M1212 RGB display colors...rotated?

[evan]

Hey all. I recently acquired an M1212 Macintosh 12" Color Display. It works fine, but the colors seem to be off in a peculiar way. Has anybody seen this?

https://imgur.com/a/RGb98

In my example image, the M1212 is on the left and a regular VGA LCD is on the right. It looks like what should be green is coming out blue, what should be blue is coming out red, and what should be red is coming out green. I monkeyed with the adjustment screws but none had the desired effect. Any tips?

 

[defor]

This looks like one of two symptoms:

1) hue adjustment's been fiddled with (adjusting the hue on monitors will usually cycle the colors like that)

2) the RGB signal connections to the monitor may be in the wrong order if it's been serviced before.

also the M1212 should be the "Macintosh Color Display" - I think this is a 13", capable of 640x480, not 12" capable of only 512x384

 

[techknight]

These type of monitors usually dont have a hue adjustment as far as I am aware of. 

My immediate thought is the CRT face has been magnetised enough to shift the entire purity of the raster. I have seen this before if the degaussing circuitry is faulty in some way. Thats my thought outside someone fiddling with the RGB signals internally. 

 

[evan]

Defor, I think you're probably right. My 12" estimate was based on holding a ruler up to the display and saying "yep, twelve".

I did a little studying up on CRTs. My understanding is that the term for my issue is called "purity", and one possible cause of purity issues is that elements in the tube have shifted enough such that the electron streams hit the next subpixel over on the phosphor.

Apparently Trinitron displays (this is one of those) are known for having purity/convergence issues, since the grille mask is heavy, vertical, and secured with fine wires within the CRT. If the mask is knocked around and shifted within the tube, then it can potentially cause purity issues. The color rotation I'm seeing is consistent with the subpixel order I can see in the image at https://en.wikipedia.org/wiki/Trinitron . This site https://www.repairfaq.org/sam/crtfaq.htm has an impressive wealth of information.

The one thing that gives me pause with the "bumped mask" theory is that the color shift is so complete and uniform across the screen. Granted, I did fiddle with the convergence settings, but you'd think a bumped mask would cause weird gradients, dark regions, etc. This seems like a complete and clean subpixel shift.

I didn't find any immediate evidence of previous servicing within the monitor. The only connector I could find inside that looks like it might be possible to install backwards is this one:

https://imgur.com/a/0oE4P

I'm extremely hesitant to reverse this connector, as I don't think it fits the evidence. It would swap red/green, and blue/black, which doesn't fit my observed color "rotation" of red -> green, green -> blue, and blue->red.

I'm a little chicken about loosening the yoke and doing coarse repositioning, as the display is otherwise perfectly crisp and I'm worried it would never be the same. At this point I'm tempted to just buy a DB-15 extension cable from Monoprice and spice the color channels around to compensate. That ought to work, right?

 

[techknight]

As I said before, you need to inspect and/or attempt to degauss the face of the CRT before you start jumping down rabbit holes. 

 

[evan]

Hi techknight.

I did take a peek around inside to see if anything looked off related to the degaussing plate or coils. The reason I'm thinking it's not a magnetized face is that the color shift is so uniform across the screen. Doesn't a magnetized face normally present with colorful gradients or differences at the edges? For a color shift to be so complete, wouldn't the original errant field have to be so large as to have no discernible curve or change in intensity in the field across the area of the display? That would be a pretty huge field, right?

This monitor does not have a degauss button, but I do hear the classic "twang" when I cold power it on. I do not hear the twang after a warm power cycle. So I suspect it's degaussing on power on.

I brought a small fridge magnet near the monitor face, and managed to magnetize the face just slightly, so there were two slightly dimmer areas in the middle of the screen. I then powered off the monitor and waited about 15 minutes for it to cool down. When I powered it back on, I heard the twang, and the discolorations were gone. So I believe the degausser is working.

You can see the purity is quite consistent in this photo (these colors are supposed to be red, green, blue):

https://imgur.com/a/Kbe0g 

And here's a slightly magnified version... a little bit of convergence issue at the bottom, but generally very consistent.

https://imgur.com/a/nBfXL

I've also noticed the Green Drive and Blue Drive pots change the wrong colors. Wherever the swap is, it's after the control circuitry.

Does anyone else have one of these monitors? If so, would you mind verifying that the connector shown here is plugged in the right way?

https://imgur.com/a/0oE4P

 

[Trash80toHP_Mini]

Worst case scenario: does it good in grayscale?

 

[evan]

Yeah, it looks pretty good in grayscale. I suppose if my crossover extension cable idea doesn't work, there's always that.

 

[Trash80toHP_Mini]

 

[techknight]

Then the wires going to the Neck PCB have been swapped or the amplifier IC is bad, one or the other. it is strange thats for sure. 

Reason I said degaussing is because I have seen magnetization get so bad where it shifted the entire purity. I saw this a few times. But You didnt hear the loud BUUMMMP when you first turned them on either. In your case, you have so we can rule that out. 

if the drive levels are changing the wrong colors, then either the CRT has been changed and its the wrong CRT entirely, or there is a reversed connection in the Neck PCB itself. 

IF the Red is correct, but the green and blue are reversed, just swap the 2 wires at the base of the connector.