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Thread: Poly 88 case

  1. #111

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    Check that the ground lead of the 7474 is ground. It is possible that there is a bad feedthru or broken trace. It could be a failed input of one of the other chips pulling the output up. Can you measure the voltage on the two outputs of the 7474?

  2. #112

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    Quote Originally Posted by falter View Post
    Okay so I replaced the 74LS74 with a known good.. exact same problem. Pins 8 11 and 12 are all 'dead'.. no signal at all.
    I think we should make a list of places there should be clocks.
    IC30 pin8
    IC2 pin 15
    IC15 pin 1
    IC15 pin 13
    IC16 pin 1
    IC16 pin 13
    IC13 pin 13
    IC34 pin 1

    Check these and list which ones have clocks and which don't.
    Dwight

  3. #113
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    Will do when I finish work here. I can confirm that I was able to test the ground pin on IC4 (74LS74) and found it had continuity all the way down to the ground pin on the expansion edge connector. So I'm pretty confident that's AOK.

    I'm of course assuming the graphics card will produce *something* onscreen if it's just left in the unit by itself? Even just a blank pattern or blocks? That's how I've been testing it.. without the CPU card installed.

  4. #114
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    Quote Originally Posted by falter View Post
    I'm of course assuming the graphics card will produce *something* onscreen if it's just left in the unit by itself? Even just a blank pattern or blocks? That's how I've been testing it.. without the CPU card installed.
    Well there's your problem! -- unless your card has the crystal oscillator section built up (and there's no reason for it to, if it was in a Poly 88) then the board isn't getting a clock! No clock means nothing drives the shift registers, which means no video signal ever gets generated.

    Be aware that the Poly 88 CPU uses a 1.8432 MHz system clock and puts it on the 2 MHz S-100 bus line, which means that a regular 2 MHz 8080 board will overclock the video card and cause horizontal sync to be over-frequency. If your monitor has a lot of range in the H HOLD pot you can compensate.

  5. #115
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    Doh! I should have noticed there was no crystal onboard but I assumed because I was seeing pulses at a few points that it was trying to do something.

    I did plug it in with the CPU board and got nothing either. Glitch can you confirm that the composite video header has the signal pin on the left and the ground pin on the right? My unit didn't come with a composite jack and connector.. so I'm not sure if I have them reversed.

  6. #116
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    That's how mine is hooked up. Make sure the CPU card's oscillator is actually running and putting out a 1.8432 MHz signal, and that it's getting to the VTI on S-100 pin 49.

  7. #117
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    Okay. Right now I am thumbing through the manual trying to figure out what some of these jumpers on the CPU card do. For example, there is a point marked 'VT1', connected at the bottom to pin 5. It has a wire soldered into it (immediately below R5) and that goes to a point marked A right beside IC15. No idea what it does but the fact that it's soldered into something marked VT1 has me wondering. The other CPU card I have here doesn't have that.

    There's also a jumper under R9, I guesss connecting R9 to pin 9.. address? This manual is really hard to flip around in in PDF.

  8. #118
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    Okay so pin 49 is pulsing both on the bus and on the VTI card with the CPU plugged in. Interestingly, the pulse I was seeing at pin 9 of IC31 disappears when the CPU card is installed, becoming (at least to my probe) a solid 'low'.

    However nothing has changed on IC4. We still have no signal of any kind whatsoever on pins 11 and 12.

  9. #119
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    So, work backwards.

    What's on pins 1 and 2 (they're tied together) on IC5? If nothing there, how about IC15? IC19? Just keep tracing back in the schematic until you see pulses of some sort.

  10. #120
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    Checked all the pins that Dwight suggested for clock signals:

    IC30 pin8 - Clock signal present
    IC2 pin 15 - No pulse.. just solid 'low' on logic probe
    IC15 pin 1 - No pulse.. just solid 'low on logic probe
    IC15 pin 13 - No pulse.. just solid 'low' on logic probe
    IC16 pin 1 - Clock signal present
    IC16 pin 13 - Clock Signal present
    IC13 pin 13 - clock signal present
    IC34 pin 1 - Nothing at all.. no pulse, no signal.

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