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Thread: Atari 520 STm Modification

  1. #1

    Default Atari 520 STm Modification

    I'm trying to determine the purpose of a modification to my Atari 520 STm. Is it a factory or user mod? Something to do with the RF modulator perhaps?

    - Don Mankin -

    [
    TRS-80 Models I, 4, 4p, Coco 2, 100, 102. Tandy 1000 RLX. Osborne 1a.
    At
    ari 800, 520 STm. Commodore 64.
    Amiga 1000. TI-99/A. Sinclair ZX-81]

  2. #2
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    Quote Originally Posted by Fooser View Post
    I'm trying to determine the purpose of a modification to my Atari 520 STm. Is it a factory or user mod? Something to do with the RF modulator perhaps?

    Where does the other end of the red wire go? I assume the grey coax goes to the rf modulator.
    Dave
    G4UGM

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

  3. #3

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    Here's a better look.

    - Don Mankin -

    [
    TRS-80 Models I, 4, 4p, Coco 2, 100, 102. Tandy 1000 RLX. Osborne 1a.
    At
    ari 800, 520 STm. Commodore 64.
    Amiga 1000. TI-99/A. Sinclair ZX-81]

  4. #4
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    Nope, got me baffled. I thought it might be a composite mod, but the "M" models don't need that. I would try one of the dedicated Atari forums. Very active...
    Dave
    G4UGM

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

  5. #5

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    Quote Originally Posted by g4ugm View Post
    Nope, got me baffled. I thought it might be a composite mod, but the "M" models don't need that. I would try one of the dedicated Atari forums. Very active...

    Thanks Dave. I've seen a mod something like this for the ZX-81 that disables the RF Modulator and provides composite out but the STm already delivers composite via the 13 pin video port.
    - Don Mankin -

    [
    TRS-80 Models I, 4, 4p, Coco 2, 100, 102. Tandy 1000 RLX. Osborne 1a.
    At
    ari 800, 520 STm. Commodore 64.
    Amiga 1000. TI-99/A. Sinclair ZX-81]

  6. #6
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    I had a quick check on 520ST circuit diagram (see dev-docs.org). The grey cable comes from R113, that is a clock signal which is 3,58 or 4,43 MHz. I think this is the color frequency for NTSC or PAL. Originally the same clock signal goes to pin 8 of the HF modulator, but this one goes by cable inside the HF modulator. This circuit is also a part of the basic clock generator circuit of the 520ST.

    The red cable comes from +12V power suppy, very close to the source, so quite jitter free and is switched through that flyover transistor, which controlled by 74HC74 Flipflop ping 6 /Q output by it's basis input. The output then again feeds the clock generator circuit of the ST 32 MHz base frequency which later is divided by GLUE and shifter to 16 Mhz, 8 Mhz, 4 Mhz

    I think this is used to get the color frequency for the modulator syncronous with the ST CPU, chipset and bus clocks.

    I think in total this modification was done by ATARI itself, and it might help to improve HF modulator output quality of that mainboard revision.

    I have seen many versions of the 520 ST board, but that mod is new for me. Maybe because it's an early NTSC board.

  7. #7

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    For what it's worth I have a PAL unit that has this exact same modification - minus the connection to the modulator (it doesn't have one). It's an early 1985 model running TOS 1.00.

    The video output from this ST is pretty poor compared to my other machines (Mega ST and STE) with obvious jailbars even on an official Atari monitor.

    Here's a picture of mine:
    IMG_20190402_181811.jpg

  8. #8
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    Not to be a not picker, but are we talking about an STFm? Or an original 520st, and I'll assume all of those had modulators.

  9. #9

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    Quote Originally Posted by tipc View Post
    Not to be a not picker, but are we talking about an STFm? Or an original 520st, and I'll assume all of those had modulators.
    No problem at all. It seems I'm misremembering in fact. I've just double checked and mine does indeed have a modulator despite being branded on the box and the case as just a "520ST". Though oddly enough the "mod" in mine doesn't have the grey wire to the modulator that OP's has. Perhaps that's an NTSC thing.

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