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Thread: Commodore CBM 2001-16N - BIG MESS!

  1. #11

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    Thank you guys for your value comments. I really appreciate all your help!
    And i understand that i will need a oscilloscope. As i hesitate to present another big electronic device (beside my bunch of Retro Computers) in my home would maybe a special scope multimeter do the job?
    I found a Grundig SO100 in my neighborhood for 150 (ca. 170$), up to 100 MhZ but i believe it is already digital. But its a big beast

    @dave_m: The computer obviously uses still the RAM and the Character ROM on the Mainboard because when i remove it from there i get the expected result (checkerboard pattern, white screen, etc.)

    @AndyG: i will try to download the EPROM software tommorow and will post it here

  2. #12
    Join Date
    Feb 2009
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    Quote Originally Posted by miata View Post
    I found a Grundig SO100 in my neighborhood for 150 (ca. 170$), up to 100 MhZ but i believe it is already digital. But its a big beast
    I found a picture of the Grundig SO100 and it looks like a nice digital scope. The vertical channel controls look conventional, but the horizontal (timebase) controls are quite different than I have seen. If that package is too big, then it may be hard for you to find a used scope that you can live with. Can you borrow a scope from a computer friend or computer club?

    I am not familiar with the usefulness of the small scope multimeters.

  3. #13

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    The analog scope I recommend is the Hitachi V-509, because its 50 MHz bandwidth, compact, has a delay timebase and H and V sync separators built in. It also use a 10kv final anode voltage on the CRT and has a super bright sharp trace.

    I agree with everything Dwight says about scopes. An analog scope with a delay timebase function is the way to go initially.

    The Hitachi scopes often have the H & V sync separators built in. These are particularly handy working with vintage computers for issues with the composite video output signal. The prices are reasonable too. I just had a look on ebay, there are no V-509's at the moment but there are some nice looking V-650's that are 60MHZ bandwidth:

    https://www.ebay.com/itm/HITACHI-2-C...0AAOSw0ThdxJKA

    With the analog scope you do not have to worry about sampling issues that crop up with digital scopes and really, to be proficient with a digital scope later anyway, the analog one has to be mastered first to become familiar with issues related to triggering, appropriate timebase settings & operations, the effects of bandwidth limitations & delay timebases etc. So its hard to go wrong with these vintage Hitachi analog scopes. They are far superior to the cheap small digital scopes that have flooded the market at similar prices. For a good digital scope though you might be expected to pay well over $1000.

  4. #14

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    He obviously think a digital scope it better. It is different but not better, especially if you get one with a low sample rate. Like I said, look for one that specifies a large screen buffer. At least you'll get something that you won't grow out of too soon.
    Most of these cheap scopes only have enough screen buffer to fill the current display. It takes away all the advantage of using a digital scope.
    Dwight

  5. #15

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    Hi, unfortunately i have no glue about oscilloscopes (yet). Today i was looking for an older analog scope in a lower price range. Because i dont know whether i will still use it after this PET project. Therefore i hesitate to spend too much money. On the other hand i am interested in learning how to use such an equipment.

    I found a Hitachi V-212 near by and it obviously also uses a trigger (whatever it is used for) and has 2 channels. Not sure whether it has H and V sync separators? I also dont know about the screen buffer size. Is this to save the screen for later view? Sorry, i really dont know what it means and the seller also dont know.

    The unit only goes up to 20 MhZ but as my interest is about retro computers (IBM, Commodore, Apple) from the 70s and 80s i dont worry that much.

    What do you think - will this unit help me? I attached the offer and the link to the manual below.

    Its again a big box but i will probably put it in my office on the desk where i sometimes repair the controllers from my company. Maybe this baby looks a bit proffesional to my customers when they visit me for meetings

    https://www.ebay-kleinanzeigen.de/s-...49683-168-9559

    https://mpe.berklee.edu/documents/st...n%20Manual.pdf

  6. #16

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    Quote Originally Posted by AndyG View Post
    Am going to take a punt and say the graphics board most likely allows for an 80 column display on a 9 inch screen. The NEC chip looks like extra sram . If you could dump the contents of the eprom on that board for us to look at that could confirm it.
    Hi Andy, please find attached the 6845 EPROM dump File. I also attached the dump from my EDIT EPROM. Ive been told this software should contain the driver for the graphic board. Maybe someone can confirm that?

    Thank you!
    Attached Files Attached Files

  7. #17

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    Hi,

    i found an analog scope near by. Its a Hitachi V-212 with 20 Mhz and 2 channels. Unfortunately i have no glue about the screen buffer size of this unit as i am still missing the scope basics. Also the seller does not know whats about the screen buffer. Is this a kind of memory in the unit? I think i am ok with 20 Mhz as my interest is limited to Retro Computers from the 70s and the 80s.

    I have attached the link to the Manual. Is this something i can work with?

    I have also attached the Dump from the Graphic board EPROM and from the EDIT EPROM. I have been told in the EDIT ROM Software there should be the driver for the 6845 Boardf included. Can you confirm that?

    I wish you a nice weekend.
    Best regards
    Denis

    PS: I did post all this already on friday but it never showed up in the forum. It also always takes a very long time between sending a post and before being visible here - also without attachements. Is this normal?


    https://mpe.berklee.edu/documents/st...n%20Manual.pdf
    Attached Files Attached Files

  8. #18

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    Hi,

    i found an analog scope near by. Its a Hitachi V-212 with 20 Mhz and 2 channels. And it can handle an external trigger (whatever it means). Unfortunately i have no glue about the screen buffer size of this unit as I have no scope experience and knowledge at all. Also the seller does not know whats about the screen buffer. Is this a kind of memory in the unit? I think i am ok with 20 Mhz as my interest is limited to Retro Computers from the 70s and the 80s. Is this something i can work with?

    I tried to attach the Dump files from the Graphic board EPROM and from the EDIT EPROM because I have been told in the EDIT ROM Software there should be the driver for the 6845 Board included. But my posts with the files do not show up here. I tried on friday and on saturday. I zipped both files. Maybe the forum administrator keeps rejecting this? Can you please advise how to upload these bin files?

    I wish you a nice weekend.
    Best regards
    Denis

  9. #19

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    Hi,

    i am doing some progress.

    1) I de- installed the 6845 graphic board and dont get the black screen anymore but the same garbage screen as with the board. The reason was a bad contact to the board on the 74191, Pin 7 at G5. Therefore the CLK1 signal did not move on through the board circuit but did still go through the red wire which connected to the 6845 board. I will now try to fix the system without that board

    2) I familiarized myself with the retro scope i bought last week (Hitachi V-212). Still need to learn a lot how to use it efficiently but i get used of it

    3) I used 2x 40 pin sockets to prepare a NOP Generator together with my new old stock 6502 (i tested in my Apple 2 so this CPU is definitly ok!).
    Today i measured the right frequencies on all adress lines ok!
    Now i also know that my logic tester is working properly i was quickly checking some adress lines on the RAM and ROM. There seems to be a lot of life on the messy board!

    4) I am still struggling why i dont get a random garbage screen. After every power cycle it is always the same picture (see attachement). I did watch the Video from Frank when he repaired an issue on the graphic circuit and see some similarities. I have checked the input (Pin6) and the output (Pin7) of the 74LS157 (at F6) and i believe there is not a good signal on both (see attachements).
    But i am not sure. I already replaced that chip but no improvement. I also replaced the other 74LS157 chips (F5, F3) but again no change. I still stuck on the same garbage picture.

    Any suggestions how i should move on with my troubleshooting? Unfortunately i still do not understand how the graphic signal is routed.
    I still dont know whether the non- random garbage screen is due to a faulty graphic circuit or because of something else.


    Garbage without graphic board.jpg
    F6_Pin6.jpg
    F6_Pin7.jpg

  10. #20
    Join Date
    Feb 2009
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    Southern California, USA
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    Denis,
    When the two video RAM chips (F7 and F8 ) are removed, you say you get a checkerboard pattern, that says that a lot of your video data path is OK. It leads me to think that something around the video RAMs is the problem. First check the sockets and the 2114 RAMs for good signals. Check that signal 'TV RAM R/W' at F8-pin 10 is high or mostly high. Did you say you already replaced the video RAM?

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