820-01310-A iMac 27" A1419 2019 No Backlight

[JoeD94538]

Hi All, I need help. This board has no backlight. One backlight output capacitor blew out and shorted. I've replaced it and still no backlight. I test with another known good display from an iMac 27" 2015 5k, but no backlight then test the original display in another iMac 2015 and the display is good. So it's definitely the board.

Can't find schematic for this board. So I'm using schematic from 2015.

Measuring the output backlight caps, I get ~70V. working backlight measure ~58V

I took some measurement;
Fuse F4400 ~12V
At J8200; Backlight boost pin 6 and 12 measure ~70V, the others LED Returns are ~1.4V ( here I saw other post about measuring pin 1-4, 2-5,3-6 at J8200 I don't understand this, what this is)

At J4400 measurement; Already replace lcd connector J4400
Pin60 BLC-EN- 3.3V
pin2-8- 12V
pin 19-20 SMB_DP_TCON_SLA_SDA/SCL present 3.3V
Pin 24 - DP_INTPNL_MLAUX_C_P measure ~0.41 measurement from a good working unit is the same
Pin 25 - DP_INTPNL_MLAUX_C_N measure ~2.4 working unit is same value.

Pin 54 - not getting anything on TCON_RESET_L but not sure. On working 2015 model this signal was there. But this 2019 model I'm not sure it should be here or not.

Any help is most appreciate
-Joe

 

[2informaticos]

Higher voltage than normal, means open line for the panel LEDs strip.
The problem should be on the return lines; if the backlight cable is good.

Did you test other display with the same backlight cable?

BTW, confirm if you are using 820-00134 schematic.

 

[JoeD94538]

Thanks for the quick reply; I measure the return lines and got around 22V. The measurement was unstable, seems like it was jumping around, maybe its the meter or not grounding right. is there a correct way to measure these?

Please disregard the measurement previous post at j8200, those were measurement for known good iMac 2015(1.4V which is weird)

Different backlight cable was used to test. Use the whole display and cable from known good iMac 2015 model.

Yes I was using the 820-00134 schematic for reference.

 

[2informaticos]

So, do you get 70V at J8200, or not?

 

[JoeD94538]

I do have 70V at J8200.

I just replaced the backlight controller, still no backlight.

How do I measure Return LEDs.

thanks

 

[2informaticos]

Start checking the small coils FB8201-11.
Next, check all the MOSFETs from return lines; Q8201-9 and Q8211-9.

 

[JoeD94538]

So happy, got it fixed 2informaticos. You're the man.

Following your advise, I start measuring the mosfets Q8201-9 and found they didn't have the 12V in pin 3 on them which was the PP12V_S0_BKLT_PWR.
This line came out of Q8119 and there is a fuse before it, F8100, and this fuse was blown. Replace it and backlight is good.

There are two fuse with these 5K iMac, one by the lcd connector, F4400 and one behind the board, F8100.

Thanks a lot for the help 2informaticos!

This is solved.

 

[2informaticos]

Glad to hear is solved now.
Thanks for feedback solution; it may help other members.