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[HELP] Commodore PET CBM 3008 3032 Screen Issue

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    #76
    Yes, the signal is attenuated by a factor of 10 - so that a more accurate reading can be taken (because the impedance is increased by a factor of 10).

    However, the maximum withstand voltage of the probe is a function of the user's safety and the material that the probe is made from - not from multiplying the maximum voltage that can be accommodated by the oscilloscope input and the factor of 10. It may work out that way - depending upon the probe of course.

    I just checked an RS-Components part ( RS-TP 6351R - RS Part-No.: 1799558 ) and the maximum specified Voltage for the probe is 400 V (RMS).

    By feeding it 4 kV - you may damage the probe and then damage the oscilloscope and give yourself a shock in the process. Read the manual...

    L1 and L2 are the horizontal and vertical scan coils - and are built into the yoke itself. You can't get one without the other. I was proposing (before we got the completely dead monitor working) to just swap them over from one monitor to the other monitor. You may not want to do that now we have the dead monitor working again...

    Dave

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      #77
      Most common x 10 probes can handle around 400V rms or roughly 1.4 x 400V peak.

      In most transistorised VDU's of the monochrome variety, the voltage on the collector of the HOT would unlikely exceed this value, so its ok to scope there with a x 10 probe. But keep away from the neck board of the CRT with the probe as the voltages there can be a lot higher. Generally if you want to be kind to a scope, working with CRT VDU's it pays to buy a x100 probe as generally these have a max voltage rating of around 2 to 2.5kV. So with these you can scope the collector of the HOT in a color set, that often peaks to 1.2kV.

      There is no need to go under the CRT's anode cap for general CRT servicing. If you don't go under there, there is no need to discharge the CRT's stored charge as it remains safely inside the CRT. The usual reason, if one did go under there would be to measure the EHT with a special probe, or to remove the LOPT or CRT, which mostly is not usually required. In the case where you go under there to measure the EHT, the set is running in this condition and you would not want to attempt to discharge it.

      Comment


        #78
        Well guys. :'( Time for my xmas story.

        So I was getting settled with the wobble and convinced myself this is good enough and went ahead to reassemble. Adjusted yoke. Fixed the final inoperational keys. So I am done and think to myself, why not play some more with the oszilloscope and maybe learn more about the power supply stability. So I measure pins 6 & 7 from the big transformer of the PET.

        Get some readings and then suddenly something happend: Christmasly sparkle with matching sounds. I don't know if I unintentionally connected pin 6 & 7 by touching the ground ring of the probe with some open cable or if it just happend anyway. I pull out the probe and notice a strong smell like soldering. No visible smoke though. Of course I pull the plug. After resting things for a while I try again and the machine is dead but makes electrical sparking noices and smells like solder. I can't locate it well but I strongly suspect the power transformer. My power meter at the socket indicates the PET box is now pulling 400W (!). For whatever reason the fuse (0.8A) seems to have no problem with that... So there is a major short there now. Darn.

        Currently, I think I am not going to touch that thing again. Too frustrating. Thank you all it was great learning journey and I guess there isn't always a happy end to a vintage computer. Maybe spirits pick up after the holidays.

        Merry xmas to you all and have a great start into 2022!

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          #79
          That’s a shame.

          However, if you go back to my post #71 it was always a possibility...

          Some machines are easy to fix, others not so, and others are just pigs! A few members I know have had pigs to repair and, to their credit, they have persevered and chased all the badness out of their machine. After that, it has been fine!

          So don’t give up - have a few slices of turkey, some wine of your choice and regroup back in 2022.

          I learnt the hard way when I was twenty something about what the small bit of plastic was that came with the probe - it protects the circuit being probed from the ground ring surrounding the probe (as I shorted the collector (metal can) of a PNP transistor to ground via the probe and burnt it out). This was on a home-built theatre lighting controller. Needless to say we had no more light that evening until a replacement arrived!

          It is unlikely that you have damaged the transformer. I would disconnect the power cable from the PET main logic board and disconnect the power cable from the monitor and measure the transformer secondary voltages with your multimeter on AC volts.

          Examine the top and bottom of both the PET main logic board and monitor board for signs of burning. There must be some evidence of it somewhere.

          These are relatively simple tests to perform...

          Have a good Christmas.

          Dave

          Comment


            #80
            Originally posted by daver2 View Post
            That’s a shame.

            However, if you go back to my post #71 it was always a possibility...

            Some machines are easy to fix, others not so, and others are just pigs! A few members I know have had pigs to repair and, to their credit, they have persevered and chased all the badness out of their machine. After that, it has been fine!

            So don’t give up - have a few slices of turkey, some wine of your choice and regroup back in 2022.

            I learnt the hard way when I was twenty something about what the small bit of plastic was that came with the probe - it protects the circuit being probed from the ground ring surrounding the probe (as I shorted the collector (metal can) of a PNP transistor to ground via the probe and burnt it out). This was on a home-built theatre lighting controller. Needless to say we had no more light that evening until a replacement arrived!

            It is unlikely that you have damaged the transformer. I would disconnect the power cable from the PET main logic board and disconnect the power cable from the monitor and measure the transformer secondary voltages with your multimeter on AC volts.

            Examine the top and bottom of both the PET main logic board and monitor board for signs of burning. There must be some evidence of it somewhere.

            These are relatively simple tests to perform...

            Have a good Christmas.

            Dave
            That is good advice.

            Most likely if there was a brief short , it would not harm the power transformer. After all, that is what transformers are totally brilliant at; surviving brief periods of overload. If this were not the case, then our national grid power distribution system would have fallen apart long ago.

            Most likely if you shorted a terminal out and there was a component failure it points to a semiconductor based part, like a diode, series pass transistor or an IC or transistor in the power supply. So, since you are already halfway there fixing it; Look at the area you probed when the event happened .Try to consider, looking at the schematic and where you were probing, what part got stressed the most?

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