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Thread: Pet 2001 9" Monitor, spot killer.

  1. #11

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    Quote Originally Posted by AndyG View Post
    Interestingly, I tried changing C22 on the video board from 3.3uF to 47uF on my 9inch monitor and that killed the spot... (it appears the early boards had 3.3uF and the later revision 47uF) though reading various posts it seems that doesn’t always work as you say.
    Yep, that's a modification introduced by Commodore between second and third revision of the 9" circuit. Raising C22 to 47uF kills the spot on most 9" monitors, but not all. I also recommend changing R5 from 1k5 2W to 2k2 2W, that's another modification made between rev.2 and rev.3.

    HTH
    Frank IZ8DWF

  2. #12

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    Quote Originally Posted by dabone View Post
    Mine apparently is a different model that those.

    C22 is a non-electrolytic, and is labeled 001 on the layout sheet.


    The only place I found schematics/board layout is here.

    http://vintagecomputer.ca/files/Comm...une%201978.pdf

    The component cross reference list doesn't match the video board at all.
    (Just compare it to the layout on page 11)


    The board is labeled 320084, the underside of the board is blue, plain fiberglass on the component side.

    The following caps currently installed.

    C1,47uf,16V
    C4,3.3uf,200V
    C12,220uf,35V
    C14,1uf,25V
    C15,3.3uf,200V
    C21,470uf,16V
    C16,10uf,25V
    C23,47uf,16V
    C26,47uf,16V
    C25,4700uf,25V
    C27,47uf,16V
    On your board revision (never seen one in real life, thanks for the schematic), it's C15 that must be changed to 47uF 50V (or higher voltage rating). Also I'd suggest changing R7 from 1k5 2W to 2k2 2W.
    Frank

  3. #13

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    In general there are four ways which solve the problem. Sometimes a combination of them is required for any particular monitor.

    In essence it requires that at the moment immediately after turn off that the grid of the CRT swings highly negative with respect to the cathode (this is a relative potential). It can be required that this swing is greater than 20 to 30v in some CRT's to fully suppress the beam current.

    Therefore there are three ways this can be done.

    The first is by charge storage, if there is a negative supply voltage fed to the g1 grid of the CRT in the particular monitor design, and that supply is fed from a rectifier. Therefore increasing the filter capacitor's uF on this negative supply line helps. But, unfortunately, if the brightness control is on this line (varying the CRT's negative grid voltage as a technique for brightness control, as it is in these monitors) the effect is proportionally reduced by the the position of the brightness control. This means that the spot suppression is only 100% effective if the user remembers to turn the CRT brightness pot to zero before they switch off the set. This is why users of monitors with the increase capacitance modification experience variable results.

    The second way is to increase the charge storage on the video output amplifier's power supply. In this case the cathode remains positive with respect to the grid for longer after turn off.

    The third method is much more effective, especially if added to the charge storage method. This involves coupling a negative going voltage transient to the CRT's grid at turn off. This is done with a circuit which charges a capacitor, via a resistive divider to ground, and with a diode, where one capacitor terminal is connected to a +ve voltage supply (such as the the power rail in the monitor). This way at turn off, when the power supply voltage collapses rapidly, a negative voltage is generated across the resistor/s, this is coupled into the grid circuit of the CRT.

    Have a look at the last page if this article to inspect the method, which works equally well if the grid return resistor is to the slider of a brightness control potentiometer as in these cases, if there is a significant DC offset, the negative going pulse at turn off is coupled in with an additional coupling capacitor:

    http://worldphaco.com/uploads/RESTOR...SONY_MICRO.pdf

    The basic way to AC couple a negative going transient to the CRT grid is shown in the attached diagram, and a recording of the type of voltage transient injected to the CRT grid at turn off.

    The 4th method seeks not to suppress the beam at all. Instead at turn off, to keep the deflection stages running for longer so the scan doesn't immediately collapse at turn off, this way the beam energy is spread over a larger area of CRT phosphor for longer before the EHT collapses and the heater emission fades . This works for VDU's which have their own independent H scan oscillators and V oscillators, not for the simplified computer monitors that use H drive signals and V drive signals, at turn of the raster scan instantly collapses.
    Attached Images Attached Images

  4. #14
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    Some recent videos on the subject.

    Adrian's Digital Basement: https://youtu.be/jwgsaUtbH3I?t=781
    IZ8DWF: https://youtu.be/WULJxrQkQeg
    Mine: https://youtu.be/iMSQTCdC-jg

    For me, with an original revision monitor PCB, just replacing C22 with a 47uf/50v solved the spot. That keeps the brightness control voltage on G1 low for a longer time until the cathode cools off.
    I made a mistake the first time I tried replacing C22 and installed it backwards. (oops)

    For Adrian, it was necessary to replace a few other components that were allowing the cathode to leak current to ground.

  5. #15

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    Quote Originally Posted by Hutch View Post
    Some recent videos on the subject.

    Adrian's Digital Basement: https://youtu.be/jwgsaUtbH3I?t=781
    IZ8DWF: https://youtu.be/WULJxrQkQeg
    Mine: https://youtu.be/iMSQTCdC-jg

    For me, with an original revision monitor PCB, just replacing C22 with a 47uf/50v solved the spot. That keeps the brightness control voltage on G1 low for a longer time until the cathode cools off.
    I made a mistake the first time I tried replacing C22 and installed it backwards. (oops)

    For Adrian, it was necessary to replace a few other components that were allowing the cathode to leak current to ground.
    I think you'd better replace R5 to a 2k2 2W resistor if it's still the old 1k5 ohms. The 33V zener is overdriven with 1k5 and it may fail over time (though if it's still ok, I believe it's a particularly tough one).
    Thanks for taking the time to help!
    Frank

  6. #16

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    Frank, I just watched the YouTube video and I also get the delayed spot after switching the PET off when it has been left on for a while. Will change the resistor R5 to see if that sorts it.

    Andy



    Quote Originally Posted by iz8dwf View Post
    Yep, that's a modification introduced by Commodore between second and third revision of the 9" circuit. Raising C22 to 47uF kills the spot on most 9" monitors, but not all. I also recommend changing R5 from 1k5 2W to 2k2 2W, that's another modification made between rev.2 and rev.3.

    HTH
    Frank IZ8DWF
    Last edited by AndyG; November 29th, 2020 at 10:12 AM.

  7. #17
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    Quote Originally Posted by AndyG View Post
    ... Will change the resistor R5 to see if that sorts it. ...
    The reason for changing R5 is to protect the 33v Zener diode at CR7. R5 shouldn't have any effect on the spot, but CR7 can cause the spot if it's been damaged.
    If R5 is 1.5K ohm on your PET, chances are you'll need to replace CR7 also.

    There are some part numbers listed in the description of Adrian Black's Part 2 video.

  8. #18

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    Yep - changed CR7 also and the delayed spot doesn't appear anymore ......

    Thanks

    Quote Originally Posted by Hutch View Post
    The reason for changing R5 is to protect the 33v Zener diode at CR7. R5 shouldn't have any effect on the spot, but CR7 can cause the spot if it's been damaged.
    If R5 is 1.5K ohm on your PET, chances are you'll need to replace CR7 also.

    There are some part numbers listed in the description of Adrian Black's Part 2 video.

  9. #19
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    I've got most of the parts from the much earlier post, but I didn't order a new zener. Hopefully I'll get to installing things this week to see the results. If it still has a spot, I'll order a new zener for cr7.

  10. #20
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    Sorry to crash the thread, but we have just created a VT420 monitor brightness issue thread over on the DEC forum that could benefit from your combined monitor expertise if you wouldn’t mind. This appears to be a fairly common fault for this type of monitor so would be applicable to a number of members (including me!). I have located what is claimed to be the schematics and posted a link.

    Thanks in advance for any help anyone can provide.

    Sorry, back to the world of Commodore spots..

    Dave

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