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Thread: Model 4 vertical deflection: Culprits?

  1. #21

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    Quote Originally Posted by MrArgent View Post
    Wasn't C204 or C211, unfortunately. Up next is the vert IC, which is in the mail.
    It wouldn't surprise me if it was the IC being defective. It is a bit of a hard ask for the IC , without a heat sink, to fully vertically deflect the crt, it takes 3 to 6 Watts of power to do it, even for a 7 to 9" crt and the output devices in the IC have to support that in a class A-B amplifier configuration. So if you think of a 3w to 6W audio amp with no heatsink on the power devices, its going to get hot at full power & no heatsinks. Probably a lot of these vertical amplifier IC's would last a lot longer if they were screwed to a heat-sink of some sort, which the manufacturers at least considered with the hole in the heat flag that they have. Though, no doubt when it was designed the case temperature of the IC was "in spec" but as with a lot of electronic parts, heating shortens their life.

  2. #22
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    Yeah, that doesn't really seem like adequate...uh, sinkage...for something in that sort of position.

  3. #23
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    The Good news: The new 5763 works.
    The bad news: It wasn't the 5763. I'm gonna have to go through the logic board, aren't I?



    It occurs to me that I could probably hook this machine's upper half to my semifunctional TRS-80 Model III's lower half to isolate further. This motherboard is a hot mess and I am absolutely not looking forward to having to poke around it. As an aside, I almost wonder if the old 5763 took out those two caps I IDed as bad, then took out the replacements too.

    Last edited by MrArgent; November 9th, 2019 at 01:58 PM.

  4. #24
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    All the more reason to get a 'scope!

  5. #25
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    V-Drive Pin 9 to R201 is OK. R201 to R202 is OK. R202 to 203 doesn't show continuity, however. Something to crow about? Likewise, R203 to C203 doesn't show continuity, and neither does C203 to IC201 pin 1.

  6. #26
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    The missing continuity was due to a difference between my CRTC board and the similar one in the 4P manual. Everything needed for the signal to reach 201 is there, so far as I can tell. Next up: Follow the breadcrumbs from 201's Pin 8 to the neckboard.

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