Image Map Image Map
Results 1 to 4 of 4

Thread: Another VIC20 black screen

  1. #1

    Default Another VIC20 black screen

    Hello Commodore experts,

    I am trying to revive my old VIC 20 that has remained switched off for the past 35+ years.
    The problem is a complete black screen. There's a signal over the video cable (DIN to SCART cable) as the TV where I plug it in recognizes there's "something". But it stays black.

    Tests I have done so far:

    - The power supply is working, as I tested it on both 5V DC and 9V AC.
    - All main ICs get the 5V DC and feel warm/hot after some minutes.
    - Trying to blindly SHIFT + RUNSTOP with an original Commodore C2N tape connected does not have any efffect: the tape motor does not start, though I could measure a 5V DC on the tape connector too.
    - Tried to reset all the socketed ICs (only VIC and KERNAL)
    - Desoldered UAB1 (1st VIA) and tried without it
    - Soldered a socket to reseat the UAB1

    I could go on and socket also the 2nd VIA and the two RAM chips, but I feel I wouldn't get much farther.

    Any hint from you guys? I do not have an oscilloscope nor any logic probe that would allow to understand if there's a fried IC.
    Isn't there any harness I could get somewhere that would bypass the VIC and generate a video signal and run some diagnostic SW?

    In case I got a logic probe, what would be the best way to use it?

    thanks for your help

    --mrseylok

  2. #2

    Default

    Quote Originally Posted by mrseylok View Post
    Hello Commodore experts,

    I am trying to revive my old VIC 20 that has remained switched off for the past 35+ years.
    The problem is a complete black screen. There's a signal over the video cable (DIN to SCART cable) as the TV where I plug it in recognizes there's "something". But it stays black.

    Tests I have done so far:

    - The power supply is working, as I tested it on both 5V DC and 9V AC.
    - All main ICs get the 5V DC and feel warm/hot after some minutes.
    - Trying to blindly SHIFT + RUNSTOP with an original Commodore C2N tape connected does not have any efffect: the tape motor does not start, though I could measure a 5V DC on the tape connector too.
    - Tried to reset all the socketed ICs (only VIC and KERNAL)
    - Desoldered UAB1 (1st VIA) and tried without it
    - Soldered a socket to reseat the UAB1

    I could go on and socket also the 2nd VIA and the two RAM chips, but I feel I wouldn't get much farther.

    Any hint from you guys? I do not have an oscilloscope nor any logic probe that would allow to understand if there's a fried IC.
    Isn't there any harness I could get somewhere that would bypass the VIC and generate a video signal and run some diagnostic SW?

    In case I got a logic probe, what would be the best way to use it?

    thanks for your help

    --mrseylok
    With a logic probe, you would first check the clock output from the VIC to the CPU, then check the reset signal to the CPU, then check all the enables on the ROMs and that would tell you if and how far is executing something. You could also mostly tell if the address and data bus are working, but it would be better to make a "NOP generator" socket to temporarily make the 6502 cycle through a NOP sequence.
    If you want to repair it "blind", then you can only substitute chips and hope is not a multiple failure case.
    Frank IZ8DWF

  3. #3

    Default

    I would try the kernal rom first. You appear to have sync signals so the VIC chip may be good. The kernal rom is a common failure. Also be sure to check the power supply for correct voltages if the Vic-20 is a newer CR version with the DIN connector power supply.

  4. #4
    Join Date
    Dec 2012
    Location
    São Paulo country, Brazil
    Posts
    728

    Default

    Quote Originally Posted by norm8332 View Post
    I would try the kernal rom first. You appear to have sync signals so the VIC chip may be good. The kernal rom is a common failure. Also be sure to check the power supply for correct voltages if the Vic-20 is a newer CR version with the DIN connector power supply.

    I can confirm that; I have at least a couple of VICs with an eprom in place of the original Kernal. VIC roms seem to be quite weak and if the kernal is socketed, you should try to replace it.

    If you plan to fix it (and maybe other vintage computers), maybe worth to buy an eprom programmer (I got a Genius from China for few euro on eBay) and a cheap eprom eraser (should cost 10-15 euro)... plus, of course, some eproms and socket (you will need to make some eprom -> rom adapters).

    --Giovi

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •