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Thread: Color Classic no video

  1. #1

    Default Color Classic no video

    Hi all,


    Total Apple noob here with a new to me Mac Color Classic that's been unused for an unknown number of years. Physically, everything looks ok, the battery although dead is intact, and there is maybe a bit of capacitor leakage, which I've cleaned up with solvent and a toothbrush. The caps on the analog board look good as well, and all plugs appear to be seated.


    It powers on, and chimes, but there is no video, or glow from the crt neck. Is the high voltage controlled from the logic board, or should there always be a glow when powered on? What I'm really asking is, will recapping the logic board possibly fix the no video (and apparently no high voltage) issue, or is it more likely an analog board / crt issue?


    Thanks,
    Chris

  2. #2
    Join Date
    Apr 2015
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    If the CRT is powered, it should always have the heater glowing in the neck. If there is no glow, then the CRT is not getting power on the neck. There's a brightness control knob on the bottom front of the unit, make sure that is set correctly.

    The first thing to check is the connections. Check the solder joints on the neck board that plug into the pins on the CRT and look for anything like rings or cracks in the solder, which is a sign of a bad joint. If you don't see any bad joints, VERY CAREFULLY pull the neck board off the tube, be as straight as possible. Inspect the pins and the inside of the socket for damage and reinstall the neck board again VERY CAREFULLY, being sure to seat it all the way on. The vacuum port is usually between the pins on the neck board and if you crack it, the CRT is hosed.

    If that doesn't fix the issue, then something on the analog board is bad. Inspect for bad solder joints, and a recap job wouldn't hurt it. You'll want to verify all of the passive components are good, like diodes and resistors before you get into checking for bad transistors and such.

    And also the boiler plate warning, CRT flybacks have lethal voltage. Don't work with it on, make sure you discharge it when you turn it off by using a flat blade screwdriver under the anode cap with a wire attaching to the CRT frame mounting it to the front. I would suggest using a large high voltage resistor of something like 1 meg ohm in series to avoid arcing. Don't rely on the wire insulation to save you, this stuff 30 years old give or take and has had lots of time to dry rot. I've seen more than a few CRT anode wires with corona glow.

  3. #3

    Default

    I wonder if a previous owner did the common resolution mod, which is said to be abusive to the monitor circuitry.

  4. #4

    Default

    There is absolutely no need to discharge the EHT from the CRT's anode cap.

    There is no way you can get a shock from it unless you actually go under the cap and attempt to make a connection to it. The only time the CRT requires the stored charge, discharged, is if you are removing the anode cap from it, to remove/replace the CRT and you don't want to get a shock when you are either removing the cap or you are carrying the CRT across the room and accidentally touch the anode cap.

    You can pull the socket off the CRT neck, the stored charge in the CRT bulb has no way to get to the socket pins on the CRT (except on some scope CRT's with post deflection acceleration) and in those cases the charge is dissipated anyway after its turned off.

    So don't go under the CRT's anode cap, there is no need for it, especially for the inexperienced, unless you want to test the EHT with a high voltage probe. I don't know who started off the urban myth that is was wise to discharge a CRT's anode when doing general service on computer monitors, it is a bad idea and only has an application if the anode cap needs to be removed to replace the CRT or LOPT or the CRT removed from the case & replaced. When it is discharged it should be done with a current limiting resistor.

    A general recap of the PCB, off the bat, is also a very bad idea. You need to establish a diagnosis before replacing any parts. Imagine if doctors did surgery and drugged people without a diagnosis, the collateral damage would be severe, it is no different with electronic servicing, blindly replacing parts with no satisfactory reason is bad.

    The CRT heater is powered either from the DC supply that powers the VDU, or from auxiliary windings on the LOPT (Line or horizontal output transformer) Often other windings on the LOPT power things like the video output stage and the CRT's focus & grid circuits.

    So you are flying blind if initially at least, if you do not have the schematic and a DVM (digital volt meter). Initially make tests of the power rail voltages in the VDU as the first step in establishing a diagnosis. If the Horizontal output is dead (no EHT, no CRT heater supply and no auxiliary voltages) it very much helps to have a scope to examine the drive to the HOT(horizontal output transistor) and in color sets, it is handy if the scope has a x100 probe as the peak collector voltages on the HOT can be as high as 1kV. So the collector circuit there on the HOT, is a place to keep away from with your hands and the DVM.

    Also, when the set is running and you are performing tests, watch out for the high voltages on the CRT's base/socket assembly.
    Last edited by Hugo Holden; March 6th, 2020 at 03:04 AM.

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