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Thread: Otrona Attache Mystery Mod and Repair

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    Default Otrona Attache Mystery Mod and Repair

    I picked up an Otrona Attache and a box of parts at VCF-MW this year. Unfortunately the Otrona is missing the back cover and power supply, which I'll have to track down at some point.

    I have a handful of Otrona motherboards and all of them were modified with a large board that replaces the bitmap graphics RAM, a smaller board that replaces the graphics/attribute RAM address multiplexer and adder, and a cut trace going between address 4 of the character/attribute RAM and the multiplexer for address 4.

    Large board
    Has several SRAM chips on it. U426, a 3-to-8 decoder for the attribute/character/graphics RAM selector is replaced. U703, an 8-bit shift register that takes the graphics RAM data and outputs the bit data to a multiplexer with the character bitmap data is replace. The bitmap graphics SRAMs are all removed from the board and U708 is replaced. 4 bodge wires come off of it. The first one taps onto the chip select input of the multiplexer for the data row address bits DR1-DR4. The second one taps onto the LSB of the scan counter coming from the CRTC. The third one taps onto the system bus's A15 line. The fourth one goes off to pin 49 on the expansion connector (ground), which is cut away from the main board.

    Small board
    Has one bipolar PROM and one multiplexer. U422, a multiplexer for the data row address bits DR1-DR4 is replaced. U423, an adder that layers the character RAM address space efficiently is replaced. There are two bodge wires coming off the board. The first one is tied to the graphics RAM bus's A4 line, directly to the character RAM. The second one is tied to the output of the multiplexer that feeds the graphics RAM's A4 line. On the main board, the A4 line is cut going between the graphics RAM bus and the multiplexer that feeds it.

    The mods installed on the board

    My theory is that the small board uses the unused 128 ((2048 byte character RAM)-(80 columns*24 rows)) character RAM addresses as addresses for the large bitmap graphics board. For what? I don't know. Bank select maybe?

    I was told that the large box of parts came from the Otrona R&D department. Many of the mod boards have date codes and version numbers on them, written in sharpie. Several of the Attache motherboards look like they were used to salvage parts from, and are in very rough shape. Only two of the boards will actually generate video, and only one actually boots the ROM and lets me get into diagnostics mode.

    The one that does works has several problems though. I'm getting some very bad interference on the video output (https://i.imgur.com/uUORp0y.mp4) and not all of the rows on the screen work. The "graphics RAM test" does not return any errors. It just flickers a bit and throws me back to the @ prompt. I ran the "fill with test pattern" test to show off the broken rows. The broken rows scroll with the text on the screen. If I let the board sit for a couple minutes powered up, and run the "fill with test pattern" test again, the top two broken rows will get filled in with some characters. If I wait long enough the two rows will start working correctly. The four middle rows will never work.

    Test pattern

    I have already tried replacing the CRTC, character RAM, attribute RAM, and all of the tantalum capacitors on the board. The only thing that displayed anything different was trying a different test revision "small board". The other revisions seem to chop off everything but the first 4 rows, duplicate the first 4 rows multiple times, or don't work at all. I can't remember if the bad rows work with the other board revisions (specifically the duplicating boards). I suspect that most of my issues are stemming from this board. The few working "large board"s that I have do not change anything.

    I haven't tried booting the computer up off of a floppy yet. I'd like to try taking this one problem at a time.
    Last edited by TangentDelta; October 12th, 2018 at 06:21 PM.

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