fixing undervoltage protection on unknown pure sine wave inverter ZXB-800WC 24V

[marc22]

Hello,
I'm looking for help in fixing pure sine wave inverter 230V (on case there is no model) on board   ZXB-800WC.
 
I believe it's a 24V volt inverter, but internal circuitry cut off battery on 10.x V and sound alarm ad 11V so it's looking like 12V inverter.
But at 12V it's not possible to get 800W using built fuses, also fan's are 24V, and last sine wave inverter module it's not working at 12V (tiny inverter board, is feed by 7815) -  only on  24V.

Main IC on low voltage side (i believe there is a problem)
PWM driver - KA7500B
op-amp - AS324P-e1

Could someone shine light where is voltage sense for cut off?
What might be wrong?

I don't know story behind this unit, i got is "as is" - not working.
Maybe in factory someone fitted wrong resistors and set unit low voltage op-amp for 12V system?


Here is my video showing problem:


Here are pictures of low voltage part of board:
http://imgur.com/a/ujVRs

Any ideas are welcome!
Regards

[pyroesp]

The buzzer activates when the voltage is too low, maybe the same signal (or derived from it) is used to disable the output?
Or maybe tracing the optocoupler to see what's driving it and see what circuit can disable it as to not generate the output sine wave.

Your input caps are 35V, so I would agree with you that the input is not 12V. If it were 12V the caps would have a lower voltage.

[Seekonk]

I would guess that decision is made in the 324.  What I would try first is look at the 4 output pins of it and vary a power supply to see which one transitions at a suspect voltage.  That would give you the section.  Does this look new or shows use.