Most likely cause of iPhone backlight malfunction?

Sorry. I wrote a really long reply without realising this website removes all formatting and has a character count on comments.

Thanks. I may try replacing the chip and the coil. They are the cheapest (if most fiddly) components involved. However I kind of think if either overheating or shorting was the cause then more components would likely be damaged besides. I’ve tested everything else and it all works. The coil claims to be high temperature resistant and I didn’t allow the overheat to get any hotter than I have naturally felt the phone after extended usage.

The LCD ribbon is a tad dodgy since a problem a couple years back damaged the receiver on the board (only the plastic clips though). However without knowing which pins are for LCD power and which are for LED I cannot really work out if this is the issue. Or indeed if the power is simply shared between the two without separate pins for each. There are no dead spots on the screen so the display portion of the ribbon would appear to be fully intact.

How can I go about testing the ribbon and do you know which pins provide power to the LEDs? I have not yet taken apart the screen unit so is there feasibly any issues which could have occurred in there from this problem?

I have heard that a cause for this problem can be disconnecting the screen without turning the phone off. I had no choice but to do this after reconnecting things and temporarily losing the digitizer and being unable to press my faulty power button (phone is jailbroken so I can switch off without the button... if the screen works).

I am unclear however as to which component specifically this is likely to damage. The screen had been disconnected like this several times before without ill effect anyway... on the last occasion in which I had to make repairs and replacements.

I do not have an ohm meter but I guess I can crudely test the chip and coil with just an LED and power supply... provided I only give it what it can handle. I’ll have to look up what they run at.

Check pin 3 of the connector with a test light. You should have power on there. If you do most likely your backlight circuitry is ok.

Thanks for your continued assistance on this. Apologies for my very vague knowledge on the subject... my secondary school electronics teacher was punched in the face... through a window... by a student... and hence quit. Strange story but there we go.

I am assuming the first diagram there is for the ribbon connector labelled 1, which is aligned horizontally on the board. This appears to have fourteen pins each side so I assume that the topmost and bottommost are merely for the connector itself and should be omitted on the count? I guess these are the terminals labelled 25-28.

Could I simply touch an LED or bulb to this to test then? Pin 3 for the positive leg and which other pin? Apologies again but the unfortunate punching incident means this stuff is quite beyond me.