Green tint (non permanent) on iMac G3

[xeiter]

ima

Hey everyone,

I am trying to analyse what's up with my iMac G3 that quite often has green tint to the all the colours especially after being turned off for a while. When this happens it looks like none of the colours (R, G or B) are missing but they are rather washed out with green.

It is especially noticable in full screen games (640x480) with dark background where blacks become green.


On desktop in MacOS 9 and OSX it is less noticable however I noticed that amount of green tint depends on the amount of white currently displayed on the screen. If I open a Finder window and make it large all colours go back to normal (blacks look black etc). If I move the window to the corner or close it - i instantly see a change of all colours to have a green tint again.

Very often, after some time of the G3 being on, the colours go back to normal permanently until I turn it off and not use for a while.

I tried the following:

• PRAM reset
• Open my G3 and knock things around especially in the neck (I do know how to be careful with CRTs before you ask)
• Froze (with freezer spray) most components on logic and analogue boards (both small and large inside the CRT) and CRT neck

Nothing above affected the green tint.

I do not have an external VGA socket on this G3's logic board unfortunately.


The CRT tube definitely works well cause this green issue came after I installed the analogue boards together with the logic board from another G3 (whose CRT tube was VERY bad).

The reason I did the swap of components is because in the current G3 its logic board had dead VRAM (artifacts). I also suspected that the its original analogue board (before the swap) was not working 100% since I had 2 (!) logic board's VRAM die on me within 2 days. So I replaced logic board and analogue boards.

Unforunately, I do not have access original logic board (and another one where VRAM died too) because I had already (rushed it) removed the VRAM from them while waiting on new one in the mail (from Alixexpress which is taking while to be delivered) so I cant check if it is the logic board's video chip causes the green tint or not.


What I can do though, is put back the original analogue board (the big one that connects to the neck of the CRT) and see if that removes the greeness. It is just very annoying and long to disassemble and assemble the G3. And I also worried it will kill my third logic board's VRAM.

Anyway, I thought I I would share my journey here as well maybe try my luck with suggestions from other readers.

I've published two videos on YT to show what I mean with green:

MacOS 9 desktop colour change when moving window (look at top task bar colour) -

Monkey Island green background -

 

[Daniël]

It's unlikely to be caused by VRAM.
VRAM defects generally give you digital corruption in the picture, a green tint to the entire image in this way is stemming from the analog circuitry.

If you swapped analog boards, then my guess would be that it simply isn't tuned correctly for the tube.
Remember, they adjust the analog boards at the factory to get the right image out of the installed CRT, which differs from tube to tube.

There are also a few different brands of tubes used in the slotloading iMac G3s, and the analog boards to a certain extent are made for one or the other.
It could very well be that mismatched analog boards and tubes could result in the picture being out of whack, because their color drive and cutoff voltages and such differ.

The good thing is that for adjusting the color drive and cutoff values, you do not need to adjust them inside of the display, only requiring a jumper on a specific pin on the neckboard to ground.
The adjustments chapter of the iMac G3 service manual shows how you can calibrate the values using the Display Adjustment Utility, which can be found on the popular Macintosh software repositories like Macintosh Garden and Repository.

 

[Byrd]

Looking at your videos, in the second one your brightness is set too high, as evidenced by the lines. I think with this dialled down by a rear pot that will help immensely. CRT displays have to "work harder" displaying bright images, also CRTs with high use/thousands of hours struggle more doing this too.

I'd dial down just the brightness/gain, and then recalibrate the monitor using the utility Daniel has pointed you to.

 

[xeiter]

Thank you both I will try all the suggestions.

The strange thing is that the issue is not permanent. When it goes back to normal (randomly) the brightness settles too as well as green goes. The very bright green shine of the background is only there when green is visible.

I actually tried reducing brightness and I can get rid of green shining background - if I do that rest of the picture is very dark and almost all green. Once again when it fixes itself everything becomes fine brightness wise.

When i installed the new analogue and logic board la I had to crank up the brightness on the fly back - the crt was black.

So I am thinking that if the issue was permanent then yeah calibration etc. but since it is intermittent I don’t know what it could be. I will still check out Daniel’s link and see what I can adjust. The issue definitely does not look like vram. I only mentioned vram because other logic boards had issues with vram and wanted to give you chronological events.

 

[xeiter]

I

It's unlikely to be caused by VRAM.
VRAM defects generally give you digital corruption in the picture, a green tint to the entire image in this way is stemming from the analog circuitry.

If you swapped analog boards, then my guess would be that it simply isn't tuned correctly for the tube.
Remember, they adjust the analog boards at the factory to get the right image out of the installed CRT, which differs from tube to tube.

There are also a few different brands of tubes used in the slotloading iMac G3s, and the analog boards to a certain extent are made for one or the other.
It could very well be that mismatched analog boards and tubes could result in the picture being out of whack, because their color drive and cutoff voltages and such differ.

The good thing is that for adjusting the color drive and cutoff values, you do not need to adjust them inside of the display, only requiring a jumper on a specific pin on the neckboard to ground.
The adjustments chapter of the iMac G3 service manual shows how you can calibrate the values using the Display Adjustment Utility, which can be found on the popular Macintosh software repositories like Macintosh Garden and Repository.

I do have a spare tube - might try that too.

Before I removed the analogue and logic boards from donor g3 it had the same issue with green - I thought it was the crt but looks like not since it is doing the same in my main g3

 

[xeiter]

big thanks to Daniel and for the service manual link above - I now know that there are two different analogue boards as well as two different tubes used in my type or g3 . I am going to check out which one I had and which one I have now to make sure they are paired correctly. Ah looks like I need to disassemble this thing again

 

[xeiter]

Ok the plan is:

1. Open G3 and check which analogue board I installed and compare with the one that I replaced. I definititely know that old board didnt have the tube selection switch (so it was 661-2370). I need to check the one currently in G3 - I think it had a switch.
2. If the currently installed analogue board has the switch make sure the setting matches the tube make.
3. Make a call whether to attempt the calibration of the new pair: analogue board - tube OR put the original analogue board in (no calibdation required)

My only fear is that the original board could have killed two of my previous logic boards (vram started glitching out).... is it possible? That was the whole reason I replaced the analogue board. Maybe it was just bad timing. I guess I could try my luck and see if it kills the third logic board.

 

[xeiter]

Update:

1. I calibrated the board - didnt fix the green tint. G3 would switch on with green tint but after some times would fix itself until I power it down for a some time again
2. I tried looking for a potential place of an issue by using freezing spray and heat gun - no affect on green tint unfortunately.
3. I replaced the neck board and that fixed the issue.