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AppleColor RGB Monitor ( IIgs Monitor ) Pincussion adjustment

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    #16
    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.

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      #17
      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.

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        #18
        well good attempt. I suspect its an overworked transformer which is just not replaceable. Shame they didnt include an adjustment pot.

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          #19
          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.

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            #20
            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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              #21
              Hugo would love to hear your ideas. Look at post #2 for a schematic link.

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                #22
                Try replacing C408 initially with a 100uF capacitor as per the schematic, use a low voltage one, perhaps 6.3V of have or no higher than 16V, because the lower voltage range caps have a lower ESR.

                If there is no improvement, or it is worse, put the 1000uF cap back in there as it was before.

                Then to control the amount of pincushion correction make the 8.2 Ohm resistor in series with that cap adjustable, perhaps with a 10 Ohm potentiometer and a series 4.7 Ohm resistor, preferably about 1 watt rated if you can get. Then see if adjusting the pot will fix it, if it does measure the total resistance value and insert fixed resistor.

                The amplitude of the drive voltage that appears across the primary of the pincushion transformer depends a lot on the value of that 8.2R resistor, assuming that is if the capacitor has a low ESR of less than one Ohm. Also the value of the capacitor is important in the tuning of it, what is supposed to be happening is that across the course of the field scan, a parabolic looking wave, almost like a half wave rectified sine wave, is supposed to be applied to the primary of the pincushion transformer, so that it ends up on the secondary, and generates a voltage in series with the line scan coils so the amplitude of the line scan is modulated across the course of the field scan.

                ( For this what I would do in my own workshop is look at the voltage across the transformer primary with a Tek 222ps scope, that has totally isolated inputs and look for the parabolic wave, it would be expected to have superimposed on it voltages from the H scanning coupled back through the transformer. The other way to improvise a pseudo isolated differential probe is to use both channels of a dual channel scope, using the ADD and Invert functions.)

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                  #23
                  Don't adjust any magnets on the yoke, or glued to the CRT. Those are for convergence or purity corrections. Magnets are only used for pincushion correction on monochrome CRTs. On color CRTs it has to be done electronically.

                  It doesn't look THAT bad to me. It's not great, but not bad enough for me to think it's defective. CRTs always have a certain amount of geometric distortion, and it tends to be worse on cheaper monitors.

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                    #24
                    Yeah it wasnt terrible.. but it wasnt good either. Anyway, I decided to leave it as is. Its the worst of all the iigs monitors I worked on,, but still not bad enough to not be ok to use.

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