820-2936-B No video on internal display, yes on external

[Brennan]

Hey guys, won't lie, still new to this stuff. I will be making good use of this forum since I have several Macs to fix and only so much time to watch LR vids.

I have an 820-2936-B with no image / video, but works perfectly on an external monitor. History: customer replaced their own screen. I've tested with a known-good, same result.

I did diode mode measurements at the connector pins, and only found that pin 5 is OL (with the screen plugged in).

Here's my measurements:

j9000	good	bad	difference
1	0	0	0
2	535	541	6
3	535	541	6
4	322	317	-5
5	ol	ol
6	555	555	0
7	555	554	-1
8	541	556	15
9	541	555	14
10	0	0	0
11	541	556	15
12	541	556	15
13	0	0	0
14	541	553	12
15	541	553	12
16	0	0	0
17	547	554	7
18	547	555	8
19	0	0	0
20	ol	ol
21	551	549	-2
22	551	549	-2
23	ol	ol
24	582	578	-4
25	586	579	-7
26	585	579	-6
27	582	572	-10
28	583	575	-8
29	568	555	-13
30	ol	ol

j9000 - lcd connected	good	bad	difference
1	0	0	0
2	409	543	134
3	409	543	134
4	321	319	-2
5	640	ol	<---- this?
6	505	557	52
7	507	556	49
8	523	514	-9
9	523	340	-183
10	0	0	0
11	523	340	-183
12	523	340	-183
13	0	0	0
14	521	340	-181
15	521	341	-180
16	0	0	0
17	530	340	-190
18	530	340	-190
19	0	0	0
20	ol	ol
21	550	551	1
22	550	551	1
23	ol	ol
24	580	594	14
25	585	593	8
26	584	594	10
27	580	590	10
28	582	590	8
29	566	590	24
30	ol	ol

Also have no backlight, if that helps. Only damage I see is that the bottom left corner of U9000 is barely chipped.

Getting .2v on pin 1 @ R9014 (LCD_IG_PWR_EN). About the same at backlight enable.

Again, on an external monitor the computer works perfectly.

Thanks!

 

[dukefawks]

Pin 5 is backlight sync pin and is not used.

Seems like LCD is not recognized and all that is needed for that is power to pin 4 and data on pins 6 and 7. Measure these pins with an LCD connected. Also check if these pins have continuity to the end of a separate LCD connector to rule out a bad connector.

 

[Brennan]

My tech is working on this now, and this is what he told me. The screen is known-good, we tested it on another device. He says that he's not getting 3.3v at pins 4, 6, and 7. Still no image / backlight. Also, he says he's getting nothing at the backlight pins. What's next? (Sorry for the late reply, put this to the side for a long time.)

EDIT: He *IS* getting 3.3v at those pins.

 

[dukefawks]

Before you have an image there will never be backlight of course. There must be 3V on pin 4 at least, check L9008. But pins 6 and 7 should have 3V too and are connected to 3v3_s0 through pull up resistors R9008/9. Check voltages on L9008, R9008/9.

 

[Brennan]

He checked those and reports that he's getting 3.3v at all of those points. (I mistakenly typed that he wasn't in my previous post). What's next?

 

[dukefawks]

Check if there is continuity to the other end of a separate LCD cable to verify if the connector is making good contact. This goes for all pins.

 

[Brennan]

Continuity checks out on a known good cable.

 

[dukefawks]

Then it should work.....You can connect a scope to the SCL and SDA data lines and see if there is any activity. Will be a short burst, so need to do a "capture/one shot" or whatever it is called on your scope.

Also was the LCD replaced because it was cracked or was it replaced because of this problem.