Viewsonic N4785P TCON Issue

[Slow_Frank]

My viewsonic went all bright white / negative, so after reading here it looks like a TCON issue, I measured all the gamma voltages and other voltages. I used the spreadsheet made by tom66 and it seems the voltages are all out, AVDD (3.27), Vcom(1.63), Von(0.7), Voff(2.48). Vin seems a little low (11.5V) Is there any further way to troubleshoot this, or should I just board replace?

Thank you.

[budm]

Re: Viewsonic N4785P TCON Issue

Either replace the AS15 or new board.

[tom66]

Re: Viewsonic N4785P TCON Issue

The gamma is bad. But I would check you are testing the other voltages OK, because I would expect AVDD that low to result in a basically fully dark image with maybe some detail visible. Also, the top gamma voltage reference usually comes from AVDD so you could not have 13V gamma with low AVDD. I would try unplugging the white ribbon cables at top of the panel, one at a time, and see if your voltages come back.

[Slow_Frank]

Re: Viewsonic N4785P TCON Issue

Tom you are correct, I took VDD on the PCB to be AVDD and looks like on my board it is simply labeled VDA, it measures 13.7V. (attached)

The gamma IC is an i7868A, which appears to be an intersil chip, but I can't find any datasheets on it, does anyone know if an AS15 is pin compatible?

[tom66]

Re: Viewsonic N4785P TCON Issue

from another user:-

as15-f=as15-g=as15-u= as15-hf=tsl1014if=hx8915a=ec5575=sl1015i=rm5101=aat7200 = all are equal.

https://www.badcaps.net/forum/showthread.php?t=33806

Do not know if others will work, you'd have to compare datasheets.

[Slow_Frank]

Re: Viewsonic N4785P TCON Issue

Cut the legs off the IC with a razor knife, swept the legs away with soldering iron. Had someone hold the IC and hit the bottom of the PCB with a microtorch. Everything came away easy. Seems like this chip was poorly soldered to the thermal plane, possibly why it failed.

[Slow_Frank]

Re: Viewsonic N4785P TCON Issue

Couldn't find the datasheet for the i7868A, so I figured I would try a AS15, waited a month for an ebay special, soldered it on. This is what I got, also it got really hot fast, so it would seem they are not pin compatible to the i7868A. (I have since ordered a i7868A). I soldered the AS15 thermal pad by hitting the bottom of the PCB with a 250W soldering gun, seemed to work ok.