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Thread: PET 2001-N screen distorion

  1. #1

    Default PET 2001-N screen distorion

    I picked up a 32K 2001-N recently, and while everything is in working order there's some noticeable horizontal distortion on the screen. The display is correctly centred and vertical edges are all straight, but characters get gradually narrower towards the right. Characters in column 1 are probably 50% wider than those in column 40.

    I know almost nothing about PETs so I'm not really sure when to start looking...

  2. #2
    Join Date
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    This seems like a problem in the video board rather than the PET main board. This may be the video board schematic in your PET : http://www.zimmers.net/anonftp/pub/c...01N/321445.gif

    There are several guys on this forum that are familiar with this type of circuitry that will hopefully soon help.

  3. #3

    Default

    Quote Originally Posted by FishFinger View Post
    I picked up a 32K 2001-N recently, and while everything is in working order there's some noticeable horizontal distortion on the screen. The display is correctly centred and vertical edges are all straight, but characters get gradually narrower towards the right. Characters in column 1 are probably 50% wider than those in column 40.

    I know almost nothing about PETs so I'm not really sure when to start looking...
    Can you share a photo ?

    I know of this http://www.cosam.org/computers/cbm/pet/20090305.html report but Are your symptoms anything like this ?

  4. #4

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    Here's a photo, although the effect doesn't seem so noticable so I drew a slightly exaggerated picture to show the effect.

    Aside from that the display is very sharp and stable, and the distortion doesn't change over time.
    Attached Images Attached Images

  5. #5

    Default

    Looks like the horizontal linearity is off.
    Some monitors have an adjustment for vertical or horizontal linearity but I don't believe any version of the PET monitor has either adjustment.
    There are a couple of metal rings on the yoke that adjust the picture centering and might have an effect on linearity.
    I have a feeling it's likely to be capacitor aging though.

  6. #6

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    The linearity is compressed to the right, this indicates a high internal resistance in the H scanning coil circuit. The implication being that either the yoke coupling capacitor has gone higher ESR than it should be or possibly the power supply bypass/filter capacitor on the supply rail that supplies the H output stage. The latter is more likely as the yoke coupling caps are usually special quality, but the power supply caps are not.

    Alternatively there may be a saturable magnetic linearity col in series with the yoke's H cols that requires adjustments, though many of these are fixed and it unlikely has been tampered with but not impossible.

    The centering magnets on the rear of the yoke have no effect on the scanning linearity, just the overall scanning raster centering.

  7. #7
    Join Date
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    Assuming dave_m has identified the correct monitor schematic back in post #2, checkout the following components:

    C716 - power supply. I would also check C713 whilst you are at it.

    The LIN inductor is fixed - L703.

    The associated coupling capacitor is C715.

    Dave

  8. #8

    Default

    I seem to have an older display board like the original 2001 rather than the usual 2001N board, but I found a schematic and layout:

    http://www.zimmers.net/anonftp/pub/c...01/video-3.gif
    http://www.zimmers.net/anonftp/pub/c...deo-layout.gif

    C716 is C23 on my board, and C713 looks to be C24+C25. I'll replace those and see if it helps.

  9. #9
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    C715 becomes C28 then... Note that this is a non polarised (NP) capacitor.

    Also, your video board doesn't appear to have a LIN inductor in the same way that the other board did.

    If you have an oscilloscope, you will notice the traces are marked at the measurement points.

    Dave

  10. #10

    Default

    Hi again,

    If you did replace the 10uF non polar capacitor , use a polyester or polypropylene film capacitor, or motor run capacitor, not an electrolytic even of an NP type electrolytic, most are not suited and have too high an ESR.

    But looking at your photo of the actual CRT face and what you are seeing with the H linearity, is the inevitable result of not having a magnetic saturable linearity coil in series with the yoke's H scanning coil.

    The 10uF (S correction cap) in practice corrects most of the linearity error between center and the right hand side, however it cannot fix the stretching of the scan on the left, immediately after flyback. This relates to what happens after reversal of the yoke's magnetic field.

    For an explanation of how this works, one way is to look at early sets that had neither the S correction cap and no lin coil either. Read this article from page 8 onwards, it explains how these linearity defects come about:

    http://worldphaco.com/uploads/Sanyo_..._from_1962.pdf

    I think, most likely, there is no faulty component in your VDU, and you are seeing the expected result. However degraded capacitors can aggravate the problem, but most likely when you try others I expect the result will be similar, because what you are seeing is very close to what is expected, without the lin coil.

    So I'm saying that your VDU is likely "Normal" and you are a very good observer of linearity defects.
    Last edited by Hugo Holden; September 20th, 2020 at 01:54 PM.

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