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Thread: Fix amiga 1200 yellow screen

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    soviet9922's Avatar
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    Default Fix amiga 1200 yellow screen

    Hi guys, a friend of mine asked me if i can fix an amiga 1200 that he got recently for him.
    The motherboard is a 1200 rev 2b motherboard.

    Testing the bare motherboard only, no accelerator or anything just monitor and power (the psu is an atx psu and is working on my 1200).
    I always got a yellow screen at power on.

    Any tips on fixing this are welcome !.

    Last edited by soviet9922; 6th September 2020 at 07:47.

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    AmigaPassion RetroPassion's Avatar
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    If you don't have any multimeters or an oscilloscope its going to be hard, however yellow screen leads to possibly a CPU issue.
    Offering Commodore Amiga Recapping and minor repairs to the entire Commodore Amiga range including CD32 Laser Replacement service. - Check out my work on Facebook, Instagram and my website - www.amigapassion.co.uk

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    Diagrom os your friend, if you have acccess to a couple of 27C400 eproms and can burn Diagrom to them, link below
    http://www.diagrom.com/

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    soviet9922's Avatar
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    Quote Originally Posted by Retroheaven87 View Post
    If you don't have any multimeters or an oscilloscope its going to be hard, however yellow screen leads to possibly a CPU issue.
    Yes i have a multimeter, logic probe and also 100mhz oscilloscope i purchased it recently and only learning how to use it, but if you can point me to what i have to look for on the board i could follow up.

    - - - Updated - - -

    Quote Originally Posted by supaduper View Post
    Diagrom os your friend, if you have acccess to a couple of 27C400 eproms and can burn Diagrom to them, link below
    http://www.diagrom.com/

    Thanks! that sure the first thing that i will check out

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    soviet9922's Avatar
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    Ok after checking the board was full of flux everywhere after a very deep cleaning the yellow screen goes away but now i have a green screen (chip ram).
    Using the diagroms i got this:


    small video showing the colors and serial output from diagrom:
    https://www.youtube.com/watch?v=Km6a...ature=youtu.be

    inserting a fast ram expansion got me other random results tried just turning on pressing left mouse button and right mouse button:



    heads or tails online

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    Green screen is a Chip RAM fault, I suggest you check all RAM Chip connections. Why was the board covered in flux, was it repaired previously?
    The future's so Retr0bright, I gotta wear shades.....

    As rkauer said:-
    "Retro computers seems like an orgasm: it is always better to have a real one than fake it".

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    soviet9922's Avatar
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    Quote Originally Posted by Merlin View Post
    Green screen is a Chip RAM fault, I suggest you check all RAM Chip connections. Why was the board covered in flux, was it repaired previously?
    The guy that was the original owner of the board tried to "reflow" the board using a hot air station.
    He obviously covered it in 1 liter of yellow stick flux and heated the chips up, of course he don't fixed the issue.
    Now he told me that he was getting green screen before the reflow operation so think the yellow screen was caused by all the flux.

    I checked the pins on the rams and most ics using the microscope and i don't see any bad solder joints at least on the ram and main ics.

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    Have you been over the chips with a multimeter to check the VCC pins are all getting the correct voltage? You may have a bad RAM chip. Do you know anyone with an oscilloscope that can help you to check the data lines on the chips?
    The future's so Retr0bright, I gotta wear shades.....

    As rkauer said:-
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    Quote Originally Posted by Merlin View Post
    Have you been over the chips with a multimeter to check the VCC pins are all getting the correct voltage? You may have a bad RAM chip. Do you know anyone with an oscilloscope that can help you to check the data lines on the chips?
    I didn't check the vcc lines on the ram chips, checked on the powersupply line and they same to by fine. The power supply is an ATX and work fine on my amiga 1200.
    Will check the pins on each chip anyway.

    Yes i recently got a 100 mhz oscilloscope so i can look for the waveforms if you can tell me where i have to check and what to look for ?.

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    verify all pin solder of gayle and other big chips

    burn 2 27c400 with Logica diagnostic
    Last edited by piacc64; 15th September 2020 at 11:32.

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