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Thread: Apple IIc Monitor Tube displaying strange behavior

  1. #31

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    Quote Originally Posted by KC9UDX View Post
    I can't even imagine how a "modern" CRT could fail in this way. It must be the yoke.

    Regarding the OP, and Post 12 did you indeed swap yokes between CRTs? If so, did the problem follow the yoke? If so, the yoke is certainly bad. If not, then electronics in the monitor.
    anode grid collapsed can make bad distortion on all or parts of a CRT. Sometimes also the focusing grid can. I've seen a few cases in the last 30 years.
    I agree however that it's more likely the yoke, and in that case, it's a lucky situation for the vertical driver(s) to be still functioning.

    Frank

  2. #32
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    I'm surprised that people are blaming the CRT and the yoke when in post #2 the OP mentions a missing diode and a broken diode. The diodes are used to suppress ringing in the deflection coils.

    Why blame the unlikely instead of the obvious?

  3. #33
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    Quote Originally Posted by David_M View Post
    I'm surprised that people are blaming the CRT and the yoke when in post #2 the OP mentions a missing diode and a broken diode. The diodes are used to suppress ringing in the deflection coils.

    Why blame the unlikely instead of the obvious?
    You must have missed the post where I mentioned I have several monitor boards. The board missing the diodes gives me some roster but that is all. I only posted that info in the hope someone could get me a schematic or parts list. The board in question with the collapsed bad image IS NOT missing diodes.

  4. #34
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    Quote Originally Posted by iz8dwf View Post
    It is the contrary: if the image turns with the yoke, it's a damaged yoke, if the image doesn't turn with the yoke, it's a bad CRT.

    Frank
    Got it and understood. Leaving to Virginia for a week, will try turning the yoke when I get back.
    And to the post by KC9UDX, didnt swap yokes yet will when I return.

  5. #35

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    Thank you for the clarifications!

  6. #36

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    One of the interesting things about faults on CRT based monitors (and TV's) is that if everything is working well enough, to at least see some kind of image on the CRT face, then you can deduce a very large amount of data about the fault condition.

    If the beam from the gun is in focus, at some point at least, you will see a trace that is reasonably well focused and that tells you the electron gun and focus electrode voltages are at least roughly correct and that the CRT's grid and cathode voltages are about right.

    Then with the beam deflection by the yoke's coils there is a lot of information to be gained from looking at the geometry of the raster scan. For example looking at the photos the OP posted, the CRT's beam mirrors the exact magnetic field generated by the yoke coils. The Horizontal deflection (width) is about the right width (but notice it is not a stable width....see below for the reason). The vertical amplitude of the scan is reduced and distorted, giving great insight into the fault.

    We know that the drive circuitry for the yoke is ok on the other CRT and yoke assembly. Therefore the faulty vertical yoke assembly is loading down the vertical amplifier's output stage (likely shorted turns in one of the vertical coils). Due to this high loading, it causes ripple on the power supply voltage and that is why the width of the horizontal scan is altering during the abnormal vertical scan period, from top to bottom, of the scanning raster.

    So, even without the good yoke/crt assembly to compare this with, most likely you will find if you measure the DC resistance of the vertical coils separately (they would normally have about the same DC resistance) you will find that one of those vertical coils will have a significantly lower DC resistance than the other due to shorted turns. Looking at the schematic, for the IIc monitor, as is commonly done, the two vertical coils are wired in series, and the two horizontal coils are in parallel.

    So a simple check, with the ohm meter, should confirm, that one of the vertical coil assemblies on the faulty yoke has a substantially lower resistance than the other.

  7. #37
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    You're not the only one who had trouble with deteriorating diodes on their system : https://www.classic-computers.org.nz...%20monitor.htm

  8. #38
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    Quote Originally Posted by T-Squared View Post
    You're not the only one who had trouble with deteriorating diodes on their system : https://www.classic-computers.org.nz...%20monitor.htm
    Thanks T, those are the exact same diodes I am having the problems with. Unfortunately I still have no way of knowing thier( there are a few different varieties not just one) values.

    also I tried clicking on Terry's link to the forum to see what value they gave him but that link seems to be not working.
    Last edited by VERAULT; July 26th, 2019 at 07:20 AM.

  9. #39

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    Quote Originally Posted by VERAULT View Post
    Thanks T, those are the exact same diodes I am having the problems with. Unfortunately I still have no way of knowing thier( there are a few different varieties not just one) values.

    also I tried clicking on Terry's link to the forum to see what value they gave him but that link seems to be not working.
    You can replace these glass passivated diodes with the 4A rated BY448. If you strike one with particularly thick lead wires, use a BY228 that has a 5A rating. Both of these are over 1600V piv rated and have fast switching times, they are probably far superior to the diode that was in there in the first place and they are easy to get.
    Last edited by Hugo Holden; July 26th, 2019 at 11:14 PM.

  10. #40

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    Quote Originally Posted by T-Squared View Post
    You're not the only one who had trouble with deteriorating diodes on their system : https://www.classic-computers.org.nz...%20monitor.htm
    The diode pin in your case corroded because of the leaked electrolyte from the adjacent capacitor as far as I can judge from your pictures but you failed to properly analyze this despite having direct view of the PCB. The capacitor itself was not replaced...

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