It looks to me the intensity bit isn't getting handled properly. There is perhaps a cold solder or a failed component somewhere in the color processing circuitry...

In that case I expect all colors to suffer and be indistinguishable.. e.g. dark blue is exactly the same as blue... light gray is the same as white...etc.
Here they are different, meaning at least the digital signal is working for all 4 lines (RGBI).. but the interpretation to analog seems wrong..

Maybe it is just the photos, but the top and bottom have very minor brightness difference on the test images. The top rows should be approximately twice the brightness of the lower ones, and they are maybe 10% brighter. This definitely suggests a problem with intensity input handling.

Obviously, the intensity bit gets into the monitor, as there is a clear distinction between yellow and brown, so the "special brown logic" gets the full 4-bit color value. Maybe it's really due to the photos, or the photos have been taken with a bad contrast setting, but Tiido is correct that the distinction between the low-intensity colors and the high-intensity colors should be bigger than visible on the picture. As the prime issue is that "dark gray" is indistinguishable from "black", and dark gray is technically just "high intensity black", the issue might in fact be related to handling the intensity bit when generating the analog levels from the digital input.

On the other hand, my first reaction to "dark gray doesn't work" was not "check the intensity bit", but "the black level is bad". In a properly tuned monitor, yanking up brightness to maximum is supposed to make the whole background appear "dark gray" instead of black. If the background stays solid black at maximum brightness, the base brightness value of the monitor is too low, which will make dark colors appear as black. The base brightness might be too low because the tube is worn out. Adrian from Adrian's Digital Basement observed that the cathodes of a CRT may get insensitive (emitting less electrons) if they are unused for many years, but this effect is partly reversible by just using the monitor for a couple of hours. If that doesn't help, I would suggest to adjust the G2 (screen) voltage of the monitor.

Worn tube would black crush, which would make the dark grey black, but it would also make all the low intensity colors get crushed too, rather than compress down brighter tones to lower ones level.

There appears not to be any significant bloom visible on the bright bits to indicate things being being driven at limits of cathode amps and beam current possible, so adjusting the G2/screen/acceleration pot on the flyback is worthwhile... and while at it, focus should be tweaked also ~

Tiido wrote on 2026-01-06, 21:55:

Worn tube would black crush, which would make the dark grey black, but it would also make all the low intensity colors get crushed too, rather than compress down brighter tones to lower ones level.

The non-bright colors would not necessarily get crushed. The crude approximation of converting RGBI to analog is that R/G/B are 66% of the individual color, while I is just 33% of each color. So dark gray is 33%/33%/33%, whereas "normal blue" is 0%/0%/66%, and "light blue" is 33%/33%/100%. If everything below 45% is crushed to black, this would make dark gray invisible, yet the non-bright colors stay visible.

Thanks guys.. The photos are not reflecting the reality very well, but yes.. the difference between the normal and bright versions of the colors is not as expected. There IS a difference, but it should be greater. The "dark gray being completely black" is the most problematic though.. I was never able to get dark gray to show up no matter how I play with the contrast and brightness knobs. Only if I use the monitor's "Mode" knob to invert the colors, only then does dark gray becomes visible and not pure white.

I'm guessing that this mode knob is selecting between different interpeters of the digital colors. The inverted set is looking good. There are also monochrome white, green and amber options, plus a "bluish tinted" option.. all in addition to the standard colors option.. which seems to have some issues.

I guess the next step now is to open up the thing and give it a closer inspection. I'll post photos here when I get to it.

mkarcher wrote on 2026-01-06, 23:28:

The non-bright colors would not necessarily get crushed. The crude approximation of converting RGBI to analog is that R/G/B are 66% of the individual color, while I is just 33% of each color. So dark gray is 33%/33%/33%, whereas "normal blue" is 0%/0%/66%, and "light blue" is 33%/33%/100%. If everything below 45% is crushed to black, this would make dark gray invisible, yet the non-bright colors stay visible.

And because they are visible, I don't suspect a worn tube or wrong acceleration/screen/G2 setting although it is often a good idea to adjust these, along with cutoff + drive pots on the neckboard, since there is no calibration in 99% monitors that tries to compensate for aging.

Many TVs (since mid-late 80s) have cathode calibration function, where there are lines at top of raster (in overscan, although visible in widescreen mode on most TVs that can do that) where the jungle chip outputs a signal to a level which produces the required beam current(=apparent brightness) that is being measured. The measurement is stored and used throughout the frame to offset the output, in essence it shifts the black level higher and higher(depending on warmup+aging) to just above the cutoff point and maintains color accuracy until the cathode amps are no longer able to cope as the voltage rail capacity shrinks down too much to have any dynamic range (TVs often have 250V rails, monitors usually under 100V). In most severe cases you see streaking toward right on bright details, which is when the cathode amps are exiting saturation due to hard clipping.

So if for example on a severely worn tube that crushes from "40%" turns the "light blue" into 0%,/0% /60%, and normal blue into 0%/0%/26%. It is not going to be able to make the 100% level 66% levels appear similar, their relative proportion doesn't change, but the bottom end is getting eaten away, which is why the effect is called "black crush" since it affects everything. But this appears not to be happening here and it is why I suspect something in the circuit that creates the voltages for cathode amps, and intensity bit related circuitry in particular. It probably is a simple fault, likely a cold solder joint or maybe there's even a pot that adjusts the magnitude of the intensity and it has gone bad...

wbahnassi wrote on 2026-01-07, 06:00:

I guess the next step now is to open up the thing and give it a closer inspection. I'll post photos here when I get to it.

I definitely look forward to seeing those ~

Alright, so here it is. I just opened it but didn't disassemble anything internally. It has quite a few internal knobs on the boards. Here is the ones I found:

Exposed through the back:
* V-Hold, V-Size, and H-Size.

Internally on the main board:
* H-H (H-Hold?), H-Center, V-Center.
* B-Drive and G-Drive screw-adjustable pots (probably there is also R-Drive? but I can't tell because these are hidden under other structure)

Internally on a side panel (not exposed):
* Focus and Screen. These two are locked into position by hot glue.

On the board attached to the back of the tube:
* B.BKG, R.BKG and G.BKG.

I'm assuming I'll have to adjust some of these during operation? I don't want to zap my self yet, so any advice on how to approach these knobs would be appreciated.

The Mode knob on the front has many wires coming from the selector switch, and they feed into a small brown board that has only one IC chip.

Should I play with some of these knobs? Which ones? I'm curios what do the R/G/B-Drive pots do, as well as the R/G/B.BKG knobs on the back of the tube.

The tuning pots are adjusted while turned on using a plastic wand designed for this purpose, yes.

[Example tool]
https://shewfly.com/products/crt-adjustment-tool

Like Tiido suggests, you should examine the solder-side of the intensity enable signal circuitry components under a magnifying glass or microscope, and look for cracks.

RGB.Drive control strength of the signal going to the cathode amps on the neckboard, and RGB.BKG on the neckboard control cutoff level. The circuitry(or part of it rather) that generates the analog output from the digital input is in the area nearby the LGB.Drive pots.

But yes, these need to be adjusted while the thing is open and running so a number of precautions must be taken since this thing is able to bite pretty hard in a few areas. Plastic screwdrivers for trimmers etc. are highly advisable since they won't get you in trouble if things slip which can easily happen. Some gloves can be helpful if there isn't a good service position available with that monitor. Usually it is possible to take the board out far enough that these controls all become accessible for adjustment as all of these were adjusted at the factory by real people who do not want to struggle doing these adjusts 🤣. At least Focus and Screen are nice and easy to access, even with bare hands and so are the RGB.BKG ones. The ones on the yoke you don't mess with unless you are unhappy with convergence and think you can do better than the factory did (possible, but you need to know what you are doing). H-H (yes, H-Hold) you should not touch unless the monitor struggles with some video timings but H-Center and V-Center you can mess with if you want to adjust geometry, perhaps to maximize usage of screen area or correct for image shift from a particular video card.

Hopefully you don't need to actually remove the board from everything to do proper inspections on it. Mains side power supply section has a big fat capacitor in it that has the capability to stop the heart if it has enough juice left in it but luckily the PSU looks to be hidden away in a metal cage. Anode detachment from the kinescope can also be tricky, since there stored charge in the kinescope can be very nasty and without discharging it first you are highly likely to get zapped, maybe not so hard but most unpleasant regardless, and the charge comes back later just from handling as it is one giant leyden jar type capacitor. Static electricity generated from touching the front of the kinescope is able to charge things up to highly uncomfy levels and if the kinescope is not left ungrounded after the Anode is removed, your fingers are likelf to receive a zap from the Anode connection of the kinescope as you try to attach the anode cap back to it (sadly I speak from experience 🤣). I tend to use a thickly insulated wire that I tie to the metal straps on the kinescope and then carefully slide the under the anode cap, at some point there's a crack sound which means the big charge in now gone. Then you can confidently use your hands to remove the anode cap from the connection. Hopefully this won't come to needing to do that, and there may be other things to detach aswell that aren't made so easy to do.

Ooof, this is a really hard one. The main board is buried under all kinds of clutter and parts of the monitor, even blocking access to the screws holding it on its frame. It's not even a case of using one of them super long screw drivers.. some screws are completely covered by the tube. I got nervous trying to figure out how to get the board out. I need to remove the board as it's not very useful to check it in its spot because many wires connect to it at locations I cannot access... but it seems really difficult to figure out.

I decided to put it back together and keep the monitor as it is.. better than breaking it more by my attempts to disassemble the board...

I hope this was a wise choice 😀

If it is that big of a pain then it probably was the right call. Some monitors (and devices in general) can be most evil. Poor people who had to assemble them in factories...

EDIT: The Focus and Screen pot on the side could/should have been still tweaked and perhaps get a little more out the situation.