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Thread: Sanyo mbc-775 corrupted graphics (CGA)

  1. #1
    Join Date
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    Thessaloniki, GREECE
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    Question Sanyo mbc-775 corrupted graphics (CGA)

    Here it is a long story trying to describe it in a few words:

    About 6 months ago I got a non-working Sanyo mbc-775 luggable pc clone xt!

    After a lot of (hard ) work finally it became alive.

    Everything was working ok. Booting ok and even run Wheel of Fortune game and some DOS 2.11 basic demos to check it.

    I had everything disassembled over my working bench trying to figure out how I would connect an external 8bit isa HDD.

    One day after it boot properly in DOS prompt I noticed that when CGA tryies to invoke any hires mode I get garbages on screen (!!) I uploaded a video if someone would like to check it:



    Since then I tried everything from changing the 2 vram chips and even the 6845 CGA chip with no success ! (

    Next thing I will try the IBM Diagnostics disk to check some thing out...

    Any help or advice would be welcome

    Thank you in advance

    Alex

  2. #2
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    Default

    What you're seeing is CGA graphics data being displayed in text mode. This is usually the result of:

    - software misconfiguration
    - wrong dip switch setting on motherboard
    - Broken CGA card

    Did anything at all change? Did you insert any cards into your system? Your CGA card appears to be working just fine in terms of RAM and the character generator, but is acting as if it never got the port writes to switch into graphics mode.
    Offering a bounty for:
    - The software "Overhead Express" (doesn't have to be original, can be a copy)
    - A working Sanyo MBC-775, Olivetti M24, or Logabax 1600
    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

  3. #3

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    Well, what graphics hardware do you have in there? I could be wrong but AFAICT the MBC-775 was only "sort-of" PC-compatible, as was the MBC-55x; the 55x had its own not-really-compatible video hardware, and the option of a CGA card for better compatibility.

    If the MBC-775 is the same in the video department, and you do have a CGA card in there, maybe the issue is that the program in your video *expects* the Sanyo graphics hardware and chokes on the CGA? That would square with Trixter's assessment of port writes going unrecognized.
    int10h.org :: :: :: blog

  4. #4
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    Default

    Hello to all oute there and Merry Christmass!

    As I said in my first post . This is an MB-775 with onboard (kind of) CGA .

    I had run this demo before and run properly.All of a sudden weird things start appearing in hires mode (I guess)

    I suspect CGA hires mode problems . Text mode is working ok . I tried running IMB51500 diagnostics that checks CGA also, but I take (the old days well known)
    'divide overflow error' -may be its due to high speed 8mhz clock ???

  5. #5
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    Apr 2016
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    Default

    A little udate:

    I installed used an 8bit cga card in one of the 2 spare isa slot . The problem is the same. I get the same corrupted image with external card ,too!

    My system has also an add-on multi i/o card with extra 384k ram . I tried to disable this extra ram to see if there will be any progress same result!

    I found no way to disable the onboard 256k ram (there is 8 undocumented dip-switches in the main board)

    May be there is smth wrong with the logic circuit decoding (!???)

    Another option would be to salvage an existing 8bit XT mainboard and exchange the 'guts' of the original machine but this is the last option as it will totally loose the orginal hw feeling (or may be not!)

  6. #6

    Default

    DIP switches for this machine are shipped with 2, 4, 5 and 8 on. This is the setting for most software applications. Please refer to the chart below explaining the functions of the DIP switches.
    1-6 = unused. 7-8 Number of 5-1/4 disk drives installed
    7 8 #of Drives
    On On 1
    Off On 2
    On Off 3
    Off Off 4

    Normal setting is as follows:
    1 2 3 4 5 6 7 8
    Off On Off On On Off Off On

  7. #7
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    Default

    @lanmanager

    Really thank you for your infos my friend.
    As there are NO documents ib the nets your info is very usefull.
    I checked and verified the correct posiotion of the DIP switches.

    Dsc_0480.jpg

    Dsc_0482.jpg

    Dsc_0483.jpg

  8. #8

    Default

    Two things - I don't think the memory add on card is original although I have the same one. Get rid of that battery if i't the original. Also, the way the CPU board plugs into the bus plane boards on these is not exactly robust.

    The bottom bus board is supported by small tabs punched out of the aluminum frame that the screws are in, and those weak tabs have a tendency to bend when when other add on cards are inserted. That causes the pin connector between the CPU and bus boards to poorly engage, potentially causing strange and unpredictable behavior . The tabs can be gently pushed back forcing the bus card connector up into the pins on the CPU card. When inserting cards, make sure the back locking plate/cover is slightly bent in at the bottom so the key will engage in the slot. This makes insertion much easier.
    Check the voltages from the power supply. It uses quite a few 17v zener diodes for regulation. I also found several dead or deformed caps and 2 resistors out of spec. Mostly these were on the 20v rail that drives the monitor. The monitor only uses mains voltage to power the demagnetizer. The yoke, gun and flyback circuits only use +12v and +20v (labeled 22v??). The power supply uses very good components, but the actual circuit design leaves a lot to be desired compared to modern SMPS.

  9. #9
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    Default

    Two things - I don't think the memory add on card is original although I have the same one. Get rid of that battery if i't the original.
    >>Ι know this and I have already replaced with a fresh battery.


    Also, the way the CPU board plugs into the bus plane boards on these is not exactly robust.
    >>Do you thing I have better make a DIY isa-like plug+connector ?

    The bottom bus board is supported by small tabs punched out of the aluminum frame that the screws are in, and those weak tabs have a tendency to bend when when other add on cards are inserted. That causes the pin connector between the CPU and bus boards to poorly engage, potentially causing strange and unpredictable behavior . The tabs can be gently pushed back forcing the bus card connector up into the pins on the CPU card.
    >>I didn't noticed this my friend and thank you for drawing my attention on this.
    However all the test are outside computer case with all cards on-air!


    Check the voltages from the power supply. It uses quite a few 17v zener diodes for regulation. I also found several dead or deformed caps and 2 resistors out of spec. Mostly these were on the 20v rail that drives the monitor.
    >>Actually the main problem when this ΜMC came to my posession was due to PSU fault.After a lot of trouble shooting I get power drive FET blown ups after a couple of minutes psu working and I came to a conclusion that this was due a shortcircuit on the internal DC-DC transformer (?!)So I replaced thw wholw PSU with a PC-AT PSU!

    The monitor only uses mains voltage to power the demagnetizer.
    >>I noticed that and I made a diy add-on demagnetizer to go with thw newly PC-AT PSU using the original parts!

    The yoke, gun and flyback circuits only use +12v and +20v (labeled 22v??). The power supply uses very good components, but the actual circuit design leaves a lot to be desired compared to modern SMPS.
    >>I confirm that although I replaces the orignal PSU after all the components and switching-psu type is of a very good quality

    Finally may be corrupted graphics has something to do with bad CPU - Main board connection.
    I will check this out my friend!
    Thank you for all your help!

    Alex

  10. #10
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    Default

    Still trying to find a solution for 3 months now without a positive result!!

    In the weekend I finally burnt an XT diagnostic eprom according to VCF excellent Ruud's post

    The result I got is in the following foto:

    Dsc_1617.jpg

    Do you thing this test is reliable?

    Revival adventure continues!.....

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