Unexpected inverter board shutdown

Hi everybody,

I've got a strange problem that really puzzles me, concerning an HV inverter board for the CFL lamps in a noname flat panel monitor with Hyundai boards and components.

Problem: Inverter board shuts down after generating HV for about half a second.

Details: Two outputs on the board for altogether four lamps, two transformers, two XA something switchmode controller ICs, one unidentified small IC, four power transistors, a couple of discrete transistors and diodes, all SMD.

What I did to locate the fault:

1. Tested outside the monitor with known good voltage supply. Same.
2. Tested without lamps. Same.
3. Hooked lamps to one output at a time only. Same.
4. Checked all discrete semiconductors. All seem ok.
5. Replaced the only electrolytic cap. Same.
6. Checked transformer operation using an oscilloscope probe on top. Pure sine on both transformers. Independent of whether lamps are connected. No distortions. Frequency about 42 kHz.

I googled a lot but found nothing so far. Now I'm at my wit's end. Maybe anyone knows something about the shutdown logic used in such boards?

Thanks for your help!

Regards, Leo