Help me work out what I've done to my iMac

[r6mile]

I recently replaced the GPU in my iMac 2010 27 (full specs in signature) for a Quadro K2100M, and things have been going pretty well other than the temps being pretty high. This led me to think perhaps I didn't apply thermal paste properly, so I just opened the iMac, took out the GPU, reapplied thermal pads and paste, and put it all back together. But now the iMac just boots to a blank screen - there is a chime, but then the screen (which I can see does power on) stays black and the optical drive makes a sound like it's trying to read something.

SMC and PRAM resets do not appear to have fixed the issue. Unfortunately I don't have an external monitor to work out whether there is a picture out.

Can you help me diagnose what I might have done wrong? Keep in mind that in the 27' 2010 iMac replacement of the GPU does NOT (unlike the 2011 model) require to take out the logic board because the GPU is at the front not the back, so it is simpler and there are fewer steps where things could have gone wrong. It is literally just the 3 display cables (sensor, power, display), and then slide out the GPU. Could I have damaged the LDVS cable?

Many thanks!

 

[TwoH]

Make sure to check the pins on the LVDS cable going into the logicboard - these are prone to bending whilst trying to reconnect them. I believe the 2010 do also have the diagnostic LEDs.. you should also check them to see if any are off

Also

Keep in mind that in the 27' 2010 iMac replacement of the GPU does NOT (unlike the 2011 model) require to take out the logic board because the GPU is at the front not the back,

What? The GPU *is* at the back of the logicboard same as the 2011 iMac, and same as the 2009.

 

[r6mile]

Make sure to check the pins on the LVDS cable going into the logicboard - these are prone to bending whilst trying to reconnect them. I believe the 2010 do also have the diagnostic LEDs.. you should also check them to see if any are off

Also
What? The GPU *is* at the back of the logicboard same as the 2011 iMac, and same as the 2009.

Thanks, will check the pins!

On the second point, sorry yes it's at the back but you can slide the GPU/heatsink out without taking out the logic board. I just changed the GPU on a 2011/21.5 and had to take everything apart, but you don't have to do that on the 2010/27 (followed ifixit guides for both).

 

[r6mile]

Make sure to check the pins on the LVDS cable going into the logicboard - these are prone to bending whilst trying to reconnect them. I believe the 2010 do also have the diagnostic LEDs.. you should also check them to see if any are off

Also
What? The GPU *is* at the back of the logicboard same as the 2011 iMac, and same as the 2009.

I have found the diagnostic LEDs thanks to the service manual - is this the only way to look at them (through the lower vents next to the RAM)? Because I am struggling to work out how many lights there are - it looks to me like at least 3 though, which would indicate 'Power available, system is powered on, and video card found'. I have tried to take a picture but it was difficult to get one in focus.

The LDVS cable looks ok to me, what do you think? Hopefully I haven't damaged the connector itself - definitely don't have the skills to fix that. If I have, I feel like such an idiot - perhaps got too confident as have opened half a dozen iMacs like this before!

I am going to try plugging the iMac to an external display tomorrow (I don't have a cable at the moment) and see if it will output video.

 

[TwoH]

LVDS looks perfectly fine to me - when I am looking at the LED lights, I have the LCD unscrewed, and I will tilt it forwards (not too much in case you break a cable!) and will have a clearer view of the lights there, also be cautious of the PSU too, around the top left.

If there are not 4 LEDs present - it does seem to indicate an issue with the LCD panel not turning on.

 

[r6mile]

I still can't properly see the lights doing what you described, perhaps in the 2010 the positioning is different - the lights point downwards, so it only looks possible to see if looking from the bottom through the vents (this is what the service manual says), but I just don't get how anybody is meant to see anything through those! I will check if it outputs display, but if not then I'm pretty sure I must have knocked something off (capacitor/resistor/etc) inserting the GPU, which is incredibly stupid. From looking at other threads, I am not the first one this happens to...

I will keep you updated!

If that's the case, I don't think it's worth trying to get this repaired, thankfully there are plenty 2010/2011 iMacs with broken GPUs on eBay for little money so I can transfer the insides, and sell what I have for parts - shouldn't cost me very much in net terms. Just incredibly frustrating...

 

[r6mile]

LVDS looks perfectly fine to me - when I am looking at the LED lights, I have the LCD unscrewed, and I will tilt it forwards (not too much in case you break a cable!) and will have a clearer view of the lights there, also be cautious of the PSU too, around the top left.

If there are not 4 LEDs present - it does seem to indicate an issue with the LCD panel not turning on.

Managed to get to the LEDs, all four are lighting up, very strange then! Any further ideas?

 

[USB3foriMac]

Unplug and replug the cable that goes to the backlight PCB. This does not have a firm connection, so that sometimes the backlight simply doesn't work.
Also, try the same with the eDP (lvds) cable. It happens that this is not working correctly and yet you have 4 LEDs. Happened to me, I tried several times, not working, then swapped with my second unit and both are working.
If you can't get it to work any more, I suggest you buy a new eDP cable and replace it. They are not expensive.

 

[r6mile]

Unplug and replug the cable that goes to the backlight PCB. This does not have a firm connection, so that sometimes the backlight simply doesn't work.
Also, try the same with the eDP (lvds) cable. It happens that this is not working correctly and yet you have 4 LEDs. Happened to me, I tried several times, not working, then swapped with my second unit and both are working.
If you can't get it to work any more, I suggest you buy a new eDP cable and replace it. They are not expensive.

(Tagging @Jumme_Cph also as he's been helping me with this on another thread)

Update: so I tried setting and resetting both the backlight and LDVS cables, to no avail. Now I have received my mDP-HDMI adapter and can confirm the Mac does not output to an external display. But I still get the 4 LEDs on. So clearly I must have knocked something off near the MXM slot on the board that prevents any display from being shown either internally or externally, but the card is detected. In all this opening and closing of the Mac I also seem to have accidentally pulled one of the LCD temperature sensor cables (they are so flimsy!) from the logic board.

I definitely don't have the soldering tools nor skills (nor know anyone else who does) to fix this logic board, so I think I must write it off and get a cheap broken 2009/10/11 iMac to transfer the insides to - will be cheaper than any fix I think. I actually would prefer a 2009/10 27' model as they do Target Display Mode over non-Macs (which I use to connect my work PC), but will see what I can get locally.

Thanks for your help guys, I guess I got too confident having opened this iMacs up quite a few times, and didn't take enough care! I feel like a bloody idiot... At least it's not a new expensive Mac!

 

[Jumme_Cph]

(Tagging @Jumme_Cph also as he's been helping me with this on another thread)

Update: so I tried setting and resetting both the backlight and LDVS cables, to no avail. Now I have received my mDP-HDMI adapter and can confirm the Mac does not output to an external display. But I still get the 4 LEDs on. So clearly I must have knocked something off near the MXM slot on the board that prevents any display from being shown either internally or externally, but the card is detected. In all this opening and closing of the Mac I also seem to have accidentally pulled one of the LCD temperature sensor cables (they are so flimsy!) from the logic board.

I definitely don't have the soldering tools nor skills (nor know anyone else who does) to fix this logic board, so I think I must write it off and get a cheap broken 2009/10/11 iMac to transfer the insides to - will be cheaper than any fix I think. I actually would prefer a 2009/10 27' model as they do Target Display Mode over non-Macs (which I use to connect my work PC), but will see what I can get locally.

Thanks for your help guys, I guess I got too confident having opened this iMacs up quite a few times, and didn't take enough care! I feel like a bloody idiot... At least it's not a new expensive Mac!

Sorry to hear this, champ. Good luck with the exchange.