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Thread: How I fixed vertical hold / size issues on a monitor (IBM 5151)

  1. #1

    Default How I fixed vertical hold / size issues on a monitor (IBM 5151)

    Hi folks,

    I had a problem where my IBM 5151 mono monitor was jumping around all over the place. One minute, the video was squeezed down in the middle, the next it would jump up to 'full size'.

    Here's a video of how I fixed it, as well as how to open it up, and safely discharge the anode/flyback.

    Hope someone finds it useful.


  2. #2

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    Hi,

    There is absolutely no need to discharge the final anode of the CRT as you did. (unless you want to remove the anode connector, which was not indicated to do for the deflection problem you had).

    The electrostatic charge on the CRT bulb, between the external and internal aquadag and the glass bulb as the dielectric, is isolated from the CRT's base pins, the final anode assembly in the gun its isolated from the other electrodes, you can remove the CRT's socket with no risk at all of shock from the charge on the final anode connection and bulb.

    In fact, the the only good reason to discharge the capacitance of the CRT bulb is if you need to remove the EHT cap and replace either the LOPT or the CRT. A charged CRT bulb will cause a person to drop a CRT if they are carrying it across a room, due to muscle twitch. But the energy stored in the capacitance of the bulb is less than 1/50th to 1/100th of a zap from a farmer's electric fence, it is not hazardous to your health ( can calculate the energy from CV^2/2).

    In addition , if you are going to discharge a CRT bulb, it should not be done with a direct short, as the peak currents can be very high and I have seen CRT's have the internal connection of the metal anode clip to the internal aquadag go open circuit with a direct short. It is much better to use a 1M to 10M resistor and wait a few seconds.

    I'm not sure who started the urban myth that when CRT monitors are worked on, the first thing to do is to discharge the CRT with a direct short, like I say bad idea, unless you want to take the anode cap off for some reason related to CRT replacement or LOPT replacement, which for many TV VDU faults is not required for the repair. Also, in addition to this, semiconductor EHT rectifiers do not have zero reverse leakage, if you wait a few days, the CRT bulb slowly discharges this way, You can confirm this by getting CRT VDU's which have not been used for a while, and going under the anode cap.

    The professional way to discharge the CRT bulb is with a typical EHT probe, with the set turned off, slipping the probe tip under the anode cap, these have about a 100M to 1GM ohm resistor in them, that tolerates the initial high voltage well and discharges the bulb over some seconds very gently.
    Last edited by Hugo Holden; July 20th, 2020 at 10:19 PM.

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