[SOLVED] iMac 21.5 backlight flashing/flickering

"I suspect you really have a power supply problem here. From the sounds of it I think one or more of the filter caps are bad. Thats often the cause of the backlight flickering issue from my experience.

Take the power supply out and inspect the caps. Do you see any that look bulged or appear to be leaking? Try out a spare if you have one."

"Hello Michael,

Yes, the problem was a defective power supply. Instead of replacing it, made some inspection on the 12V circuit. The 12V voltage created at the capacitors are splitted into two circuits: the standby voltage output (which is unswitched and goes out straight forward) and the main 12V output which passes through two high-side MOSFETs in parallel that acts as an on/off switch. I found that one of this MOSFETs was defective (gate was short-circuited) which was keeping this switch always ON but with a fairly high output impedance. This is why i have measured 12V on the main output even with the PSU turned off but then the voltage drops when computer turned on and starts to drain some current. Apparently, the motherboard circuits were able to work fine with 10,5V, but the backlight driver board didnt.

So, i have simply replaced the defective MOSFET and voilà... everthing worked as expected. Of course, replacing the PSU would also solve the problem.

Good luck!"

With your knowledge, I would go over the board with a soldering iron first looking for a cold solder joint.

Can confirm changing the PSU fixed my problem, VGOR has to do with the power delivery to the backlight board from the PSU.

Hello today, repair this problem. Replacing the power button, remove the power button cable from the motherboard and join the 2 pins with a screwdriver

Hola para un IMAC A1311 , la falla de pantalla negra se pudo solucionar reemplazando el par de MOSFET 85T03GP, de los cuales uno estaba en corto circuito. Fueron reemplazados por su equivalente localmente en Chile y disponible IFR3205. Primeramente y como recomendación se deberían seguir los mismos pasos indicados al inicio, realizar el AHT , y verificar si el error corresponde a los del tipo indicado….4SNS / 1/40000001: VGOR- 10.xxxx….. esto se puede verificar alumbrando la pantalla en negro con una linterna y mucha paciencia…correr el TEST y verificar el código. Quedo agradecido de este sitio, ha sido de mucha ayuda….finalmente el repuesto no supero los USD 8, más unas cuantas herramientas y ya fué factible revivir el IMAC . Saludos desde Chile