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Type: Posts; User: Dwight Elvey

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  1. Replies
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    Yes, the trailing the next ROM after the trailing...

    Yes, the trailing the next ROM after the trailing edge, where it goes high, of D8-20 is D9 data. To see the square wave of D8 at the start, you'd need to move the probe to D7-20 from D8-20. ( at leas...
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    Do remember, we are only looking at the trailing...

    Do remember, we are only looking at the trailing edge of the select from D8, so we only see the beginning of the data on D9. It might be good to look for the possible failure of the other ROMs. In...
  3. DIEN* can be driven by two sources. One is from...

    DIEN* can be driven by two sources. One is from the 8080 processor from its WR* signal. The other is from the front panel. Remember what I said about the processor, it only does two things, JUMP and...
  4. The sequencer was really a tough one to write...

    The sequencer was really a tough one to write code for. All kinds of row and column jumps. It always reminded me of playing chess or checkers. Each move dominated the next move possibilities. The...
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    The question is how can one fix it. Try moving...

    The question is how can one fix it. Try moving the ground connection closer to the signal connection. As Hugo says, there is not enough information to say much of anything.
    Dwight
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    I did some careful offsets and it looks like the...

    I did some careful offsets and it looks like the traces from the left go between the pins of the 6402. If they went to the pins, some would need to go to the read data.
    It would be much better to...
  7. When you look at the second plot of the ripple,...

    When you look at the second plot of the ripple, the first thing to notice is that the peak to peak is about 600 mv. Again, it is important to note the frequency. 60 Hz on a full wave rectification...
  8. So, the problem is not the read, it is the write....

    So, the problem is not the read, it is the write. I'll look at the schematic tomorrow morning. I've got some resting to do. We'll do some more test then.
    Dwight
  9. I don't know, it doesn't look that bad. I don't...

    I don't know, it doesn't look that bad. I don't even know what instructions are being run and which source is driving the bus. I other words, it is not useful information. Also, is the little 1 arrow...
  10. Measure the ripple with a good capacitor and...

    Measure the ripple with a good capacitor and you'll see why I was so sure it was a bad capacitor. Post the picture so that we can have it for others to see. Use the advance edit to get the picture...
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    I suspect there may be a clock wire and possibly...

    I suspect there may be a clock wire and possibly a data ready wire and a reset. I see lines going directly to the 40 pin chip. Is there a data circuit we can look at for this chip or is it one of...
  12. That is a lot of ripple. It is way too much. That...

    That is a lot of ripple. It is way too much. That is why I asked you to check it at the large capacitor. Use the negative post of the capacitor as your ground. There are two possibilities for that...
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    Yes, that is what I was referring to as the least...

    Yes, that is what I was referring to as the least power adapter.
    Dwight
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    But what is the driver chip. The signals look...

    But what is the driver chip. The signals look like they might be rs485 or rs422 instead of rs232.
    Dwight
  15. What is the vertical scale? What is the vertical...

    What is the vertical scale? What is the vertical volts per division?
    It looks like both diodes are there but i don't see a what the volts per division is.
    Is it 2 volts per division?
    Is this...
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    Does that mean you have the little board in the...

    Does that mean you have the little board in the back? The keyboard it self looks to be a parallel keyboard. I don't know what the board in the back does. It doesn't have enough wires to both power...
  17. Earlier, you said you were reading random values....

    Earlier, you said you were reading random values. I guess it was not what I meant. FF is not random. When you do a write the data has to go onto the cpu side of the buffers.
    I'll try to describe the...
  18. The fact that most stuff looks good and you are...

    The fact that most stuff looks good and you are reading random data means the RAM is there. ( you might check that a particular address doesn't change.
    Anyway, I suspect the buff buffers or most...
  19. First look at the rectified and filtered side....

    First look at the rectified and filtered side. That should have small 120Hz ripple. You need to measure the frequency. 60 Hz bad. 120 Hz better.The regulated side should have almost no ripple.
    Dwight
  20. When you have the chance, always tighten the...

    When you have the chance, always tighten the screws on the power transistor.
    Dwight
  21. Replies
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    They should be connected to the SEL signal on pin...

    They should be connected to the SEL signal on pin 20 on the main board but not 18 on the main board.
    If pin 20 of the EPROM is connected to ground, the EPROM pin 18 should go to the main board pin...
  22. CR4 is used as a full wave bridge recifier CR2...

    CR4 is used as a full wave bridge recifier
    CR2 is not used this way. It is used as 2 transformer center tapped rectifiers. I don't believe this can be test in circuit with the transformer still...
  23. Although, not truly relevant to this post, most...

    Although, not truly relevant to this post, most meters do not read out true RMS. They rectify the AC signal and and take the peak multiplied by 0.7071 for the display. This only works for a perfect...
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    First to your question about the 2 lines late in...

    First to your question about the 2 lines late in the sweep. No, it is not from the D8 EPROM. It is only active on the first part of the sweep it is turned off by the signal on D8 20. When a TTL...
  25. Measure J8 9 to 8 AC Volts J8 1 to 3 AC volts...

    Measure
    J8 9 to 8 AC Volts
    J8 1 to 3 AC volts
    J8 5 to 3 AC volts

    J8 2 to 3 or 6 DC volts
    J8 4 to 3 or 6 DC volts

    Dwight
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