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Thread: Poly 88 case

  1. #141
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    And here I was going with what I was told was the schematic. So, is there a schematic for rev. F?

  2. #142
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    I thought it was the right schematic.. it was very close... I didn't realize there was such a huge difference between 1.2 and F when it came to pin 49. It looks like they had many different revisions over the years. I have looked and looked but have not found any schematics for F. May not exist.

  3. #143

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    Usually it is more random on the values but it looks relatively good. IC1 and IC2 are configured for the 1.8432 frequency, so that is good. The image you have does not seem to indicate any particular failure at this point. When you have the V4.0 monitor and keyboard connected, we'll know more. You should still cut the lead going from pin2 of IC1 to the IC next to the crystal. You'll be loading down the driver on the CPU card and possibly shortening its life.
    In general I would not remove jumpers. Often these are factory installed so to solve routing issues. The boards is still very similar to the original. Can you note which pins have wires and which IC number and what 74XX part they are, in case the IC number doesn't match the schematic. The boards should be almost identical in function, except a little extra circuit for the additional columns.
    Dwight

  4. #144
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    Like Dwight said, cut the trace and rejumper to there -- you can't just hook pin 49 into one of the crystal lead inputs!

  5. #145
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    Ok. Just fyi that pic was taken with the cpu board installed..

    So just for clarity.. I want to cut the trace leading from IC1 pin 2, and then jumper from pin 49 to there? Or to the IC pin it was leading to?

  6. #146
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    You know, I'm looking at it here.. I don't see *any* traces leading from IC1 pin 2 to anywhere. On the back side it's not connected to anything. On the front side, I can see a trace running beside it.. but that's it. Unless the trace runs backwards under the IC... I'm not seeing it connected to anything.

    EDIT: NM.. I found it.. it does run under the IC and then out to IC45 pin 7 (74LS04)

  7. #147

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    Cut the trace and run the wire from 49 to IC1 pin 2. Are you sure it is 7? that is ground!!!!!
    I suspect it is pin 8. Pleas check!!!


    If the CPU doesn't have the right boot code, at most it should clear the display. As I recall, you have to do something to get into the monitor. I forget what.
    I'll be off to lunch soon so won't be able to check on it until after I get back.
    Dwight

  8. #148
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    Might be pin 1. Not at home but I'm looking at my picture of the back of the VTI.. I see the passthru under IC1.. and the trace goes to the bottom right most pin. I see the ground pin (I think) at the other end of that row. So maybe pin 1? I get confused when I flip these things over.

  9. #149

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    There is a feed through, under IC1. I'll bet that is connected to pin two as it goes in the desired direction, towards the crystal. Use your ohm meter! Don't guess.
    Dwight

  10. #150

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    One other thing I recall, the System88 used a different memory location than the Poly88 V4.0 code did. If it came from a System88, the address decoder would be set wrong. I don't recall what it was for either.
    a more readable schematic at: http://bitsavers.informatik.uni-stut...anual_1977.pdf
    Dwight

    I did some digging. I think the System88 was at D800 and the Poly88 was at F800. The jumpers may have to be changed to match.
    I'll look some more when I get a chance.
    Dwight
    Last edited by Dwight Elvey; March 14th, 2018 at 04:00 PM.

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