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AppleColor RGB Monitor ( IIgs Monitor ) Pincussion adjustment

VERAULT

VERAULT

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Im doing a restoration on a typical IIgsRGB monitor A2M6014. This one has noticeable pincusion on the left and right side. I cant for the life of me find an adjustment for that. Anyone have any idea? Its not terrible, but I know I can tweak it better if there is an adjustment.

There is a very simple PCB on the CRT Yoke labeled Wacom WA-03V0 with two small adjustment pots on it but I have no idea what this board does.
 
That pcb adjusts the shadowmask turns out. Still doesnt answer pin cusion adjustment. Im gonna assume there isnt one and its done with magnets.
 
I couldn't find anything for the Mitsubishi monitor part number either. You could mark the current positions of those pots and try adjusting them slightly in either direction to see what they do. I didn't see anything marked pincushion or horizontal linearity on the main PCB either.
 
T502 is the side pincushion correction transformer. I would check C408 and R415 which would affect the amount of side correction.
 

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So this is strange. Unless my eyes deceive me. The schematic shows C408 as a 100uf 6.3V cap but what I have in the system is a 1000uf 6.3v cap. I assume the schematic is wrong?
 
So R415 measured 8.6 Ohms so its within spec. Cant find my cap tester so I just swapped C408 with a 1000uf 10V cap. Video is still exactly the same. Its not terrible its just enough that it bothers my perfectionism. But the pincussion is still there.

Could T502 be the issue? I dont just like doing recaps on things that dont necessarily need them. The only issues with this monitor besides the pincussion is the brightness and contrast were not idea so I increased the internal subbrightness and sub contract so the knobs made more of an effect ( i realize that is over driving slightly but I do this on other old monitors,,, thats what they are for. The pincussion was there before I made these changes. The color and brightness are now quite nice..
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Could be. Does the raster seem the right width ? L552 in that schematic is the width adjustment. A slightly weakened flyback could need some increased width compensation which would also
increase the pincushion correction. That width coil is probably a ferrite core that can be turned but there would be a danger of cracking it if it's frozen after all these years. Or you could try decreasing that cap to 100uf. It is tuning T502 and might make the pincushion better or worse. It won't hurt anything to try it.

Larry (Allen) G
 
I played with the width coil ( I always do when tuning the picture) It makes no difference in pincushion but it does increase width. I played with the raster width as well. I realize it looks a bit compressed in the photo but it looks much better in person.

I dont think its the flyback. There is no arcing or popping. IT seems like a working strong flyback.

Oh this monitor is such a pain to take apart (poor design for sure) Just to get at that board. I really dont think I want to do it just for a let-me-see approach. I think Ill just deal with it as is. I have worked on alot of these IIgs RGB monitors. This is the first to display an incorrect pincussion so their circuit must be pretty good.
 
Yea your photos look pretty good so don't push your luck. Are the top and bottom curved in at all ? If all 4 sides are curved in then the yoke may need to be pushed ahead on the tube
but if the color convergence is lined up (no colored edges) probably leave it alone since pushing the yoke will shift the convergence and need ring adjustments.
 
The convergence is fine. Turns out those two pots on that yoke pcb change the convergence . but yeah the convergence is fine.. The bowing is only on the right snd left side. Top and bottom are fine.
 
I have the same issue, especially the upper left corner. The text was apparently distorted. It looks worse in person than the pictures. What did you decide to do?
 

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neosunrise said:
I have the same issue, especially the upper left corner. The text was apparently distorted. It looks worse in person than the pictures. What did you decide to do?
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yeah like mine your left side is slightly worse. The upper left where the apple logo and text is.. I think you have a magnet issue. Mine yoke has this weird foam glue yellow orange in color holding a few magnets to the yoke. Also a couple plates on some coils in the middle of the yoke. The glue deteriorated and I had to glue them back on. Looks like you either lost a magnet or need on in the upper corner. I would take off the back cover and look at the white plastic part of the yoke and see what the magnet situation is. Are any missing? Any floating around inside?

Im just going to leave mine. I dont really feel like mucking about with it anymore and its only a slight bowing its not bad. I cant win every battle.
 
VERAULT said:
yeah like mine your left side is slightly worse. The upper left where the apple logo and text is.. I think you have a magnet issue. Mine yoke has this weird foam glue yellow orange in color holding a few magnets to the yoke. Also a couple plates on some coils in the middle of the yoke. The glue deteriorated and I had to glue them back on. Looks like you either lost a magnet or need on in the upper corner. I would take off the back cover and look at the white plastic part of the yoke and see what the magnet situation is. Are any missing? Any floating around inside?

Im just going to leave mine. I dont really feel like mucking about with it anymore and its only a slight bowing its not bad. I cant win every battle.
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I think the apple logo and the text are fine, they are just a little slanted. I was just wondering what you would do to the pincushion distortion. If it can’t be adjusted easily, I’ll just live with it.
 
Im suspecting the Transformer is failing. So find an exact replacement, or a new monitor board which isnt going to happen. So living with it seems like the likely option.
 
So today I opened up the monitor and inspected the resistor as well as the capacitor. I can confirm that C408 is 1000uF and my multimeter showed that its capacitance was around 960uF. I replaced it with a 1000uF capacitor but just like the OP said, it didn't make any difference. I also tried replacing it with an 100uF one as retrogear suggested but it made the screen out of sync. The resistor measured around 8.6-8.7ohm. Don't know what else I can do.
 
well good attempt. I suspect its an overworked transformer which is just not replaceable. Shame they didnt include an adjustment pot.
 
neosunrise - in your case with only one corner affected it is more likely yoke positioning, purity rings or the corner of the shadow mask coming loose inside the tube.
Where the text is distorted, is it mis-converged ? (RGB not lined up together) . That would definitely indicate what I listed and not pincushion problem. Could need to be demagnetized
but degauss usually clears it after a few cold power ups.
 
In the case where there is fairly symmetrical pin-cushion defects in the 4 corners, it means that the amplitude of the parabolic shaped drive to the the pincushion transformer is incorrect, pretty unlikely a faulty transformer but not impossible.

If there is a "pincushion -like" error just in one corner, this normally means that the field correction magnets around perimeter of the yoke (close to the CRT bulb) that one is out of adjustment or has moved or been lost. Some yokes had these glued in, others you could rotate them.

So for the VDU shown where the pincushion defect is affecting the scan fairly symmetrically, it is a matter of adjusting the amplitude of the drive waveform to the pincushion transformer to correct it, I could make a suggestion how, if there was a schematic of the area,
 
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