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Mac SE CRT ghosting/Bleeding to the right

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    #31
    Originally posted by Hugo Holden View Post
    When there is a frequency response issue in the video amplifier/CRT gun driver, it will always seem less severe at low contrast and low brightness levels.

    It is pretty difficult to make any helpful suggestions, without the schematic of the CRT neck board and he circuit that feeds that from the main board.
    Hi Hugo, Thanks a lot for your help, I tried my best but didn't find an exact schematic of the Mac SE video board, but I found a general mac repair guide, which seemed to use the Mac Classic. Classic II's video board as example and stated: "The horizontal and video circuits are also virtually identical to those used in the SE and SE/30, but the vertical and power sup- ply circuits are completely different." attached in the following link:

    https://imgur.com/a/8MPXmeC

    It is really simple, it seems that the core part is just the NPN transistor.... but since this is the video board schematic of Mac Classic not SE, the number of diodes cannot correspond to the actual SE board (4th pic) which has 4 diodes, nor did I see the inductor on the actual SE video board (which is drawn on the schematic), but the core part 2N3904 was there.

    I did tried to find the photo of Mac Classic's neckboard and Mac SE's neck board, upon comparison of the two boards, they really look similar, even the positions of the components, what seems have changed are just the names of the components, eg. C1 -> CC1

    Apart from the previous schematic of the Classic found in the repair manual, I also found a copy of the classic's neck board from as follows:

    https://imgur.com/a/rcrAAXP

    (I'm sorry I have to use imgur, because this forum doesn't allow upoading attchment more than 256KB which will require me to compress the image to a blurring unreadable quality)

    Assuming that the statement of this manual is true: the SE and Classic video board are similar, Could you please help me identify in this circuit which part is most probably faulty that I should try to replace? Thanks
    Last edited by ivannudem; June 4, 2021, 05:58 PM.

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      #32
      Hi, everybody,
      Just an update on this issue. I spoke with a family member (who used to work in this field) to ask him to help pinpoint where the problem was, and sent him some photos of this smearing problems, even before I explained further, he immediately told me this was a bandwidth, frequency response problem in the amplifier circuit (I forgot the exact phrase, but like Hugo said.) So the cause I think is quite clear, something wrong with the amplifier circuit. Since I have the "good" FDHD, and that there's a NPN amplifier transistor on the neckboard/video board (the one connected to the neck), I did a video board swap, I installed the known good FDHD's video board onto the neck of this bad SE's CRT...Unfortunately It didn't solve the problem, there were still smearings to the right after a long black line..
      https://imgur.com/a/JvLwAMJ

      So I assume there must be something wrong on the analog board, But I didn't find any suspicious amplification circuit there?
      https://imgur.com/a/eBipQLz

      I do agree that a CRT defect is rare, and I hope it was not that which caused the problem...
      This family member lives in another city... and I don't think he still keeps a scope at his home... LoL
      Sigh...Facepalm...
      Last edited by ivannudem; June 6, 2021, 09:22 PM.

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        #33
        Originally posted by ivannudem View Post
        So I assume there must be something wrong on the analog board, But I didn't find any suspicious amplification circuit there?
        It's pretty obvious where the amplification circuitry is for both the horizontal and vertical drive by tracing back from the deflection drive outputs.

        The TDA1170N and the associated circuitry between it and the flyback is for the vertical drive, while the GBU406 and area around it are responsible for the horizontal drive.

        If you don't have access to an oscilloscope, do the old fashion method of testing components out of circuit. It's not difficult to test resistors, diodes and transistors with a multimeter. Capacitors need a capacitance meter or LCR meter.

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