in all the way and didn't fall out partially during shipping.
The microprocessor drives a transistor that in turn enables the 12 Volt
coil of the relay. You could look with a meter or scope across the relay
coil and/or the drive transistor from the micro. Should be a single resistor
from the micro to the relay drive transistor. The relay is a big Deltrol relay.
Another thing to look for is a signal from the AC line input which drives a
small transformer, the secondary of which (low voltage) goes into the A/D
converter in the microprocessor. The A/D input sees the whole AC input
cycle from the secondary and has a 2.5 Volts DC bias at the A/D input.
Capacitively coupled with another series resistor. If there is no AC
swing at the input of the micros A/D converter pin, but there is 2.5 Volts
DC at that pin, the little transformer ~might~ have an open winding.
The micro must see at least so many volts of swing and can't be too far
off in frequency, but does have quite a good range as I remember. (It's
been many years now)
This might be totally off, but hopefully helps a teeny bit.
boB
K7IQ