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Thread: Homebrew TVT I picked up

  1. #191
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    Does the Rigol have an "AUTO" button? Start there--hook a probe on something and hit AUTO and see what gets displayed.

    But you should probably start by adjusting the compensation on your probes. The user manual tells you how to do that.

  2. #192

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    I'm looking at the online manual for the 1000 series. For the most part, I don't see anything with the Math button that will be useful in debugging your board. Pay more attention to the Vertical, Horizontal and Triggers. Those will be the most important things to deal with. The Math operations are post processing of the signals you've sampled. There are a number of really nice feature in the Math stuff but nothing you might want to use on your board.
    Dwight

  3. #193
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    Okay. Let me work on undoing my mistake here and then I'll hook up and we can see where this goes.

  4. #194
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    Dwight, I didn't know that you had a Rigol 1000Z-series scope.

  5. #195

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    First, look at the character generator chip( https://www.atariarchives.org/cgp/Ch02_Sec04.php ). You can see the bits it expects to send are set up as rows and columns. It has outputs that correspond to the rows to send as the dots. Note that the columns are addressed backward from D6 to D0. This is simple to handle, D6 will be the first dot to send to the video and D0 will be the last. These need to be serialized to the video. This would be done by the dot clock ( most likely the clock from the crystal or divided ). It could be serialized 2 ways. One would be to have a shift register that can be parallel loaded into a shift register but I don't see any specific shift chip for that. It could be done another way, using a 6 to 1 mux and a counter. I don't see anything that looks like that either. A shift register can be made with individual flops and some gates. There are some 7474s or 7473s that could be used for that purpose. Since these need to run with a high speed clock, you might look at these with your scope.
    There are a number of counter chips. There would be a video character counter. This needs to run at the next lower speed than the dot counter. I can see some 7485 chips. Those are comparator chips. The input character counter would be compared with the video character counter to create the cursor on the last row of a video character. I suspect one each of the input channels of these, 7485, chips are used to determine the cursor position. You might trace these inputs to see where they go. It is a little complicated as to how the video counter works. For each character, it needs to count across the row for each character and then increment the character count for each of 64 characters. It must then increment the row counter for the next row of dots. It must stay on one character until it completes a row of dots from the character ROM. It must also reset the LSBs of the video character counter such that it repeats the video character LSBs counter for each of the 64 characters, the number of rows for the character ROM dot rows. When it has finished the last row of dot bits, it must then increment the MSBs of the video character counter. To start the next row of characters. 64 character are counted with 6 LSB bits. The rows of characters are counted with 4 MSB bits. Since these counter run continuously because they keep the display running.
    The other side of the 7485s would then only change when one type a character and then only once per character. By scoping out the 7485s, we should be able to determine which chips are for what.
    The chip(s) connected to the output of the character ROM ( 6571A ) should either go to a shift register ( possible multiple flops ) or to a mux ( 74157 or gates ).
    Once we know which is the input character counter, we can use the trigger on the scope to see what each of the data bit is coming from the RAM. I suspect the RAM bits go to one of the EPROMs near the UART. That would leave one EPROM left to figure out.
    Once you have those pieces identified, we can do some experiments to figure a lot of things out and see why things are working so poorly.
    Find and mark what you think are the video character counter chips and the input character counter chips.
    Dwight

  6. #196
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    The chargen is connected to a 74165. There are also two 7495s on the same board that seem to interact with it somehow. I'm trying to sort through the tangle of wires but it's tricky. I'm wondering if it'd be easier to use the other logic probe I have with the pulse function to suss these out.. if I understand that one correctly, you can hook it up to a given point on the board, and then send a pulse around that you can then find with the probe?

  7. #197

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    Your really better of using your scope. You can't see if a signal is cyclic of not symmetrical. Clocks will be symmetrical data not. Oops I just edited. it was a 74165. That is the right one to serialize the output of the character generator.
    Dwight
    Last edited by Dwight Elvey; July 5th, 2019 at 09:42 PM.

  8. #198

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    The Row inputs to the 6571A should go to the dot row counter.
    Dwight

  9. #199

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    Quote Originally Posted by Chuck(G) View Post
    Dwight, I didn't know that you had a Rigol 1000Z-series scope.
    No, I was looking at the manual online.
    It has a bunch of math and logical operations. On the math side, you can do things like FFT or exponential. Logically, you can to things like ANDing, Oring or XORing saved patterns. These would be useful for some digital operations but generally of little use during normal debug.
    As an additional note the best doc for 6571A is:
    https://bytecollector.com/archive/ma...cket/M6570.pdf
    Dwight

  10. #200
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    I have been working on documenting the circuit. It is tedious work, and a bit mind bending because of how hard it is to distinguish the wires from one another.

    In terms of troubleshooting I've decided to order a replacement card socket before I go further. As you can see by this picture, the socket took a heavy blow while this unit was in transit, as did the keyboard, which broke the PCB in a few places. The socket was literally broken into two pieces. Because the two sides of the socket were held by screws, I realigned the sides, pulled out broken plastic and make sure the 'teeth' were lined up. From a visual and continuity perspective they seemed to be, but maybe not. That cordless drill battery you see in the photo causes everything to work just as well as if I'm probing with the logic probe - I realized the amount of force I was using with the probe was bending the board down, so I tried simply putting weight on it, and.. it works.

    I'm thinking either the broken socket or possibly a cold solder joint somewhere on the card might be giving me issues. Anyway, I'll do due diligence and replace the socket before going further on diagnosis.

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