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Thread: Vic 20 - Powers on, but no image displayed

  1. #1

    Question Vic 20 - Powers on, but no image displayed

    I have a Vic 20 that I purchased over 10 years ago second hand. At the time that I purchased it, I am quite confident that when I tested it, it worked. I cannot recall whether the image was was OK or not.

    The Vic 20 has been stored in a cardboard box inside and no longer functions correctly.

    The problem is that the machine does not display an image.

    I have ruled out the power supply and the display cables, as these work with a Commodore 64.

    The machine still somewhat functions. When I switch it on, there is a brief flash on the screen as the screen receives a signal, typical of what you would expect. If a Datasette is connected, and Play or Fast Forward is pressed, the Datasette will begin to play/fast forward about a second or so after switching the machine on, which I recall is what is expected.

    If I type "LOAD", the machine seems to load a program from a tape. That is, the Datasette will automatically stop at the point in the tape where the program ends. A subsequent "LOAD" command seems to load the next program.

    I was able to, using POKE generate sound. I had all three voices producing sound, although the sound was distorted and weak and unreliable. It did not sound correct, but I was using a cheap USB video capture device at the time.

    When an AM radio is held nearby tuned to the lowest frequency, I do pick up interference as expected, and I can hear the machine starting.

    So the machine is not completely dead.

    I have tried the following

    - Reseating the VIC chip (twice) and reseating all other socketed chips (VIA chips 1 and 2, 6502 and, KERNAL? and Character ROM

    - Run the machine without the Character ROM, VIA chip and VIC chip. These made no difference, except without the VIC chip I think the machine did not start BASIC at all

    - Tried three different displays (A TV, a CGA monitor with Composite In and the USB video capture device). All have the same results (these work with the C64)

    - Adjusted the variable capacitor next to the VIC chip. No change.

    I do not have an RF modulator, and have to use the composite video connector, which I believe should NOT be an issue. I see no reason why I wouldn't be able to get an image this way.

    The board is clean, no evidence of damage or dirt and the Vic 20 otherwise looks pristine. Electrolytic capacitors look OK.

    The only possible defect I noted is the 0.1um ceramic capacitors. They all have a white residue, and a couple which are pictured, appear to have some small deformities. The deformities are where the capacitor was sitting against a chip. As far as I am aware, ceramic capacitor generally don't go bad.

    Seeking advice as to where to go from here with regards to troubleshooting and repair. I suspect the VIC chip, but I also suspect the capacitors.

    Attached are pictures of the VIC chip area, as well as two capacitors which look damaged. The one next to the VIC chip appears to have a hole in the surface. This is in the first picture. Although hard to see, it was resting against the chip, and the chip has a slight peeling on it, and there is a corresponding cavity in the capacity, which feels soft and brittle.

    IMG_7460.JPGIMG_7462.JPGIMG_7463.JPGIMG_7464.JPG

    Any help appreciated.

  2. #2

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    Hi and welcome!
    I think you're in EU, since your VIC-20 has a PAL VIC chip.
    UNfortunately the syptoms are pointing to a bad VIC chip, the MOS6561. It could be a bad Q2 and/or associated components, but since you have a flash on the screen, it means the sync signals are present and probably the Q2 circuit is working but getting a bad signal from the VIC chip.
    Do you have any test equipment (like an oscilloscope) to see what's going on? Can you swap a working 6561 from another VIC-20?

    HTH
    Frank IZ8DWF

  3. #3

    Default

    Thanks,

    I don't have test equipment, nor a working Vic-20. I've found that you can buy VIC chips, and a couple of guys on the Lemon 64 have stated they are working chips as they have purchased.

    I vaguely remember the image not being that great when I bought it, but that was over 10 years ago. I may just buy a replacement PAL VIC chip and see how I go. The scratchy sound is probably also indicative of the faulty VIC. It will be cheaper just to buy the chip and see how I go, than buy the equipment.

    BTW, I'm in Australia.

  4. #4
    Join Date
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    Quote Originally Posted by boraxman View Post
    BTW, I'm in Australia.
    To add 'Australia' (or 'Land of the kangaroos', or 'Land of koala burgers', etc.) to your profile, the navigation is: [My Profile][About Me][Location].

  5. #5

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    I just wanted to add that I tried the machine on a different TV.

    It does produce seemingly correct sound, its just very, very, very quiet. The distortion is actually due to having to turn the volume all the way up to hear it. With the volume all the way up, there is obvious background noise, but the VIC voices are there making the sound they should.

    It seems like a very weak signal is being sent to the TV, but it is generating sound OK.

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