Image Map Image Map
Page 1 of 2 12 LastLast
Results 1 to 10 of 13

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
    853

    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

  5. #5

    Default

    Thank you Frank,
    since I am quite a newbie in this, could you give me some further hints as to how I should use the logic probe (TTL mode, I guess) to check the clock and the enables?

  6. #6

    Default

    Thank you norm8332,
    yes the kernal is a pretty common failure for Vic-20. It is a "newer" CR version and I have already checked all voltages on all MOS chips, RAM, etc.

  7. #7

    Default

    Hi Giovi,
    thank you for your suggestion. Could you point me at some samples of eproms to buy, other than the programmer? I have no experience at all at programming eproms, so it would be very helpful to get some more hints.

  8. #8

    Default

    Thank you all for your suggestions and sorry for being so late back on this thread!

    Since I am a complete newbie in diagnosing and fixing old retro-stuff, please feel free to add any further hint/links about eprom programming or how to play with the logic probe to further inspect what's wrong with the vic.
    I have already checked that the CPU is completing the reset sequence and that all ICs do receive the correct voltage.
    Also the CPU address lines seem to be active.

  9. #9
    Join Date
    Dec 2012
    Location
    São Paulo country, Brazil
    Posts
    853

    Default

    Quote Originally Posted by mrseylok View Post
    Hi Giovi,
    thank you for your suggestion. Could you point me at some samples of eproms to buy, other than the programmer? I have no experience at all at programming eproms, so it would be very helpful to get some more hints.
    You will need the programmer, of course, and I suggest to buy an eprom eraser (UV eraser), they're quite cheap and useful.

    The VIC-20 uses ROMs of two different size:


    CHARACTER ROM : original: 2332 ROM (4K) -> can be replaced by a 2732 + an adapter (32k bit = 4 kBytes)
    BASIC ROM : original: 2364 ROM (8K) -> can be replaced by a 2764 + an adapter (64k bit = 8 kBytes)
    KERNAL ROM : original: 2364 ROM (8K) -> can be replaced by a 2764 + an adapter (64k bit = 8 kBytes)

    You can build your own adapter out of a couple of sockets:

    usare una eprom 2764 al posto delle ROM C=2364.gif
    This is for 2764 eprom (you can find a great tutorial here: https://ist.uwaterloo.ca/~schepers/sockets.html).
    It's a little tricky because of the different size between the eprom and the original rom.




    eprom_adapter-2732-2532.jpg
    and this is for the 2732. Quite simple. I uses two sockets because I don't like to bend and solder directly to the eprom, but check there's enough space (height) into the VIC... IIRC there should be.


    Be sure your eprom programmer can program the eprom you planned to buy. I have a chinese, cheap Genius G540 that works quite well (at least mine requires WinXP).

    Then you can go to zimmers.net and download the firmware you need.

  10. #10

    Default

    Thanks Giovi.
    Today I have checked a bit with the multimeter and the logic probe over the various ICs on the motherboard.
    Turned out that the UF8 (MOS 65245 equivalent to 74LS245) was kind of stuck on "isolation" mode, as if blocking communication between Data bus and Address bus.
    It's just an hypothesis, of course, but I ordered a spare for it, which I intend to use to replace it and see what happens.
    The other two 65245 (UE8 and UD had quite different signals, so this made me suspect that something could be wrong with UF8.

    In case nothing changes, I will try and get another kernal IC to replace UE12 (901486-07) which is also socketed.
    I do not feel very confident with all that work to adapt the eprom sockets to the original pinout on the Vic-20, so I would rather go for some spare original ICs.

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
  •