Help with an old computer with no video signal

Discussion in 'General Electronics Chat' started by capacapa, Sep 3, 2013.

  1. capacapa

    Thread Starter New Member

    Mar 31, 2012
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    1
    I'm trying to fix an old computer (MSX), but so far, no luck

    On one msx, I have no video from video out or RF, I used multimeter, and compared with another MSX, it seems encoder on the no-video msx, shows reading of 4.58v on pin #30 while the working msx, shows varied number usually in the 2.xx v, I measured r g b pins both in and out, they give varied voltage same as the working MSX , same for cpu clock and the power voltage both are correct,

    Does this shows that the encoder chip CXA1145 faulty? though RGB shows values and goes and comes from same chip. the video out comes directly from the chip's pin 20 to the encoder pin out #30.

    Are there other possibilities beside the chip?

    Encoder connection
    [​IMG]

    [​IMG]

    Encoder details:
    [​IMG]
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Those old video encoder chips ran hot and aften failed. That would be my guess.

    Your schematic shows composite sync going into that PCB, pin8 "C sync" which then goes into pin 10 of the IC.

    I would check the c sync looks good at the entry to the board (before Q1 and that inverter gate) and check it is still good going into pin 10 of the IC.

    If the IC is getting good sync into pin 10 I would expect it to produce some waveform out of your pin 30. As there does not appear to be any type of "blanking" control to the IC it should output video waveform whenever the C sync into the IC is working.
     
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  3. capacapa

    Thread Starter New Member

    Mar 31, 2012
    17
    1
    Thanks a lot for helping me with this.

    I measured pin 8 and 26 c-scan in, out
    not working msx
    pin 8: 4.60
    pin 26: 1.97

    working msx
    8: 4.61
    26: 2.69

    so this shows more that the chip is likely at fault, right? one thing I don't understand, if composite video signal delivers 5v , is that interpreted as no signal by the digital TV instead of some noise?
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I think your measurement procedure is holding things up here. Do you have access to an oscilloscope to check the video waveform?

    Just measuring a DC voltage on a pin and guessing it's not the right waveform is not going to cut it. :)
     
  5. capacapa

    Thread Starter New Member

    Mar 31, 2012
    17
    1
    I don't have access to an oscilloscope, seems pretty pricey. I guess replacing the chip is cheaper.

    I guess another possibility for fault is the VDP chip which is where the c-sync comes from. but it seems uncommon thing to happen, so I ll try to replace the cxa chip first and see. problem it is going to be pain . I have to desolder the encoder board, then can desolder the cxa1145 chip. and I'm beginner, it's going to be a tough job :eek:
     
    Last edited: Sep 7, 2013
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Good luck! :)
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,452
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    What RB is saying if you want to start tinkering with electronics at that level an oscilloscope becomes essential.

    Look into picking up a good second hand scope which you can find for under $50, sometimes less if you're lucky.
     
  8. capacapa

    Thread Starter New Member

    Mar 31, 2012
    17
    1
    Not sure if you mean good luck with that, or just wishing me luck :D either way, thanks, man! I appreciate your and other people responses, put me on the right track.

    I'll keep an eye on used but good ones, I searched on auction sites, mostly got ancient ones or broken so far. but might wait for a good deal, I just spent a lot recently on several hobbies, need to charge my wallet.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It was to genuinely wish you well in the repair.

    I've replaced a lot of those video chips and they can be tricky, often on single sided PCBs with fine track spacing and there's a lot of careful unsoldering needed and sometimes a track will lift etc.

    Re a 'scope, you can get little colour handheld scopes on ebay for $60 which will do this kind of job fine. Video waveforms are only about 15kHz, so you don't need a big expensive RF scope;

    [​IMG]
     
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