Way of testing LG main board to find odd issue with it not turning on.

[Xandros]

I have this Apple display, it's an ADC display, connects to a Mac using a special ADC connector gets it's power from the computer via the same connector. I'm trying to troubleshoot it's mainboard which is made by LG model LA291Z.

The odd problem is, that when the monitor is plugged into the Mac, the Mac won't turn on (nor will the monitor of course). I've ascertained this is because of the mainboard in the monitor as I swapped the mainboard with another and that one works fine, I just don't know what on the board is causing it to happen. If I unplug the monitor and then plug it back in it "trips" the Mac and causes it to switch on by itself but it ends up in open firmware (essentially the Mac version of a BIOS) which is not normal of course. After this it all seems to work fine.

I can't really get my head around what could cause such an issue? There's two pins of 28v input and two pins 28v return that go to mainboard of the monitor. Here's a picture of where the power input is on the board. I'm guessing, that the reason it won't let the Mac turn on is because there's something wrong with returns?



First off, a very technical question: What's that black thing with the white dot on it?

[budm]

Re: Way of testing LG main board to find odd issue with it not turning on.

Are you getting 28V between A & B, C & D? IF you do, then nothing wrong with that returns.
How about the pictures of the whole board?
Since you have good working board then you can do the Voltage reading comparison.

[Xandros]

Re: Way of testing LG main board to find odd issue with it not turning on.

Thanks for the response. Sorry I forgot to upload the full shot of the board.

Between those points I'm getting 25.1v on both sets, which I believe is OK since although the pinout for the ADC states it's 28V the service manual for the monitor states anywhere between 24 and 28 is OK when testing.

Though while I was putting it back together to test those points I seem to have inadvertently found (or rather narrowed it down to) why the Mac won't switch on. The monitor has built in USB and as you can see I left the cable for USB (coming from the Mac end) unplugged by accident and low and behold the Mac booted up. So whatever is wrong with the board, it's something to do with the USB.

[knoxrebel]

Re: Way of testing LG main board to find odd issue with it not turning on.

Through the proprietary connector ADC (which is essentially a dvi-D only with use of some extra communication pins.) The cinema displays can be used to power on and put to sleep the machines so if there is a problems with the monitors i wouldn't be surprised if it was causing other issues with the desktop.

[Xandros]

Re: Way of testing LG main board to find odd issue with it not turning on.

Whatever is wrong with it, it's the USB circuitry on this mainboard. I need to test the pins later of the plug that wasn't connected. It goes from left to right if I'm to understand USB wiring correctly, Ground (black) Data + (green) Data - (white) +5V (red) +5V (orange) Data - (blue) Data + (yellow) Ground (brown).

I can only assume that in order to actually prevent the Mac turning on, it must be shorting out the 5v.

[Xandros]

Re: Way of testing LG main board to find odd issue with it not turning on.

OK so I was mostly wrong in my assumptions about that plug, turns out it does carry the USB wires, but only three of them (ground, d+, d-), the rest are other things to do with the monitor. I took the cable off and looked at a pin out for the ADC connector and then used a continuity test to find out which pin on the ADC plug corresponded to the wire in the plug that went in the mainboard, and here they are. I also tested the voltages of each wire while the computer was plugged in (but not turned on, as it won't turn on with this plug connected to the mainboard):

black = pin 23 USB return = 0.000v
green = pin 21 USB Data + = 0.813v
white = pin 22 USB Data - = 0.003v
red = pin 13 Soft Power = 1.142v
orange = pin 3 LED = 0.000v
blue = pin 9 DDC Data = 0.219v
yellow = pin 19 DDC Clock = 0.219v
brown = pin C5 Analogue video/DDC return = 0.000v

Does anything there ring any alarm bells to anyone? Also if there's nothing out of the ordinary about that, does anyone have any suggestions as to why the computer might not turn on when this monitor is connected, now we know it's something to do with one of those pins?

Thanks.

[fzabkar]

Re: Way of testing LG main board to find odd issue with it not turning on.

I would disconnect the USB, DDC, and LED signals in turn until you find which signals, if any, are hanging up the boot process. You may find that one or more signals are shorted to ground or to the supply rail.

BTW, I found the schematics for the LA291Z PCB in the service manual for the M8536. See pages 25 and 26 of the PDF.

"Soft_Power" is an open collector output from the mainboard, so I can't imagine that this could be the culprit (although it could stop the monitor from powering on).



(The component with the white dot is an inductor.)

[fzabkar]

Re: Way of testing LG main board to find odd issue with it not turning on.

A more direct link to the service manual ...

https://cdn.badcaps-static.com/pdfs/...3ed0d109d3.pdf