2001 seems like it is new enough that you're probably not gonna see manual adjusts inside of it. A service manual of a Viewsonic Q71 showed there are no adjusts that you can play with either. There was no mention of a service menu in it, so it is unlikely you can fix it without doing some physical modifications like changing a resistor or two to manually create the corrections.

Tiido wrote on 2024-12-22, 14:43:

2001 seems like it is new enough that you're probably not gonna see manual adjusts inside of it. A service manual of a Viewsonic Q71 showed there are no adjusts that you can play with either. There was no mention of a service menu in it, so it is unlikely you can fix it without doing some physical modifications like changing a resistor or two to manually create the corrections.

Yeah, an external resistor or pots on the RGB lines was the first thing that came to my mind.

If there are no internal adjustments another thing that came to my mind is switching the control for red to green, so I would have red as "FIXED" and use that slider to control green instead, but I would need to trace the circuit and know the pinout of the OSD chip.

I know a TV repairman who had a lot of CRT experience in the past, I'll give him a call later this week to get his opinion on what is going on. I prefer not to open these things more often than I have to.

I finally got around to opening it up.

The RGB circuit contains the following ICs:
TDA4886 - "I2C controlled video controller"
LM2439T - "triple CRT driver"
NT6828 - "I2C OSD"

I took a quick look at the datasheets and it seems this board should be capable of green control, seems they just artificially disabled the control in the OSD, as it seems TDA4886 would have been able to adjust green as well, it has a full set of 3 compliments of inputs and outputs for the color channels. Obviously it would probably be possible to jack into I2C to adjust the green, but I don't really love the idea of dangling wires from a running CRT.

It seems this board can accept other OSD chips besides NT6828, this one is 16 pin, but there are 20 pins in the IC position. Ideas what other OSD chip I can replace it with to enable green adjustment?

Other ideas how I can modify this circuit for better color adjustment?

Right now green is the strongest, blue is strong, but red is a little weak.

PS I had the idea of swapping around green and red channels so I would get green adjustment instead of red, leaving red as fixed, but maybe there's a better solution?

The OSD chip is only an OSD chip, it doesn't contain the MCU that runs the show. You find the MCU on the main board, probably somewhere near the front where buttons are but there's no replacing one since they all got model specific internal ROM based code+data etc.

Simplest way is likely inserting a pot into the video signal path, perhaps between the TDA4886 and LM2439 (pins 6, 7 and 9 are the inputs). Wires are gonna have to be really short or ideally none at all.

Tiido wrote on 2025-06-30, 16:39:

The OSD chip is only an OSD chip, it doesn't contain the MCU that runs the show.

Yep, you're right. I was looking in the datasheet some more and was scratching my head why I only find text generation functions and not the video adjustments. Also, those 4 additional pins on the OSD position are the RGB output lines going to the CRT driver, I guess the other circuit variation could have blanked the controller and have the OSD output directly to the CRT driver. Anyway, that's mystery solved.

Tiido wrote on 2025-06-30, 16:39:

Simplest way is likely inserting a pot into the video signal path, perhaps between the TDA4886 and LM2439 (pins 6, 7 and 9 are the inputs). Wires are gonna have to be really short or ideally none at all.

Yeah, I see a good place to add resistors or pots, but for a pot, what value would make sense here? 1K? 10K? Something else? I want this resistance between the output of the controller and input of the driver, or between the output of the controller and the ground?

There's a 390 ohm resistor to ground in the LM2439 datasheet reference circuit on each of the RGB lines, it looks to be absent in this circuit. Actual resistance of that net to ground is about 8.7K... If I divide it down, say to 5K on green, will that weaken the green?

I think I should look at the feedback reference voltage for each of the color channels on the controller, maybe there's something there to tweak. I'll look into the TDA4886 datasheet some more.

r00tb33r wrote on 2024-12-20, 09:02:

If I turn up brightness all the way, the black of the screen glows cyan hue, meaning green and blue are strong.

I kept thinking about this observation, wondering if I should instead be boosting red, which would allow me to retain more overall brightness. I traced the RGB input from the VGA cable and only two resistors are present in the path, 33 ohm on the controller input, and 10 ohm on CRT driver input. These values are so small that I don't think (small) tweaks to them will have much effect. I did see a solder splatter blob right on the pads of the red signal path that flaked off with a mutimeter probe. I'm not sure if it was a solder blob from factory or when I was removing the shielding.

Short update. I traced the color channel feedback circuits and decided to modify the resistors there. The 1K resistor on the red feedback measured slightly higher than the other two, so I decided to solder in a 10K resistor in parallel to drop the total 10% to see if I can see a difference on red.

I reassembled the monitor but had a slight mishap with one of the many ground wires. I lost one in the rats nest inside and it touched something in the AC input area and blew the fuse. I found and hooked up the wire and replaced the fuse, and repaired the jumper it burned.

After that the monitor powered up and showed the OSD display. All 3 colors present, not enough red on it to tell if it made a difference.

Unfortunately it does not sense VGA input (does not sense vertical/horizontal sync). I don't know if something got damaged during the mishap, if something got disconnected while I was closing the cover... On one hand it's showing something, on the other hand it's not working as a monitor at the moment. I'll let it sit for a few days before I touch it again. I respect the angry pixies inside.

Was looking forward to some good news. Really needed it. 😒

Aww, that is such misfortune 😒

Sync goes from neckboard to the mainboard, so maybe there's a cable issue. Since OSD came on, the MCU is living and most rest of the monitor is functional.

As far as resistor value goes, it is really something that depends on what already is there etc. and I would just be doing things experimentally but only because I have a substantial stash of any kind of resistor I might need.

Tiido wrote on 2025-07-02, 16:46:

Aww, that is such misfortune 😒

Yeah, but when I opened it this time I found another ground wire popped off the neckboard shielding and was close to touching the CRT deflection coils. And I know for a fact this wire I did connect. So it seems it's possible I didn't just lose a ground wire the first time around, it might have just popped off the connector like the one this time did. So that mishap may not have been due to my negligence after all.

Tiido wrote on 2025-07-02, 16:46:

Sync goes from neckboard to the mainboard, so maybe there's a cable issue. Since OSD came on, the MCU is living and most rest of the monitor is functional.

Learning terminology. Yeah, I traced and toned the sync signals, they do indeed just connect on the neckboard from the VGA cable to the ribbon cable going to the MCU on the primary board. I reassembled the monitor again but this time I got a white picture on the screen, no OSD visible. Still, I connected it to VGA to see if it's some kind of signal grounding or termination problem, and I saw the monitor switch refresh modes as the computer was booting. So it's picking up sync signals this time, but something happened to RGB guns... Perhaps I didn't seat the neckboard onto the CRT quite as well this time, gonna need to try again. But it is good news that there was no damage on sync. The connections in the monitor are dodgy, without the glue and zip ties that used to be in there it doesn't seem to hold together nearly as well. I was hoping if I just carefully put it together while experimenting with the RGB circuit modifications it will just work... Clearly it's a new problem each time. Gonna need to let it sit a few days again, then I'll open it again to see what's not seated well this time.

Tiido wrote on 2025-07-02, 16:46:

I have a substantial stash of any kind of resistor I might need.

Yeah, I have a wall of components in the workshop at work. I pluck whichever resistors I need from it.

r00tb33r wrote on 2025-07-05, 05:20:

white picture on the screen, no OSD visible.

Same thing again. Granted I didn't really disconnect and reseat any connections besides G2 and the neckboard CRT tube connection.

Gonna have to let it sit for a couple days again. Then will take it apart more thoroughly.