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Thread: A1200 No RGB, and composite colour wrong

  1. #1
    8bitplus's Avatar
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    Default A1200 No RGB, and composite colour wrong

    Hello.
    I hope someone here might have some hints and tips to fix this problem.
    I'm fixing up a pile of A1200 motherboards, and found one that initially didn't work. After changing an obviously bad cap (C459) I got it working, but no image on RGB.
    Click image for larger version. 

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    I could tell the board posted by the floppy idle clicking sound. No error flashes on the LEDs either.

    I had the board connected to a TV via RGB which flickered slightly when powering on, but no Kickstart image.
    So I connected the Composite and got an image.
    Click image for larger version. 

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    The picture was very off colour and had interference in the middle of the picture. I removed the RGB cable and this improved the stability, but it still remained off colour.
    Click image for larger version. 

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    I will of course continue replacing all the caps, but is there something else I can check regarding the lack of RGB output? I didn't try the RF output yet, will do soon.
    Could the Composite and RGB sync cause this issue?
    www.8bitplus.co.uk
    A1200 Desktop 030/33mhz, 32mb Ram.
    A1200 Tower 060/80Mhz 64mb Ram, Voodoo Mediator.

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    piacc64's Avatar
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    no rgb, verify fuse near db23

  3. #3
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    Since you have picture on composite the Video DAC is OK. But the picture shows something wrong with the CXA1145 as well. Do you have a CSYNC/VSYNC/HSYNC on the DB23 port? I have never seen a board with all three (R, G, B) lines being gone at the same time.

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    8bitplus's Avatar
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    I should have time to take another look tonight or tomorrow. How am I best checking the sync lines? I have a multimeter but no oscilloscope. I'll test the RF output too, that would confirm CXA1145 problem if the colors are wrong wouldn't it?
    www.8bitplus.co.uk
    A1200 Desktop 030/33mhz, 32mb Ram.
    A1200 Tower 060/80Mhz 64mb Ram, Voodoo Mediator.

  5. #5
    CrazyBob's Avatar
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    Also have a look at / replace the 3x 0.22uf SMD caps at the underside of the board near the CXA1145.
    These are labeled for v1.x revision: C211, C212 and C213.
    For v2.x revision: C211A, C211B and C211C.

    Both the video DAC and encoder chips are probably OK.
    If the mentioned caps are bad it changes the impedance of the RGB lines and you have no/bad video output.
    Last edited by CrazyBob; 2nd March 2017 at 07:19.
    => Commodore A600HD/40 <=
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    Did you replace the other capacitors too?

    it looks like there is some oscillation occurring in the video output section, often this is caused by bad bypass capacitors. It's always wise to replace all electrolytic capacitors in an A1200 (or A4000). Even the ones that appear to look OK.

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    Any luck with the repairs?
    => Commodore A600HD/40 <=
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  8. #8
    Ya' Like it Retr0? Inactive Member
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    @Thread

    There is alot of fettling of ideas of to what the problem, part of the repair job is to diagnose and fault find - I would humbly suggest reviewing the manual and checking out the feed to and from the DAC.


    In fact I have been reviewing a lot of new youtubes lately and this one is very pertinent -


    RetroGameModz

    I have really enjoyed these videos, very precise and logical fault finding, diagnosis and repair and would go so far as to say this is the minimum I would expect from a professional repair.

    If I have but only one gripe it would be the commentator can digress - especially if the board had received previously poor work - sadly this can distract from the core focus of the videos he does.

    Still its entertaining, informative and enjoyable in all the right ways
    Last edited by Zetr0; 11th March 2017 at 09:48.
    8)Flux Mastah' Zee8)
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    8bitplus's Avatar
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    Quote Originally Posted by Zetr0 View Post
    @Thread

    There is alot of fettling of ideas of to what the problem, part of the repair job is to diagnose and fault find - I would humbly suggest reviewing the manual and checking out the feed to and from the DAC.
    Thanks I'll take a look.

    I finished the recap, no differance. The same image on RF.
    I've ordered a digital capacitor tester to check the remaining smd caps. Only problem is finding the time to tinker with stuff atm. only have about an hour a week of amiga time lol
    www.8bitplus.co.uk
    A1200 Desktop 030/33mhz, 32mb Ram.
    A1200 Tower 060/80Mhz 64mb Ram, Voodoo Mediator.

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