Late 2009 iMac - LCD backlight issue (intermittent)

I changed my caps and its been working at full brightness now for a few hours. Previously it would go black (display still active if seen under strong external light source) after some time. Over time that period got shorter and I started to notice high screen brightness shortened the period. In the end, any brightness above very low would shut it off. I checked my caps for capacitance and they seemed ok. replaced anyway. Didn't have the correct values or sizes (2x35v 270uF 8mmx15mm, 2x63v 120uF 8mmx20mm) on hand so I used 1x35v1000uF and 1x63v 470uF. I knew the larger caps would not fit in situ due to other users trying that. Simply adding some extensions to the legs and some insulation on the legs so they dont short on things allowed me to tuck them a little further over where the imac is thicker. Obviously the right size caps is better but don't be afraid to try desoldering your bad old caps and testing some new ones! Its really easy. Even if this is your first go and taking components off the board. These big caps are the easiest to solder things you will find on a mac. Little surface mount resistors are far worse! Give it a crack people. A replacement inverter board is going to have old caps too. Repair for the win :-)

Worth also mentioning is that I have upgraded the GPU to K4100M. The machine had a failed GPU in the past. I'd had about 2 years of slowly increasing frequency of screen going black since the GPU upgrade and had thought it was related to that initially. External monitor has worked perfectly throughout and never any graphical issues otherwise. Ill report back if the issue arises again but I am pretty confident it was the caps that were failing.

Exact same problem on a 27” 2009 iMac. I use the same workaround of keyboard strokes to force the display asleep and then wake with another keystroke.

The screen runs OK for very long periods at low light settings and is more prone to failure when set at mid to higher light settings. I replaced the inverter board with a used one but the problem got much worse!! I could hardly keep the display illuminated for a second before it went black at any light settings.

Electrolytic capacitors on the inverter board might be the culprit. Caps are notorious for failing with age. I see four large electrolytic caps on my board.

Perhaps I should acquire some spares and see if that fixes the problem. Anyone had success fixing this issue?

I bought a new inverter board and a rebuild power unit for my late 2009 21.5 inch. The back light works again, but still has problems. Dont know if the sync cable might be the cause or the last high sierra update. Please help!

It may also point to bad solder connections on the LED backlight strip. It was a common issue on the 27" Cinema LED Display, the solder seems to degrade over time...