iMac 27in A1311, mid 2011 - no image or backlight

[vzgherea]

I've got a mac that chimes and boots up just fine if using an external monitor, however when using the inbuilt LCD panel there is no backlight or image on the screen. After opening it I found that the LVDS cable was ripped off. Replacing it has not changed anything. What I have done so far:
1. replaced the LVCD cable
2. replaced the EFI battery
3. performed SMC and PRAM reset
4. replaced the video card
5. analyzed the main board (the area around the LVCD connector and the connector itself) for burnt elements
6. verified that the LCD panel is receiving 12V for backlight and another 12V for TCON module
7. analyzed the TCON module (the area around connector) for burned fuses/elements

There is still no image on the internal screen, even when using an external source of light. All 4 diagnostic LEDs are lit. Unfortunately I don't have another compatible LCD panel to try it. Is there any way to test the LCD panel separately?
What else can be done?
Thank you!

[vzgherea]

Re: iMac 27in A1311, mid 2011 - no image or backlight

...the LCD panel is receiving 12V for backlight...

This is wrong! The output voltage should be at least 3 times higher.

[mon2]

Re: iMac 27in A1311, mid 2011 - no image or backlight

hi.

The video image (LVDS cable driven) is separate than the issue of the backlight.

Please inspect carefully again the LVDS cable, the contacts on this cable, the seating inside the blade connector and the actual mating connector itself.

Any liquid damage? Does not hurt to take some alcohol on a Q-tip and wipe the gold plated contacts to clear any clear liquid / sugary glue, etc. - just to be safe.

From memory, there are silver taped up black cables on the backside of the display screen. One cable to the LEFT and another on the RIGHT backside of the display. Each is used to power the internal LED strips (LEFT and RIGHT side and at the bottom of the display).

What we have seen is that when customers attempt to do their own upgrades, they rest these rather sensitive areas against a hard surface and destroy the mating harness. That is, the male & female connectors separate and therefore there is no more backlight. The source backlight cable may be ok but this mating cable harness does not transfer the voltage to the display.

On this note, carefully (stay away from the open backside of the open frame power supply - it can be LETHAL!!). To the LEFT of this dangerous power supply is the backlight power supply. At the BOTTOM of this large black circuit board will be the BACKLIGHT cable harness cables. This is from memory. Check the voltages on this cable harness region. What are the values here?

See attached picture.

You can in voltage mode of your meter with black on 'LED-' and red probe on 'LED+'.

Or black probe on GROUND on this board and red probe on 'LED+'

What value(s) do you see here?

As you have noted, an external flashlight on the front face should allow you to see the image on the display. If you do not and the LVDS interconnect is 100% ok or appears to be then the issue could be T-con related.

We did face a similar issue and took off the aluminum foil tape to access the T-con circuit board. Once we had this access, found a number of onboard tantalum polarized caps. Each of the 5-10 caps were replaced (bought from Arrow). After this exercise, the LCD panel worked fine. Brought back from the dead.

This is assuming your video card is ok which you said you have replaced.

[vzgherea]

Re: iMac 27in A1311, mid 2011 - no image or backlight

I understand these are two separate issues.
My LED driver board is different, please see attached. The output is 12V (same as the input).

The LVCD cable and the connections are fine so I would suspect the TCON board or the LCD panel itself. Would it be ok to temporary replace the tantalum caps by electrolytic ones?

[mon2]

Re: iMac 27in A1311, mid 2011 - no image or backlight

OMG - this is very eerie. Have exactly the SAME issue with a local client. Trying to debug as we type. Same, LED out is @ 11v8 on the bottom side of the backlight connector.

[mon2]

Re: iMac 27in A1311, mid 2011 - no image or backlight

We have a defective i-pex 30 pin lvds connector. Have a new one on order for this repair. Will update this thread tomorrow.

[mon2]

Re: iMac 27in A1311, mid 2011 - no image or backlight

hi. just got our iMAC display working. Was not fun. Will document in a new thread but check the following:

1) be 100% sure that the LVDS 30 pin ipex connector is not shorting and that each pin is soldered ok. Given that you have 4 LEDs ON for the POST, I think we can agree that your LVDS cable is ok but check anyways.

2) be sure that the LVDS cable is not pinched from the logic board to the backside of the LCD panel itself. Would help if you can test another cable but again, if 4 LEDs are ON, I think you are ok here.

3) Confirm that the SYNC cable and connector to / from the display are mated ok. We found that one of the 4 pins on the SYNC cable was not seated properly which kept the screen DARK!! Got scared, calmed down and reviewed the cables and found this error. After correct seating, all was ok for the backlight.

4) This LCD panel had a dark LEFT side. Inspection revealed that the SMD connector for the backlight was broken off due to a physical bump of handling the LCD panel by this DIY customer.

See attached.

[vzgherea]

Re: iMac 27in A1311, mid 2011 - no image or backlight

I've already tried 2 LVDS cables - same result.
I thought that even complete removal of the SYNC cable should not turn off the back light. Will check it again.
What about the LED driver output, is it 12V or more?

[mon2]

Re: iMac 27in A1311, mid 2011 - no image or backlight

See attached.

Interesting in that:

1) when the display was dark, the voltages at the same test points were @ 12v0 (approximate)

2) when display is ON, voltage readings are at +35v5.

Q: Is there a possibility that your backlight cable in the same bottom corners (as per our repair today) are broken? This is a very common issue.

Here is a full video on this issue and repair - we did the same as Solution # 1. It was not too difficult to break open the white plastic parts (carefully!) with sharp blue shears we purchased from chinaphonefix in Shenzhen for $2 or so. Then we had the room to solder on the 6 wires for the repair using wire-wrap wire.

https://www.youtube.com/watch?v=LM3zW35BC20

Suggest next for you to carefully remove the aluminum foil tape on the backside of the display that leads to the bottom cluster of black wires. There is another similar cluster on the other side of the display.

Just remove carefully so you can apply it back in the same way. These metal foil tapes are for EMI shielding for possible noise.

Once you have this open, the black and white mating SMD connector will be visible. Try to wiggle this connector - does it move at all? If yes, then this cluster of backlight LEDs will not light up. Repeat for the other side of the display. The SMD connector should not move if it is soldered onto the LED strip properly. See the video for full details.

If these connectors are still solid and do not move around, then I believe the issue is the T-con board on the display itself. As noted earlier on this forum, not impossible to fix. We just replaced every tantalum onboard the board and this fixed the issue.

Why we are leaning towards the T-con board is that this backside board is responsible for the sync signals with the inverter board. If the inverter board is not working properly with this T-con board, the display will remain dark. We experienced this issue today.

The T-con boards are something like $10 USD new / Grade A on aliexpress. Just review the full display panel # and ask the shop for the replacement. For fun, you can try to repair it - parts can be sourced from Arrow - very likely you can receive for tomorrow.

Chime back with the full display details and I will try to find the Aliexpress shop we chatted with about this topic. The same shop sells OFFLINE tools to test the display. I think we will be purchasing a stand alone LCD tester in the near future.

BTW - the voltage readings are from the inverter board when the system was turned ON and display was LIT up.

[mon2]

Re: iMac 27in A1311, mid 2011 - no image or backlight

If out of ideas, carefully review the t-con board on the display panel and share pictures. This pcb is the one that connects to the main logic board through the lvds cable.

Once the pcb is exposed, place your meter in resistance mode and black probe on ground. No power to the display!!

Red probe on every + pin of the tantalum caps.

What are the resistance readings?

We are checking for low resistance values which indicates one or more caps may have failed due to an internal short.

For our first t-con repair we just replaced every tantalum cap and got the display to work.

[mon2]

Re: iMac 27in A1311, mid 2011 - no image or backlight

Also,

https://www.ifixit.com/Answers/View/...D+-+what+order

[vzgherea]

Re: iMac 27in A1311, mid 2011 - no image or backlight

It turned that the image and the backlight issues were caused by a single faulty board - the LED driver one.
I've just installed a working driver board and got the backlight and the image on! It looks like when the LED driver board is faulty the video signal is not even being sent to the LCD otherwise I would have been able to see something on the screen using an external source of light.
Tried disconnecting the SYNC cable and it made no difference - the image got displayed on the screen.
The only remaining problem in my case is one dark side of the LCD, going to follow the procedure described in post #7 of this thread.
Thank you!!!

[vzgherea]

Re: iMac 27in A1311, mid 2011 - no image or backlight

There is a way to test the LED driver board without connecting it to MAC. Supply the ground, 12V and 3.3V to ENA and PWM pins and the board should provide about 42V on the output pins. All the input pins are marked on the board. Used DC-DC converter to get 3.3V as a resistor voltage divider did not work. The 42V at on the output pins comes on for 1-2 seconds and then goes down until the 3.3V gets disconnected and then connected again. On the working board I am able to maintain 42V by connecting/disconnecting 3.3V indefinitely long. On the faulty board I can get 42V on the output pins once, then connecting/disconnecting 3.3V has no effect and the output voltage decreases.

[mon2]

Re: iMac 27in A1311, mid 2011 - no image or backlight

Thanks for the update and test ideas. Good to hear of your progress.

Attached are a few more pix from our last repair.

As a suggestion, cut open the (white plastic housing) area of this SMD backlight connector with sharp cutters to allow for more soldering tip room.

Then use wire wrap wire and consider to start in the middle of the SMD pads - there are 6 of them. Each much end up on the same pins as the original connector.

We just soldered extensions and then onto the broken connector + applied kapton tape to secure. See the pix and the youtube video which was of great help.

Also, if needing the LVDS i-pex connector, we found that the original is inexpensive in Asia but is a 2 part stamped assembly. It is almost impossible to remove this part off an older donor board and attempt to solder onto another PCB. In this process, the compression contacts became warped due to our hot air gun. This led to unable to see any video / missing LED # 4 on the POST. Perhaps will be ok if using only FLUX and a soldering iron while gently pulling up on the connector. Then there is the risk of peeling the SMD pads off the board.

We verified that our donor connector was not making contact with the inserted blade cable using a meter and probing the OTHER side of the LVDS cable. Just a nasty nasty connector.

In addition, this same connector was impossible to find here in Canada and in stock. A few list the part but showed out of stock for at least another week++.

After a few hours of research - found that the T-con board had a much higher grade i-pex connector that was all aluminum (single piece) on the frame. This component was stamped with JAE. JAE calls this the LCD connector line. Yup, very appropriate.

We ended up using the JAE connector off our working T-con board to service this client the next business day.

The same connector can be purchased from Digikey and in stock. Although still waiting for the connector to ship after 3+ days of waiting due to this covid nightmare (so they say).

Here is the link for the same footprint - same function connector from Digikey but is much higher grade than found on the iMAC logic board:

JAE:

FI-X30SSLA-HF-R2500

https://www.digikey.ca/en/products/d...-R2500/3674236