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Thread: Commodore 1702 real schematic?

  1. #1

    Default Commodore 1702 real schematic?

    Hi all,
    I'm trying to repair a 1702 monitor, with a Toshiba CRT. However, no schematic I've found in the preliminary service manual that can be found on the net is close enough to have a real idea of what's wrong (well, unless I start reverse engineer the schematic from the actual PCB). There's a fuse T630mA that's blown on the main PCB, it's labelled as F501 and there's no track of it on the schematics nor on the part lists. It's paralleled by a small diode and is across two pins of the flyback horizontal transformer. The monitor has no sound and no video working, but the horizontal oscillator is running and the CRT's anode voltage is present.
    Before I try to power it with a new fuse, I would try to understand if there's something else wrong as usually fuses don't die of old age.
    Maybe anyone out there has the correct schematic? The prints on the main PCB showed no useful result on google.
    Thanks in advance.
    Frank IZ8DWF

  2. #2
    Join Date
    Oct 2016
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    Cincinnati, Ohio USA
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    Fuses do sometimes die of old age due to oxidation of the filament. Fuses are cheap, so I would try changing it. If that fixes it, great, if not you then know you have another issue.
    PCjr, DTK PC-XT Turbo, 386DX 33, 486 laptop, Pentium 120, Pentium III 500, various old laptops, Commodore Colt, all working. I also have a 286 that I need to see if I can repair.

  3. #3

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    Quote Originally Posted by KenEG View Post
    Fuses do sometimes die of old age due to oxidation of the filament. Fuses are cheap, so I would try changing it. If that fixes it, great, if not you then know you have another issue.
    I know it. It would be good to have the real schematic anyway.
    Changing that fuse solved the problem this time.

    Frank IZ8DWF

  4. #4
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    Oct 2016
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    I'm glad the fuse fixed it. Yes, it is always nice to have schematics. The other reason I try changing fuses first is that how it blows can help you tell if you have a dead short or a more minor issue to look for. It is usually easier to find a short that a minor issue causing a slightly high amperage across the fuse.
    PCjr, DTK PC-XT Turbo, 386DX 33, 486 laptop, Pentium 120, Pentium III 500, various old laptops, Commodore Colt, all working. I also have a 286 that I need to see if I can repair.

  5. #5

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    Quote Originally Posted by KenEG View Post
    I'm glad the fuse fixed it. Yes, it is always nice to have schematics. The other reason I try changing fuses first is that how it blows can help you tell if you have a dead short or a more minor issue to look for. It is usually easier to find a short that a minor issue causing a slightly high amperage across the fuse.
    This fuse connect power to a secondary winding of the flyback transformer, it's really a peculiar design. So DC-wise is a dead short anyway (well, not so "dead" but close enough that I gave up trying to understand if there was an unintended short in parallel to the intended one).

    Frank

  6. #6
    Join Date
    Oct 2016
    Location
    Cincinnati, Ohio USA
    Posts
    336

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    From what I read, you cannot directly read the resistance with a volt meter. You put a DC source across the winding and read the voltage drop across the winding. I assume the DC voltage used depends on the AC voltage that it is rated for.
    PCjr, DTK PC-XT Turbo, 386DX 33, 486 laptop, Pentium 120, Pentium III 500, various old laptops, Commodore Colt, all working. I also have a 286 that I need to see if I can repair.

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