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

  1. #1

    Default Commodore CBM 2001-16N - BIG MESS!

    Hi,

    Greetings from Germany

    i already read some posts here about troubleshooting the old Commodore PET. I really love the design of it and was able to find an old unit (likely 197 in my area. But it was in a horroble shape especially inside. The computer was obviously a home for mice for years! But i could not leave it there. After cleaning up a bit i started troubleshooting following some advice what i found across the Internet. The unit is somehow special. The pre owner (died some years ago) did a lot of modifications on the unit. He stacked 2 ROMīs on each other for each slot and routed the associated VCC supplies to a switch on the front of the unit. Additionally he added a Board with the MC6845P on the extension port. From there the Display is connected. He also added another 16 kb of RAM.

    My plan is to put it back to an original state - whatever this could mean....

    Before deeper touching the electronics i was able to power up the CBM and got the garbage display on the Screen. At least the Display is working Obvioulsy the Character ROM is also working othertwise i would not see any proper characters, right?

    First i measured the Voltages: 17,36 VAC / +5,00 VDC / -5,42 VDC / +11,90 VDC
    This is not so bad, correct?

    I have checked the 6502 in my Apple 2 - got only question marks in that unit - so obviously dead
    I already replaced a known good one - CLOSED

    I was able to download the ROM Software from the Top EPROMS (BASIC 4.0) and verify with the files on Zimmers.net
    They all look ok apart from the EDIT Rom. This seems to be special. However i downloaded a backup from it and put this back in the CBM. I feel confident all ROM Software is ok.

    I proactively replaced 8x RAM by 4116 units to have some spares left for the further troubleshooting.

    I also ordered 2x 2114 Video RAM to play with it.

    I did check continuity across all 16x RAM chips and found 7 bad sockets and underneath lifted traces (also happened to me while removing them!) Now, the continuity check is ok - CLOSED

    I did check continuity across all 7 ROM sockets and found on bad socket. Replaced and ok now. Just found out that the pre owner did cut the jumper between Pins 21/24 (VCC +5VDC) on D3, D4, D5 - still need to fix that

    I was hoping that i get at least a different garbage screen but NOPE! The same picture, same characters
    I was hoping to use Davidīs PETTESTE2K (Version 03 beta) to get now a better idea - NOPE Same story

    But i get a different picture when i replace the Video RAM by the 2x new ones i received thius week. The screen looks different but still not what i would expect according Davidīs Manual.

    NOW - my question is how to go on with the troubleshooting. Can you please tell me what screen looks more "normal" thus which Video RAM is the better one? I have attached some pictures.

    Best regards
    Denis

    Garbage new VIDEO RAM.jpg
    Garbage original VIDEO RAM.jpg
    original as get.jpg
    display board.jpg

  2. #2
    Join Date
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    Default

    Hi Denis,
    Welcome to the Vintage Computer Forum - Commodore Section.
    It looks like your main board has been modified. Your ROM sockets seem to have adapters to use perhaps 2732 EPROMs, and the video control is using a special board using a 6845 CRT Controller to replace the existing video control logic on the board as it perhaps failed in the past.

    Neither of the video photos look like the proper random characters that should come up so your character generator may still be in question. What were the original ROMs 6540 or 2316 or 2332 chips? The fact that you have a raster scan is a good sign your video mod is basically working, but a lot of checking may be needed.

    If your video RAMs are on sockets, remove the 2114 chips and see if you get a proper checkerboard pattern. With the character generator removed you should get a white screen or a blank screen but the raster will be seen. Is there a schematic for the special video board? Be careful removing ICs from sockets. It is easy to bend the pins. Do you have a good chip removal tool? The board looks in bad shape. Was it stored outside?

  3. #3

    Default

    Quote Originally Posted by dave_m View Post
    Hi Denis,
    Welcome to the Vintage Computer Forum - Commodore Section.
    It looks like your main board has been modified. Your ROM sockets seem to have adapters to use perhaps 2732 EPROMs, and the video control is using a special board using a 6845 CRT Controller to replace the existing video control logic on the board as it perhaps failed in the past.

    Neither of the video photos look like the proper random characters that should come up so your character generator may still be in question. What were the original ROMs 6540 or 2316 or 2332 chips? The fact that you have a raster scan is a good sign your video mod is basically working, but a lot of checking may be needed.

    If your video RAMs are on sockets, remove the 2114 chips and see if you get a proper checkerboard pattern. With the character generator removed you should get a white screen or a blank screen but the raster will be seen. Is there a schematic for the special video board? Be careful removing ICs from sockets. It is easy to bend the pins. Do you have a good chip removal tool? The board looks in bad shape. Was it stored outside?
    HI Dave, thanks for your suggestions. I found the CBM stored in a old house under construction. It looked like it was there for longer time. Unfortunately i dont have any history as the pre- owner already passed away.

    The Character ROM seems to me the only original ROM chip. All the others are 2732 and 2716 EPROMīs.
    When i removed the Character ROM one leg broke of. But i was able to download the code from there and put it on a 2716 EPROM and this one is giving me the same picture. I would not expect the Code itself is broken? I did a test by downloading the Character ROM file (characters-2.901447-10) from zimmers.net. But that gave me a blank (green all around) screen.

    And yes, when i remove all 2114 chips i get the checkerboard pattern. With the MPS2114 (the ones i found istalled) i get a lot more characters on the screen than with my "new" ones from Ebay. These are L2114-2CB GTE with 200ns. What picture looks more normal?

    I really want to take that weired graphic board out of the game (i dont found a schematic of that) but when i unplug it then i get a black screen. I did not understood (yet) how the original graphic is set up. Interestingly i found out it does not matter where to put the disply cable to - directly to J7 or to the graphic board outlet (and the inlet to J7). As long the graphic board is connected to the extension terminals J4/J9 i will get the same picture as always. When disconnecting the graphic board from J4/J9 the screen becomes blank.

    I also tried several EDIT ROMīs: the "original", the one from zimmers.net (edit-4-80-b-50Hz.german) and also Davidīs PETTESTER - always the same picture!
    This is really concerning me most that i dont see much changes when i do something. The graphic seems to be bypassed in some way. By the way there is a +5 VDC supply wire going directly to that graphic board. And some flying wires to the pins of a vew chip. I attached a photo showing that. On the back side of the board there are also a lot of flying wires.

    Attached there are some photos from the back side. One interesting spot is the socket G4 (yellow circle). The documentation says spare socket. The previous owner routed a some wires to the extension pins two drive the 6845 board . Does it make sense to anybody? Another spot is inside the red circle. The guy did bridge some CPU Pins with the 6522 VIA chip.

    Maybe that Board is too much modified so better to trash it?

    Mainboard top view.jpg
    Mainboard back side.jpg
    G4 back side.jpg
    6502_6522_Bridge.jpg

  4. #4
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    Well, it'll certainly be a challenge that could keep you (and some of us) busy troubleshooting for a while

    I'd probably start by carefully documenting all the modifications to try to figure out why they were made and the ramifications of removing them. Interesting and probably a relatively good sign that you get some video with that 6845 board.

    When referring to ICs it's a real help if you mention the location.

    What test equipment do you have, if any? Oscilloscope, logic analyzer, 2532-capable EPROM burner? Do you have the schematics for that board? Should be these:
    http://www.zimmers.net/anonftp/pub/c...01N/index.html

    I have some relatives in Dresden, BTW; lovely city!

  5. #5

    Smile

    Quote Originally Posted by MikeS View Post
    Well, it'll certainly be a challenge that could keep you (and some of us) busy troubleshooting for a while

    I'd probably start by carefully documenting all the modifications to try to figure out why they were made and the ramifications of removing them. Interesting and probably a relatively good sign that you get some video with that 6845 board.

    When referring to ICs it's a real help if you mention the location.

    What test equipment do you have, if any? Oscilloscope, logic analyzer, 2532-capable EPROM burner? Do you have the schematics for that board? Should be these:
    http://www.zimmers.net/anonftp/pub/c...01N/index.html

    I have some relatives in Dresden, BTW; lovely city!
    Hi Mike

    Oh yes, i already started to get the feeling this is a bit too much mess for me 😃
    But on the other hand I do learn a lot on this project. Just bought some equipment just for this Pet including programmer (can burn also 2532!) And a logical probe. No Oscilloscope.
    And yes I am reading through the 2001 schematic from zimmers.net.
    So, I have some basic understanding of that stuff and I like soldering

    Trying to understand the behavior of the logic probe. When I started to troubleshoot I tested on some data lines at the CPU, ROM and RAM. I only got Low and High. But no pulse. After replacing the faulty CPU and sockets I still get no pulse light on the probe but now I get on some pins a low AND a high and the sound is different. But still no pulse light on. I use CMOS setting.
    Is this showing some life circuits or is my cheap logic probe confusing me?

  6. #6
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    Quote Originally Posted by miata View Post

    I use CMOS setting.
    Is this showing some life circuits or is my cheap logic probe confusing me?
    The PET has a lot of small bipolar TTL and the 40 pin chips are NMOS. It would be better to use TTL rather than CMOS.

    There should be pulsing on many signals (clock, etc) check instructions.

    How is the J7 video connected to new board? So the new board is connected to the J4/J9 expansion connectors to get access to all the bus and address lines. I wonder how much of the PET 4000/8000 video logic is duplicated on the new board. Where are the video RAMS located? Are they still on the old main board? Has the character generator PROM been relocated to the new board?

    You will probably need access to a scope before you can fix this system. A very interesting problem to fix. Later we may have to think about removing the new video board, and just fixing the old video circuit. The old video circuits are hard to fix which is why perhaps the original owner went to the extreme of adding a new board. Or else it was for a special purpose??
    -Dave

  7. #7

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    As Dave says, you'll likely need an oscilloscope. It is just a fact of life. A lot can be done with a good logic probe but it is useful for the simpler error, of a stuck signal, not on a bus. The most common failure in ICs is an open pin unless we are looking at an over voltage failure, in which case it will be a short someplace. Most single failures are the open pin types. For TTL, an open pin float to a 1 if it is an input lead. NMOS parts don't always pick a direction but often float high or just become noisy. Knowing this, when analyzing the likely failing part can save some time.
    As for a scope, do not buy a 1Ms scope for this use. It will constantly restrict your trouble shooting. A minimum DSO is a 15Ms to 25Ms scope. Even then, a 50Ms DSO will be better. Look for one that states the size of the frame buffer. There are features associated with the frame buffer that can be quite useful.
    Another option is a logic analyzer. These are great for looking at bus related issues. These are often used most in development work as well. I tend to not use these as they require more setup time. To get a nice textbook picture of what is going on may require 10 to 15 connection. Having a pod for your specific processor is a great additional item.
    I still recommend an analog scope. Many working 100MHz scopes are available at around $100 USD. I recommend getting one with a delayed sweep function as well. As you become more proficient, you'll find valuable uses for this feature.
    Dwight

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    Quote Originally Posted by Dwight Elvey View Post
    I still recommend an analog scope. Many working 100MHz scopes are available at around $100 USD. I recommend getting one with a delayed sweep function as well. As you become more proficient, you'll find valuable uses for this feature.
    I totally agree. For an old 1 MHZ computer like the PET, a used analog scope of 100 MHZ bandwidth is all you need. I like the Tektronix 454 or 465 as they are fairly compact and have nice a delayed sweep function. Whatever you get, make sure it works either by a demonstration or, if buying online, at least pictures of it functioning showing both channels working with a bright trace. It should come with probes.

  9. #9

    Default

    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.

  10. #10

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    Quote Originally Posted by dave_m View Post
    How is the J7 video connected to new board? So the new board is connected to the J4/J9 expansion connectors to get access to all the bus and address lines. I wonder how much of the PET 4000/8000 video logic is duplicated on the new board. Where are the video RAMS located? Are they still on the old main board? Has the character generator PROM been relocated to the new board?
    -Dave
    When you look on my initial post i have attached the photos how i did get the system. There is a purple/grey/white cable which connects between the 6845 board and the J7

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