Uneven color tone on Color Classic's screen.

[Marconelly]

I purchased a non-working Color Classic with an intent to repair it, and after fixing up some fairly extensive logic board damage due to capacitor leakage, and of course doing the logic board recap, the computer was back up and running! Two problems remained however: there was some picture flickering during the warmup period, and the picture had a mild color tone shift, in the right third of the screen. Most of the screen has a slight bluish/greenish tint, but the right third had a reddish tint.
I decided to do a full analog board recap, and also touched up any suspicious looking larger solder joints on the analog board and the CRT neck board. This solved a flickering problem completely, and the picture now looks pin-sharp and stable. The color tone shift remain unchanged however. Now, this is pretty mild, and after some use I practically forget it's there - but I'd still like to eliminate it, if possible.

It's very difficult to take a good photo of this, but here's my best attempt. That's kind of what it looks like in real life, minus the moire noise of course:

Are there some pots I can try turning to correct this, or is it a sign of a permanent tube damage?

 

[misterg33]

You might already know all this, but just in case... Color shift in just a corner of the display could be a gauss issue (magnetic field). Make sure the built-in degauss coil is connected since you've had the computer taken apart previously. It's connected to analog connector BP2 according to service source

If it continues, try moving the computer to another location in case there's something magnetic near by that you're not aware of. Power cycle the computer to try to run the built-in degauss.

If you have access to a degaussing coil, you can try using it, but there is a built-in coil that's suppose to do this.

If it was more than a "splotch" in the corner, I would think there would be a display POT you could adjust for it, but from your description it sounds like gauss. If colors flicker at startup, that's possibly the pin connections at the CRT neck. They have a tendency to lose contact with the little box that sits on the back of the CRT. Sometimes reseating it will fix. Sometimes letting it simply warm up will fix. DONT MESS WIH THE CRT UNLESS YOU KNOW WHAT YOU ARE DOING. There's high voltage involved.

If you do an internet search for "color classic service source pdf" you can probably find more info about the degauss connector and the display adjustments.

 

[Marconelly]

Thanks! I don't think there is anything magnetic in the environment, as I have another Color Classic which I've been repairing - that one has other problems, but the screen looks perfectly uniform when the computer sits in the exact same spot where I'm testing the CC with non-uniform screen tint.

Degauss wire is connected where it should be, and it's worth noting that this isn't just a corner - it's a whole right side of the screen where this shift gradually happens. This problem has been in the screen even before I did an analog board recap, and the recap hasn't changed it (it fixed the flickering though)

 

[misterg33]

As an aside -- can I ask what that crazy icon is in the extensions list on boot? Is it some sort of fax software?

 

[Marconelly]

As an aside -- can I ask what that crazy icon is in the extensions list on boot? Is it some sort of fax software?

Yes, it's a fax software extension. It's on the system that's on the HDD that was in the computer when I got it. The extension even makes some noise that's probably supposed to sound like the paper being spooled out of the fax machine. But it's really more like static noise, and at first it made me think the computer's speaker was broken.

 

[Paolo B]

I have 3 of those fancy machines, all restored and tuned at different specs, including one with 640x480 mod.
All of them are showing the defect you report, to a different extent.
My guess is that the CRT just degrades unevenly over the years.

 

[Marconelly]

I have 3 of those fancy machines, all restored and tuned at different specs, including one with 640x480 mod.
All of them are showing the defect you report, to a different extent.
My guess is that the CRT just degrades unevenly over the years.

I see I guess I will just have to accept it, and feel lucky that I have another one that doesn't have this issue.

 

[JDW]

But is it really the CRT which degrades, or rather degradation of PART of the degaussing coil surrounding the back of the CRT, and/or the magnets in the vicinity of the aforementioned discoloration? Or would it possibly pertain the the Yoke board or its connectivity to the CRT neck?

 

[Marconelly]

But is it really the CRT which degrades, or rather degradation of PART of the degaussing coil surrounding the back of the CRT, and/or the magnets in the vicinity of the aforementioned discoloration? Or would it possibly pertain the the Yoke board or its connectivity to the CRT neck?

I was wondering about that, but I don't know if any of that is repairable or adjustable. I see that the CRT has those plastic strips that can be moved around - aren't they in control of some magnet positions? In any case, on mine they seem to be factory glued, so I don't think anyone ever touched them.

 

[Paolo B]

But is it really the CRT which degrades, or rather degradation of PART of the degaussing coil surrounding the back of the CRT, and/or the magnets in the vicinity of the aforementioned discoloration? Or would it possibly pertain the the Yoke board or its connectivity to the CRT neck?

I was wondering about that, but I don't know if any of that is repairable or adjustable. I see that the CRT has those plastic strips that can be moved around - aren't they in control of some magnet positions? In any case, on mine they seem to be factory glued, so I don't think anyone ever touched them.

Proper degaussing is probably the first thing to test. In “another forum” it is reported that the CRT degaussing coil connector and / or thermistor may be at fault. Can’t further check on my machines, as I am now in the midst of a relocation.

 

[JDW]

I see that the CRT has those plastic strips that can be moved around - aren't they in control of some magnet positions?

These strips...

I've never tried moving them with the machine on, but I suspect something will check on the screen when you do touch them. Just be careful you don't zap yourself if you do decide to touch them because the coil is nearby. Rubber gloves would be safe though.

 

[Paolo B]

As far as I know, those magnetic bands are used for fine tuning the geometry of the frame. They seem to me typical factory sealed stuff not meant to be touched.

 

[JDW]

@Paolo B
You earlier said you have 3 of these machines. Did you do the VGA mod on any of them? If so, was there a change in the color cast, before/after the VGA mod? I wouldn't think there would be, but I'm curious nonetheless.

 

[Paolo B]

@Paolo B
You earlier said you have 3 of these machines. Did you do the VGA mod on any of them? If so, was there a change in the color cast, before/after the VGA mod? I wouldn't think there would be, but I'm curious nonetheless.

Yes, I have a Mystic, too. Sourcing a donor LC575 has been relatively easy in Japan. I actually made the VGA mod following your excellent tutorial, even though I am kind of proud I made the whole thing in a non-destructive, reversible way. I remember I choose the machine with the cleanest looking CRT (still some shadowing), and didn’t notice any major difference before and after the mod.

 

[Marconelly]

Yes, I have a Mystic, too. Sourcing a donor LC575 has been relatively easy in Japan. I actually made the VGA mod following your excellent tutorial, even though I am kind of proud I made the whole thing in a non-destructive, reversible way. I remember I choose the machine with the cleanest looking CRT (still some shadowing), and didn’t notice any major difference before and after the mod.

Would you mind sharing some photos of how you've done this? I've been thinking about that exact thing, as the entire process appears to be disconnecting things from one another - so it appears it could be done in such a way that one switch could control the mod on and off.

Maybe this could be a basis for one of @JDW future videos?

 

[ClassicHasClass]

My Mystic has a pretty good tube (though I've not done the VGA mod to it), and even it shows some degredation. They're not young machines anymore.

 

[Paolo B]

I did the mod a few years ago, so I don’t quite remember the details by heart. I need to open the machine and compare side by side what I have vs what @JDW suggested. Surely, though, I wanted to have something reversible and non destructive, so I had to find workarounds for the trace cutting steps.
I just remember I was satisfied with the result.
Now my machines are packed for a relocation which will take months, so I can’t access them.