A2485 - No Backlight - 820-02100

[applejuice95]

Have functional A2485 unit, all things work but backlight is out.

There was some liquid corrosion around UP800's Caps and Res(s) - Cleaned all well . No change.

I measured resistance to ground of all UP800s surrounding components on another working board.

The components that are not correct on this board, with backlight out are:

CP810 - should have 250K resistance at Pin 1, its reading 80 Ohms
CP811 - Should have 25K at Pin 1, its reading 80 Ohms
RP805 - Should be 30K on both Pin 1 & 2 - Pin 1 is 50Ohms and Pin 2 is 200ohms
CP812 - is actually Correct, 3.5K on Pin1
CP870 should be 300K on pin 1 and .8(ground) on Pin 2.. Pin 2 is fine, but Pin 1 is only 190ohms.

The above Caps also get 1.2V when board is plugged in on bench.
Except CP870, that ones getting 5.1V , as well as CP817 & CP818

The Circuit as a whole, is getting 12V from CP803 and CP 802




On the FLIP side of the board where the 4 switches are..

On my good board: Display disconnected, board plugged in, CP823, and CP822 both get 4.8V on Pin 2. - Also (measuring with resistance) both Read OL on Pin 2, and about 10M ohms on their Pin 1(s)

On the no backlight board. Plugged in, on bench - CP823 has 1.6V on Pin 1, the same 4.8V on Pin 2, but CP822 only reads 2.7V on Pin 2, but still gets 1.6V on Pin 1.. Then in resistance for these two.. CP823 Pin 1 - 15ohm (practically ground) - and Pin 2 is OL like on the good board... CP822 15ohms on Pin 1 just like the other one, but this one reads 30ohms on Pin 2 instead of OL like CP823...

Someone please help me figure this out. Should I replace CP822 with another 25V X5R 402 Cap?

Also diode mode across CP822 and CP823:
good board - 0.47V on both.
bad board - 0.47V on 823, but 822 only .013V

[CasasElectronics]

Hi applejuice.

Try to inject low voltage on low resistance lines (the ones differing from good board), If you can touch anything hot, the problem may be from that side.

[applejuice95]

So injecting into the power side of any of these capacitors or resistors gets nothing. We do have thermal but nothing glows, and injecting through the ground side of the capacitors just makes the cap glow.. We started at 0.8V 1A and dialed up to 1.1V but no higher for this area.. God forbid.

Anyways. When we plug the board in, on the bench. UP800 heats up, in the upper right corner.. And cycles.

[mon2]

Suggest to flux and remove C822. Then test in diode mode if the short condition changes. From the shared measurements, this leg is shorted.

[applejuice95]

I replaced C822 (before you had replied) - The numbers are all the same still.

Did you want me to remove C822 and test the Pins or test the Cap?

Yes, Reading only anywhere from 70 Ohms to 200 Ohms on VDDA from the IC at CP811 and CP 870 is low.. I've already replaced RP805, and CP870.. I haven't replaced CP 811 but, I don't think it will change anything. None of the values for resistance or voltage have changed..

The BGA UP800 is only $6 from MSentrix.. I'm going to replace it tomorrow.. Theres likely corrosion beneath the IC that I can't see.. I'll update tomorrow if this doesn't fix it..

[applejuice95]

Replaced UP800 No visual corrosion but, now C822 also has 4.8V which is great. We still however have no voltage at the end of the circuit (CP830-835) Should have 10V.

I'm not sure if with the board out of the machine and on bench, if RP830 should have voltage at Pin 1 from UP800 but its not there at all.. So as of right now, LP800, and CP823 and CP822 are all getting the right stuff, according to my good board. This is comparing both boards on bench nothing connected but power, they get 4.8 V on CPs and the Inductor gets like .223V on both sides. For both boards.. But when the good board has display connected and its on, those to CP823/22 go up to 11V or so. LP800 gets like 6V on both sides.. It looks like RP830/31/32/33 all send voltage to the switches.. But I'm not sure if it send voltage based on the display being connected or not. But they seem to have 0V at their receiving pin from UP800

[applejuice95]

Checked both BKL connectors, on good and bad board.. JP900.. All 56 pins, all the way around, on both are showing the same values.. What does this mean? Why is my circuit not boosting up to the right voltage? Should I be looking at VIN or EN in UP800?