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

  1. #171

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    If I recall correctly, the wire was related to blocking the read of the video RAM while the external board video board RAM was attached.
    It sounds like you might need some soldering lessons. You should be able to tell the difference between a gob of solder and a proper joint. Just because it is working is not a good criteria.
    To do a good job requires a clean tinned tip and fresh flux ( not plumbers flux please ).
    Dwight

  2. #172
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,800

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    Yes, you don't want to go connecting that pin to GND without a good reason . As Dwight correctly states, that pin being low prevents the CPU from reading the contents of the video RAM. No wonder my PETTESTER was complaining...

    By the looks of it, a WRITE by the CPU would have occurred to both the PET original screen memory AND the bolt-on video card. But a CPU READ must only come from one or the other (otherwise you would get bus contention and the CPU would read garbage). This must have been the way the original owner had disabled the PET video RAM.

    As to the cassette read/write issue. The cassette write for both of the recorder/players is sourced from pin 13 of the VIA (6522) UC5.

    The READ lines, on the other hand, are processed separately.

    Cassette #1 (J3) PIA (6520) UC7 pin 40.

    Cassette #2 (J6) VIA (6522) UC5 pin 18.

    If you checkout the schematic (http://www.zimmers.net/anonftp/pub/c...N/320349-3.gif) you will see there are a couple of links on the PCB. Are these intact or not? Just a thought.

    Have you tried writing a tape via (say) cassette port #1 and then trying to read it via both cassette ports #1 and #2?

    SAVE "FRED"
    LOAD "FRED"
    LOAD "FRED",2

    You could also hit 'PLAY' on the cassette drive and monitor the appropriate pin(s) with an oscilloscope or logic probe (PIA and/or VIA) to see if any signals are present. The signal level from the cassette should be 5V or so - so it should be obvious. To get a comparison, perform a SAVE from the PET and monitor the cassette WRITE signal. It should be similar on a LOAD/READ.

    Keep your girlfriend well away ! They have a nasty habit of trying to dispose of vintage computers...

    Dave
    Last edited by daver2; March 26th, 2020 at 11:23 AM.

  3. #173

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    Hi.

    I am considering this Pet as FIXED now although i could not check the IEEE-488 Interface and Parallel User Port yet. Both of the last issues have been solved by replacing the 6522 VIA Chip. The system is now running its original Firmware (Microsoft Basic 3.0) and can read/ write from/ to cassette tape.

    The tape reading issue i was able to solve by:

    - cleaning the tape recorders read/ write head
    - Adjusting the Tape recorder Azimuth screw using the C64 and the SW "CASSETTE-AZIMUTH by H. Diebeck"
    https://csdb.dk/release/?id=49944
    - using another tape

    Again i want to say a BIG THANK to you all. Especially to Dwight for his patience to teach me in electronics and scope handling (lets say i now reached the next grade). Special thanks also to Daver2 and his nice Petester. And also to dave_m, MikeS, AndyG and all the others who gave me nice support in repairing my old Commodore
    For some of you I may appear quiet obtusely but please note my background is mainly mechanics and i started to work on electronics in this level only few years ago.

    I did really enjoy this journey of troubleshooting and i learned a lot. You are a great forum!!!! I will certainly stay here and will try to help others so far i can.

    And one more thing....about my soldering skills
    I feel confident in de- soldering and soldering and i like this kind of tasks because i am a mechanic. For me it is very satisfying. And yes i always got a clean tinned tip and fresh flux. I know the Basics! And yes, i messed up one important soldering joint and Dwight (as my new Internet teacher) has the right to criticize me. But my other joints are perfect and conductive. I did tell my story to help others not to repeat my mistakes!

    I hope, after this crisis i will have the opportunity to come back to California to show him. This is not unlikely as my company got a factory in Carlsbad


    Space Invaders running.jpg

  4. #174
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,800

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    You can never go wrong with space invaders!

    Nice job!

    Dave

  5. #175

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    I enjoyed the hunt.
    Here and at work, I always try to clearly convey my thoughts, in a clear way. It is difficult but I am learning as well. As I'm getting closer to retiring, my boss wants me to pass on my thinking and problem solving to the next generation. I need to pass on information clearly and without getting frustrated. When first asked to do such, I didn't know how to begin.
    Anyway, I can sum it up in a few words.
    First what is it suppose to do that it is not.
    What do I expect to see at each point in the circuit, based on what I saw last? Remember, it isn't working so I shouldn't expect to see that it is.
    When possible run experiments to better understand the problem.
    Working with some parts of computers is relatively easy. One sees, such and such on one side of a device one thing but on the other side it isn't what is expected. This works looking forward and backwards.
    Micro processors make it a little more difficult. Being able to write and execute test code is always one of the main ways to deal with these machines ( remember, experiment! ). Dave's test ROM is great for this.
    Years ago, I was responsible for test procedures of a particular board at Intel ( what was called the analog board for the floppy controller ). I was tasked with showing the technician on the line how to trouble shoot them using a pile of what was called "yellow dot board inventory". ( we put a yellow dot on failed boards ) I used the meter and scope to show the tech what to look for at each point of the design. I thought I was doing really great as we were zipping through the boards as many had the same problems but I though I was doing great on showing what to look for. When done, I realized that I'd wasted my time. The tech was just writing down a list of things to have reworked on the board. No symptoms, just a list. D45, D16, U34 and so on.
    He was creating an easter egg replacement list.
    This is why you'll see me get frustrated when I see things like "I replaced all the capacitors and it is still broken". It is like saying "I replaced all the tires on the car but it still won't start".
    When I help, I try to make it a learning experience for me and for the person that is doing the looking. I try to not do too much of "just show the picture" but also make sure to include what it is suppose to be doing or what is expected and why.
    I suspect that is what I forgot to do with the tech. I should have been spending more time analyzing the design with him so he'd learn how each part worked or didn't.
    So, as I say, it is a learning experience for me as well. It is helping me at work to be able to teach others how I solve problems so I won't be just leaving a big hole when I leave to retire.
    Dwight
    Last edited by Dwight Elvey; March 27th, 2020 at 06:55 AM.

  6. #176
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,910

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    Quote Originally Posted by miata View Post

    Again i want to say a BIG THANK to you all.
    I am proud of you. You did good saving a classic Commodore PET.
    -dave_m

  7. #177
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,910

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    Quote Originally Posted by Dwight Elvey View Post
    I enjoyed the hunt.
    Dwight,
    Excellent article on the 'Art of Troubleshooting'. I have saved it as a reminder not to jump to conclusions and 'shotgun' so much! A little analysis and experiment early can save a lot of time.
    -Dave

  8. #178

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    I thought it would be nice to share some impressions from the Unit before, during and after repair. The first photo shows the Pet at its new permanent Home - just below my Home Office desktop! Always ready to play a Space Invaders session
    My next project is on the second photo. The "Volks Computer"

    Pets new home.jpg
    Father and son.jpg
    Mainboard after restauration.jpg

    And hereby some older photos:

    mainboard before #1.jpg
    mainboard before #2.jpg
    mainboard before #3.jpg
    Pet in Dishwasher.jpg
    dead 6502.jpg
    workbench.jpg

    best regards
    Denis

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