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Discharging a CRT

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  • Kelly Leavitt
    Now that VCF is over, I need to fix a couple of broken bits. Both happen to be video driver boards for the old tandy equipment. What is the proper method for
    Message 1 of 7 , May 15, 2006
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      Now that VCF is over, I need to fix a couple of broken bits. Both happen to be video driver boards for the old tandy equipment. What is the proper method for discharging a CRT? I've been zapped twice by the one unit after I thought I'd discharged it. Not fun at all. My wife could hear me swearing all the way upstairs.

      Where the Flyback attaches to the glass there is a "suction cup" looking insulator over the anode. I'm guessing that I need to short this (the wire under it) to ground, but what ground? Should I be connecting to the local house ground (ie the ground on a neaby outlet), or the circuit gruond on the system I'm working on, or the shield ground (like the case)? Am I risking other houshold appliances by discharging to the ground of the house, or even to the frame, which eventually connects to the house ground?

      Should there be a resistor in the discharge line, or just a piece of jumper wire. I've seen setups using alligator clips and a long scredriver. If there should be a resistor, what size?

      How long should I hold the discharge "tool" to the anode? Is there any way to verify the unit is discharged before I start working on it again?

      Most will probably say stay away, but I need to get these units repaired, and I don't know anyone local that would repair them other than me. I'm pretty good otherwise about troubleshooting circuits, just not the video driver side.

      Also, anyone have a spare set of 20MHz oscilloscope probes they'd part with?

      Thanks,
      Kelly
    • macmothership
      Kelly, Here ya go: http://www.macmothership.com/maccontent/info/Safety.pdf -Jim ... driver boards for the old tandy equipment. What is the proper method for
      Message 2 of 7 , May 15, 2006
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        Kelly,

        Here ya go:

        http://www.macmothership.com/maccontent/info/Safety.pdf

        -Jim


        --- In midatlanticretro@yahoogroups.com, "Kelly Leavitt" <kelly@...> wrote:
        >
        > Now that VCF is over, I need to fix a couple of broken bits. Both happen to be video
        driver boards for the old tandy equipment. What is the proper method for discharging a
        CRT? I've been zapped twice by the one unit after I thought I'd discharged it. Not fun at all.
        My wife could hear me swearing all the way upstairs.
        >
        > Where the Flyback attaches to the glass there is a "suction cup" looking insulator over
        the anode. I'm guessing that I need to short this (the wire under it) to ground, but what
        ground? Should I be connecting to the local house ground (ie the ground on a neaby
        outlet), or the circuit gruond on the system I'm working on, or the shield ground (like the
        case)? Am I risking other houshold appliances by discharging to the ground of the house,
        or even to the frame, which eventually connects to the house ground?
        >
        > Should there be a resistor in the discharge line, or just a piece of jumper wire. I've seen
        setups using alligator clips and a long scredriver. If there should be a resistor, what size?
        >
        > How long should I hold the discharge "tool" to the anode? Is there any way to verify the
        unit is discharged before I start working on it again?
        >
        > Most will probably say stay away, but I need to get these units repaired, and I don't
        know anyone local that would repair them other than me. I'm pretty good otherwise about
        troubleshooting circuits, just not the video driver side.
        >
        > Also, anyone have a spare set of 20MHz oscilloscope probes they'd part with?
        >
        > Thanks,
        > Kelly
        >
      • Kelly Leavitt
        Jim: Thanks. Does anyone know the pros and cons of having a discharge resistor in line? I see procedures that call for one, and others that say not to. From
        Message 3 of 7 , May 15, 2006
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          Jim:
          Thanks. Does anyone know the pros and cons of having a discharge resistor in line? I see procedures that call for one, and others that say not to. From the Mac one that Jim sent, they suggest using one with a resistor, then one without.

          Any chance of damaging a tube without the resistor in line. The circuits I see are either a clip lead from chassis ground to a scredriver, or a clip lead from chasis ground to a 10M (or 100M) ohm resistor to a screwdriver.

          If a resistor is used, should it be a 10M or 100M?

          Kelly

          -----Original Message-----
          From: midatlanticretro@yahoogroups.com on behalf of macmothership
          Sent: Mon 5/15/2006 11:41 AM
          To: midatlanticretro@yahoogroups.com
          Cc:
          Subject: [midatlanticretro] Re: Discharging a CRT



          Kelly,

          Here ya go:

          http://www.macmothership.com/maccontent/info/Safety.pdf

          -Jim
        • Mike Loewen
          Here are my pictures - apologies for the quality: http://ripsaw.cac.psu.edu/~mloewen/Oldtech/VCF-East2006/ Mike Loewen mloewen@cpumagic.scol.pa.us Old
          Message 4 of 7 , May 15, 2006
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            Here are my pictures - apologies for the quality:

            http://ripsaw.cac.psu.edu/~mloewen/Oldtech/VCF-East2006/


            Mike Loewen mloewen@...
            Old Technology http://ripsaw.cac.psu.edu/~mloewen/Oldtech/
          • Bob Grieb
            Kelly, In the latest Marlin P Jones catalog, you can find 20 MHz scope probes for $8 each. The have higher bandwidth ones as well. Not sure what the quality
            Message 5 of 7 , May 15, 2006
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              Kelly,

              In the latest Marlin P Jones catalog, you can find 20 MHz scope
              probes for $8 each. The have higher bandwidth ones as well.
              Not sure what the quality is like...

              Bob Grieb

              http://www.mpja.com/productview.asp?product=15085+TE

              --- Kelly Leavitt <kelly@...> wrote:

              > Now that VCF is over, I need to fix a couple of broken bits. Both
              > happen to be video driver boards for the old tandy equipment. What is
              > the proper method for discharging a CRT? I've been zapped twice by
              > the one unit after I thought I'd discharged it. Not fun at all. My
              > wife could hear me swearing all the way upstairs.
              >
              > Where the Flyback attaches to the glass there is a "suction cup"
              > looking insulator over the anode. I'm guessing that I need to short
              > this (the wire under it) to ground, but what ground? Should I be
              > connecting to the local house ground (ie the ground on a neaby
              > outlet), or the circuit gruond on the system I'm working on, or the
              > shield ground (like the case)? Am I risking other houshold appliances
              > by discharging to the ground of the house, or even to the frame,
              > which eventually connects to the house ground?
              >
              > Should there be a resistor in the discharge line, or just a piece of
              > jumper wire. I've seen setups using alligator clips and a long
              > scredriver. If there should be a resistor, what size?
              >
              > How long should I hold the discharge "tool" to the anode? Is there
              > any way to verify the unit is discharged before I start working on it
              > again?
              >
              > Most will probably say stay away, but I need to get these units
              > repaired, and I don't know anyone local that would repair them other
              > than me. I'm pretty good otherwise about troubleshooting circuits,
              > just not the video driver side.
              >
              > Also, anyone have a spare set of 20MHz oscilloscope probes they'd
              > part with?
              >
              > Thanks,
              > Kelly
              >


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            • Kelly Leavitt
              Bob: I ordered a couple of sets. I ll let you know how well they work. Since I only use them 5 or 6 times a year, it shouldn t be too bad. I have a dual trace
              Message 6 of 7 , May 16, 2006
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                Bob:
                I ordered a couple of sets. I'll let you know how well they work. Since I only use them 5 or 6 times a year, it shouldn't be too bad.

                I have a dual trace scope and no probes. The fastest thing I work on is around 8MHz so the 20MHz scope should work well.

                Kelly

                -----Original Message-----
                From: midatlanticretro@yahoogroups.com on behalf of Bob Grieb
                Sent: Mon 5/15/2006 5:55 PM
                To: midatlanticretro@yahoogroups.com
                Cc:
                Subject: [midatlanticretro] scope probes



                Kelly,

                In the latest Marlin P Jones catalog, you can find 20 MHz scope
                probes for $8 each. The have higher bandwidth ones as well.
                Not sure what the quality is like...

                Bob Grieb

                http://www.mpja.com/productview.asp?product=15085+TE

                --- Kelly Leavitt <kelly@...> wrote:

                > Now that VCF is over, I need to fix a couple of broken bits. Both
                > happen to be video driver boards for the old tandy equipment. What is
                > the proper method for discharging a CRT? I've been zapped twice by
                > the one unit after I thought I'd discharged it. Not fun at all. My
                > wife could hear me swearing all the way upstairs.
                >
                > Where the Flyback attaches to the glass there is a "suction cup"
                > looking insulator over the anode. I'm guessing that I need to short
                > this (the wire under it) to ground, but what ground? Should I be
                > connecting to the local house ground (ie the ground on a neaby
                > outlet), or the circuit gruond on the system I'm working on, or the
                > shield ground (like the case)? Am I risking other houshold appliances
                > by discharging to the ground of the house, or even to the frame,
                > which eventually connects to the house ground?
                >
                > Should there be a resistor in the discharge line, or just a piece of
                > jumper wire. I've seen setups using alligator clips and a long
                > scredriver. If there should be a resistor, what size?
                >
                > How long should I hold the discharge "tool" to the anode? Is there
                > any way to verify the unit is discharged before I start working on it
                > again?
                >
                > Most will probably say stay away, but I need to get these units
                > repaired, and I don't know anyone local that would repair them other
                > than me. I'm pretty good otherwise about troubleshooting circuits,
                > just not the video driver side.
                >
                > Also, anyone have a spare set of 20MHz oscilloscope probes they'd
                > part with?
                >
                > Thanks,
                > Kelly
                >


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              • Mike Loewen
                http://sturgeon.css.psu.edu/~mloewen/Oldtech/VCF-East2011/ Also, pics from the museum area:
                Message 7 of 7 , May 16, 2011
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