2 
1 no backlight, video ok, 4 diag leds lit
Still no hints on my issue with an iMac 21.5” EMC2638 A1418, which has no backlight, the video is ok and all 4 diag leds are lit.
I found out it’s not the fuse F8100, and there is 12V arriving on Q8102 on PP12V_S0_BKLT_FILT, just not passing through because Q8102 is either turned off or blown.
At the moment I’m not calling Q8102 blown, because its gate is telling it not to turn on, with BKLT_EN_DIV close to 12V.
I couldn’t find a datasheet or any other info on the backlight driver IC LP8561. Perhaps it’s telling Q8102 to not turn on with the shutdown signal BKLT_SHUTDOWN, and I was wondering how overriding that signal would be potentially dangerous to the driver IC’s output, given it’s only linked to the mosfet via a zero ohms resistor (am I wrong about R8122 being zero ohms?).
Since all 4 diag leds are lit, the driver is supposed to turn on the backlight, but it’s not.
What do they mean by “NOSTUFF” on some parts on the schematics, like for example R8120? Shouldn’t that mean they’re not on the logic board? But that seems odd.
That iMac had 2 blown caps in the bank of 20 of 2.2uf on the BKLT_BOOST output, but the fuse F8100 on that circuit didn’t blow. The caps were causing a short, and once removed from the board, that short was gone and the backlight power hasn’t returned.
What is the actual voltage supposed to be fed by BKLT_BOOST? It does get 12V coming from Q8102, if turned on, via L8100 and from what I gather from the circuit, the backlight driver IC (LP8561) switches a mosfet Q8100 by BKLT_GATE, which I assume generates a higher voltage on BKLT_BOOST.
How can the backlight driver not turn on the backlight power when the 4th diag led pretty much says it should? Is the driver IC blown in some way?
Is dit een goede vraag?
@spkdd "What do they mean by “NOSTUFF” on some parts on the schematics, like for example R8120? Shouldn’t that mean they’re not on the logic board? But that seems odd" that is correct. It means that there should b no part. This is done because of revisions during the development of the board and may have been present for diagnostics during that period.
door oldturkey03
That sounds logical and that's pretty much what I thought, however, there are parts flagged that way that are necessary, such as the ones related to the BKLT_EN signal for example, which makes no sense, as those should be on the board or some things won't work.
door didier godefroy