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Thread: Poly 88 case

  1. #131
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    Yeah I can't find a manual anywhere that explains this board. It's either Rev 0 or 1.2. I think I'm gonna close up this machine and put this all this aside for now. Maybe hope a Rev 1.2 or something closer to the schematic I have shows up one day. Kinda flying blind without a good schematic.

  2. #132
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    One thing I did note as I was putting stuff away... there is a spot on the board marked 'Y1'.. and on the other side are the remains of the legs of whatever was there... maybe that's where the crystal was? I don't know what Y means in the nomenclature of parts placement.

  3. #133
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    Answered my own question. There is definitely supposed to be a crystal there. Check out this photo of same revision board, in that area (crystal is top, just to the left of the second IC from the left):

    s-l1600.jpg

    So that explains a lot. Now to find out what crystal it is.

  4. #134
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    There is a board jumper to select either the crystal oscillator or a clock signal off the bus.

  5. #135
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    But my board has no connection to pin 49... what other pin could it get a clock signal from?

    I'm wondering if I could patch the clock from pin 49 somewhere on this board for now..

  6. #136
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    Look at the schematic, note "7" which says "Cut trace from 49 for on-board crystal". If your board has a 74S124 installed, then that's what someone's done.

  7. #137
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    Yeah. But that schematic is for rev 1.2 of the board. I am rev F. There is no trace to cut. Pin 49's trace has been eliminated in this revision. Pin 49 goes nowhere. I imagine I could try jumpering from that pin to where the onboard crystal feeds in..

  8. #138
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    Quote Originally Posted by Chuck(G) View Post
    Look at the schematic, note "7" which says "Cut trace from 49 for on-board crystal". If your board has a 74S124 installed, then that's what someone's done.
    Chuck, it's a different board, the schematic doesn't match. It doesn't look like there ever *was* a trace to pin 49, from the pictures.

    I'd say you've found the issue with the crystal being gone. You can run a jumper down to pin 49, or put the proper crystal back in. Putting a crystal in would probably be the easiest way forward, since the schematic doesn't 100% match up to your board. You can probably use either a 2 MHz or 1.8432 MHz crystal, as Dwight pointed out there's apparently a jumper for selecting the prescale in the clock divider. Both are common frequencies (1.8432 MHz is common as the input to a baud rate generator as it divides nicely into standard x16 baudrates).

  9. #139

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    Can you measure the pins 3, 4, 5 and 6 of the two IC's, IC1 and IC2 to see which ones are connected to ground and which go to a pullup, with your ohm meter. With that I can tell you what frequency you need. I am suspecting it is a 2MHz, like the original had. The board also has a 50/60 Hz jumpers but that effects the vertical signal. Yours also looks to have the RAM for 64 column.
    I suspect we can wire it to pin 49 but I need to see which pins are effected by the jumper between IC1 and IC2. I suspect the trace jumper is for the 2MHz crystal. Also, you can see a trace from the feed through below IC1 going to the IC next to the Y1 crystal. That would be the wire to pin 2 of IC2. We should be able to make it work with two cuts and on jumper to pin 49.
    I do need to know what the current configuration is of the pins on IC1 and IC2, pins 3,4,5 and 6.
    This way we can run it like the original POLY88 used, without the crystal.
    Dwight

  10. #140
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    IC 1:

    Pin 3 - 2.4k
    Pin 4 - 2.4k
    Pin 5 - 2.4k
    Pin 6 - GND

    IC2:

    Pin 3 - 2.4k
    Pin 4 - 2.4k
    Pin 5 - GND
    Pin 6 - GND

    Just for kicks, I ran a jumper wire from pin 49 to the left side of Y1 and got this:

    20180314_092806_001.jpg

    I'm wondering if I should remove all the extraneous wiring on the back. Other Rev F and H boards I've looked at don't have any of that. Not sure what purpose the original owner had in doing that.

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