Gateway Profile 4 LCD Computer
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Project: LCD Repair Gateway Profile 4 LCD Computer |
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The Gateway Profile computers, specially the Profile 4 model, were/ are notorious for the inverter going bad. The Gateway Profile 4 PC doesn't look as good as Apple MAC's, but does have a nice-looking design. Unfortunately, it also has a a design flaw. This model uses Samsung LCD, which is considered to be one of the best ones out there. Nothing wrong with that. Unfortunately, the LCD screen module was supposed to be used in normal monitors. If you open it up and look inside, you'll find that Samsung provided appropriate openings for air circulation and cooling, but they were all blocked in the Gateway Design. In the second picture, you can clearly see. All the ventilators are for the main mother board, and it sits right behind the LCD module. The only opening is where the wires from the video card come into the LCD module. The inverters are supposed to get very hot. As you can see in the picture, Samsung has provided ventilation holes in the metal enclosure right where the inverter sits, but it's been blocked. This results in the inverter getting heated up, and the components degrading over time, much more quickly than they do on normal LCD monitors.
The inverter shown above had already been repaired before. From what I could see, the capacitors, the transistors driving the transformers (under the metallic covers), and the round ferrite core inductors had been resoldered (possibly changed). I removed the transistors, and one of them had totally burned out. This one had been certainly replaced, or was the original one, because it had a different marking than the other three. As you can see in the picture, it's leg has come off :-) That's not because of my clumsy desoldering. It's because having burnt out, it had become brittle. I couldn't find Toshiba's RN6003 from mouser.com or digikey.com. I'll replace it with simple SOT-89 NPN power transistor with similar ratings, and see if that fixes it or not. In the first go, I ordered PNP transistors from mouser.com. I've reordered NPN transistors, and will get them hopefully by next weekend. So that's a project for the next weekend. I'll see if it fixes the problem or not. I was thinking of replacing the entire inverter by a generic one, but normally, apart from the 12volts power, they also have a dim control (0 - 5 volts) which adjusts the brightness, and and an enable signal (ON or OFF, 5 volts). Samsung design seems to be different. It uses a Genesis GM2120 LCD controller, which can provide direct PWM signal to the inverter to control brightness. Normally, the PWM waveform is generated by inverter itself, where the inverter controller chip compares a sawtooth waveform with the supplied dim control voltage to generate a PWM signal. I'll have to carry out some checks (unfortunately, I can't do them right now, since my oscilloscope and other equipment is in storage, and I only have a portable multimeter to do all my testing), but I think that the Genesis chip is providing PWM directly to the inverter. That means that the brightness control can't be used on a generic inverter. I've order three of these, and if I use a generic inverter on this guy, then I'll have to install a separate potentiometer for brightness control. Update: Turns out that I was right. I read the Genesis GM2120 datasheet, and turns out that unlike the LCD controllers that most of the LCD manufacturers use, it does indeed provide a PWM signal directly to the backlight inverter instead of just a 0-5v brightness control voltage signal. This means that if someone wants to use a generic LCD inverter, an external potentiometer will have to be used. One more note. If you want schematics for the reference design for the entire controller including the inverter, you can sign a NDA with Genesis Microchip Inc., and get those. No point in chasing after Gateway or Samsung for that. In my experience with the design teams that I've worked with, except for minor modifications, the designers use the reference design as is, so you'll be able to get either the exact details, or the equivalent parts for this inverter. Update2: The GM2120 datasheet also states that the PWM can be connected to an RC integrator to generate variable DC voltage for brightness control. So that means that an external potentiometer will not be required. However, the values for the resistor and capacitor will have to be experimented with. The datasheet doesn't give reference values. Perhaps the reference design includes these values, but I don't have access to it. Update3: I tried replacing the transistors, but it didn't work. So that meant that I had to see if one of the op amps that were being used in the circuit were busted. I'd already tested all the other transistors, and they were fine. I'd have had to test the different signals to isolate the problem, so I suggested to my wife that I take my oscilloscope out of the storage. She suggested to me that if I did that, then either your oscilloscope is going to live in this apartment, or you are. I decided that I'd rather live in the house myself. Oscilloscope can wait until we move to a bigger place. But this also meant that unless I was willing to replace all the components, there wasn't any way I could repair this guy. I'd already bought three generic inverters, two of which I'd used in L297P's, and had a third one still available. So I decided to make use of that. I traced the connector pins to the GM2010 pins. These are shown in the picture below.
The pinout for the generic board is given in the page for L297P computers. Using this information, I made the cable shown in the picture below. I just cut the connector at one end, and soldered the wires together and brought them into the generic inverter connector. Heat shrinks were used to insulate the soldering joints between the wires.
This fixed the backlight issue. However, the problem didn't finish there. Seems that XP Professional that I installed on the PC (since it had a Microsoft authentication tag for installing XP Professional on it) didn't quite like the hard disk that I put in, and I had to spend hours to set the BIOS settings right and bring the computer up. Since that is not related to electronics, I'm not giving the details of that adventure here, but before you repair this monster, please keep in mind that it's a black-hole, which even eats up your time. I'm quite sure that the problems are not finished here, and it will keep me busy for many more hours. However, I do have to admit that since the LCD panel is from Samsung, the display is superb. |
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