iMac black screen, external works

[MrBojangles]

The computer (iMac A1312, late 2009) will start fine but after a random amount of time the screen will turn black. Sometimes I get to the login screen but most often it shuts off during startup.

Sounds remains and external screen works fine.

I've been searching all over and I'm hearing inverter and power supply a lot. What do you guys think?

 

[cyabro]

Can you still see the image on the screen if you shine a bright light at it?

 

[MrBojangles]

Can you still see the image on the screen if you shine a bright light at it?

Nope

 

[cyabro]

Won't be the inverter then as you'd still see an image on the display.
Does an external display work fine?

Edit: Sorry just saw in the subject that an external does work fine.

That means most likely no issue with the graphics card either.

That really only leaves the power supply.

 

[phaZed]

Bad video chip, common in 2008-2011 Macbooks.

 

[TECCS]

I found one Imac like this had a problem exactly the same the LCD has a sensor for overheating that turns the screen off. it was absolutely clogged with dust by the left hand fan. I took the glass and screen off and unplugged everything carefully then with canned air cleaned all the dust out now works fine. its worth a try because mine was intermediate as well and worked on external.

Plus it doesn't cost as much to try.

 

[MrBojangles]

I found one Imac like this had a problem exactly the same the LCD has a sensor for overheating that turns the screen off. it was absolutely clogged with dust by the left hand fan. I took the glass and screen off and unplugged everything carefully then with canned air cleaned all the dust out now works fine. its worth a try because mine was intermediate as well and worked on external.

Plus it doesn't cost as much to try.

Thank you for your tip. I went ahead and did that. I dusted it all off and noticed also that some connections on the inverter board were oxidized (see pic).

Inverter board: https://imgur.com/AkTGYCb

Oxidized connection: https://imgur.com/esln6dT

So I cleaned the parts I could see were bad and put it all together again. Now I get this,

Video: https://imgur.com/dMxx8dw

Looks like the the computer is about to open up a portal to another dimension...

Anyway external screen still looks fine.

Surely this confirms that it is indeed the inverter board at fault, not the power supply?

 

[phaZed]

Tellin' ya - Still looks like a bad video chip/solder ball issue to me. That is not an oxidized connector. The crud on there is flux from the soldering process and is normal.

If it were the inverter board you would only be experiencing a 'lit-unlit' situatiuon, however, you are having screen glitching. The glitching, to me, looks like the panel is losing sync or picture periodically which is causing the effect in your video. It *could* be the panel, but more likely the video chip itself.

EDIT: Also, simply because it works on an external doesn't mean it's not the video chip... the internal LVDS screen and external connectors are physically different - the output from the chip is a physically different output.

 

[MrBojangles]

Tellin' ya - Still looks like a bad video chip/solder ball issue to me. That is not an oxidized connector. The crud on there is flux from the soldering process and is normal.

If it were the inverter board you would only be experiencing a 'lit-unlit' situatiuon, however, you are having screen glitching. The glitching, to me, looks like the panel is losing sync or picture periodically which is causing the effect in your video. It *could* be the panel, but more likely the video chip itself.

It can't be the chip if external screen looks fine though!

 

[phaZed]

It can't be the chip if external screen looks fine though!

Unfortunately, it sure could be!, check the edit above