HP w19b - screen fades to black

[momaka]

Hey everyone, I have a HP w19b 19” wide screen monitor I'm trying to repair. When turned on (not connected to a computer), it initially displays a white screen and then quickly fades to black in a few seconds. After this, the backlights stay on for another few seconds before the monitor goes into standby mode. Fuse on the panel board is not open.

I think the problem is caused by Q3 on the panel board (see first picture). Q3 is marked “126E” with the “E” underlined. Diode/resistance test gives me about 2 ohms between the pins circled in red, regardless of the orientation of the multimeter probes. I'm guessing this is a transistor and it's shorted.

So, does anyone know what this component is exactly? I'm having a hard time finding a datasheet for it. I looked at a few SMD code books online, but none seem to have it. Figured the E is probably a date code or a manufacturer logo, so I tried just the string “126” and still get nothing. However, I do get quite a few results with the strings “12”, “26”, and “123”. They all say it's a transistor, but some say PNP others NPN. The SMD code book I'm looking at currently is this one. It says it's been revised January 2006, whereas the monitor was manufactured October of 2006. Would this matter?

Basically, I would like to find a compatible replacement for Q3 if possible, since I'm thinking about removing it from the panel board and testing the monitor (chances are, though, it will likely break when I unsolder it).

As for the rest of the monitor, everything else seems to be fine. I checked the 3.3v and 1.8v regulators on the video board as well as the 3.3v regulator on the panel board, and they all seem to be putting out the right voltage. Disconnecting the video cable between the video board and the panel board gives a white screen, as expected. I've also partially recapped the PSU/inverter board (with proper equivalent low ESR caps) just in case, even though none of the original caps seemed failed or heat abused. The caps on the video board are a mix of Rubycon and United Chemicon.

Here are the board numbers in case anyone needs them:
Panel: HannStar HSD190MGW1
Panel board: HannStar 6p18v00015 A2

And I'm guessing these are internal HP numbers:
Panel Board: 7051s00a83 – b5 0637
PSU/Inverter board: 715G1899-1-HP
Video board: 715G2176-1

Thanks for reading.

[momaka]

Re: HP w19b - screen fades to black - RESOLVED

Okay, the update for this thread was long overdue, so here it is.

The monitor was fixed about two months ago and has been working great ever since. In fact, I used it as a second monitor for about a month until my friend finally came to pick it up. It’s quite a nice monitor, actually – great colors too, which I find is a rare thing to see with LCDs. Panel for this monitor is a HannStar HSD190MGW1-B00 Rev:1.

So, forget everything I said about Q3 in my post above. It turns out that Q3 was fine. The reason it fooled me is because the two pins circled in red in my first post (Gate and Drain) are connected across the inductor “6R8”. A big part of what helped me find the problem was that U3 (MAX1748) on the panel board would overheat every time I tried to power the monitor. So, I looked up the datasheet for the MAX1748 IC, and it turns out the panel board pretty much has its own power supply and this chip is responsible for controlling it.
Thanks to the MAX1748 datasheet (U3 on the panel board), I was able to understand how this monitor works and therefore troubleshoot it. Overall, the power supply on the panel board of this monitor is quite similar to the sample circuits on pages 14 and 15 of the datasheet. The MAX1748 IC is a triple-output TFT LCD DC-DC converter. Basically it takes 5v from the video/logic board and uses that to produce several different voltages – in this case a main boosted voltage rail between 10v and 13v as well as two other voltage rails derived from this rail (referred to as negative and positive charge pumps in the datasheet). The positive charge pump can produce up to +40 volts while the negative charge pump can produce down to -40 volts. The main boosted rail is used to power the buffers (U5 on the panel board) while the positive and negative charge pumps are used for the transistors in the display.

Using the datasheet, I began checking various components on the panel board.
The problem with this panel board was that C33 was shorted - a tiny multi-layer ceramic cap. I thought these don’t go bad, but apparently they do. I stumbled on it when I was checking the SOT-23 diode packs; D2, D3, D4, and D5. These diode packs (3Jv) have 2 diodes inside connected in series (see image in the link below) – pin 1 is anode of diode 1, while pin 2 is cathode of diode 1 and this is also connected to the anode of diode 2. Finally, pin 3 is the cathode of diode 2.
https://www.badcaps.net/forum/attach...1&d=1294381123
Now, if you look on page 10 of the datasheet, you will see that these diode packs are used in the negative and positive charge pumps. In this case, C33 was the capacitor for the diode pack D5 for the negative charged pump (marked C5 in the datasheet) – I could determine this because one end of C33 is connected to pin 10 of U3 (DRVN).
So, when I checked D5, it showed a normal diode reading between pins 1 and 2 and pins 2 and 3, just like the other 3 diodes. However, when I reversed the probes on my multimeter for pins 2 and 3, I got a diode reading again (only slightly higher). I then checked the other diode packs again, but none of them exhibited this behavior. Moreover, when I decided to check the MOSFETs in the negative charge pump of the MAX1748 IC, I put the (-) probe of my multimeter on pin 10 of U3 (DRVN) and the (+) probe on ground; this resulted in the same slightly-higher diode reading I got for pins 2 and 3 of D5. So, finally I decided to check C33, and there it was – my multimeter showed complete short circuit across C33.

I removed C33 the next day and installed another ceramic capacitor of the same size that I took from a dead Radeon 9700 video card. Checked resistance again on C33 with the (-) probe on pin 2 of D5, but this time I got a nice open circuit (using 2000 Ohm scale). Crossed my fingers and turned on the monitor – immediately, I noticed that U3 was no longer overheating. Then I looked at the front of the monitor, and amazingly, I was greeted with a nice image on the screen (I think it was the HP logo) .
…
So this is the whole power supply circuit for this panel-board, more or less. I don’t think anyone will have this exact same problem, but I still wanted to share this info just in case. This was actually the first monitor I fixed that had a bad panel board and the techniques I used here are likely applicable to other monitors as well, especially HannStar monitors with the same problem as mine and same MAX1748 chip.
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Okay, this post is long enough, I'll stop here .