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Thread: Composite picture from a CRTC PET

  1. #71

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    Quote Originally Posted by giobbi View Post
    The PET to VGA through the CGA -> VGA adapter still doesn't work, despite all the tests I did with jumper in every position and both edit ROM (standard and ntsc).
    Conversion adapters can be a little finicky about the exact timing and pulse widths. A CGA monitor itself would not be. One thing is if you could get your hands on a real CGA monitor to test your basic CGA signals. Or look with the scope closely might help. If you were going to put a replacement monitor in the unit a vintage monochrome CRT could be more authentic looking. What happened to the original monitor (if you have not already mentioned that on this thread?) The reason I ask is that a lot of the time an equivalent CRT can be found and the boards are almost always repairable. But I see you need it mainly for servicing.

  2. #72
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    Quote Originally Posted by Hugo Holden View Post
    Conversion adapters can be a little finicky about the exact timing and pulse widths. A CGA monitor itself would not be. One thing is if you could get your hands on a real CGA monitor to test your basic CGA signals. Or look with the scope closely might help. If you were going to put a replacement monitor in the unit a vintage monochrome CRT could be more authentic looking. What happened to the original monitor (if you have not already mentioned that on this thread?) The reason I ask is that a lot of the time an equivalent CRT can be found and the boards are almost always repairable. But I see you need it mainly for servicing.
    The original crt was broken by the parcel service... Yes there are replacement CRTs, but they would be too expensive,
    Considering expedition to Brazil, import fee and the risk to receive another broken CRT...

  3. #73

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    yes, it can be a risky business shipping CRT's. They always need double boxing and then some luck too.

  4. #74
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    Quote Originally Posted by MikeS View Post
    Glad it's working!
    Mike,
    Yes, and it hasn't escaped my notice that your reminder about PET polarity jumpers got Giovi to check his jumper configuration and fix his problem. Daver, Hugo and I do all the work and you steal the glory. Good job!

    P.S. Would a European 50 Hz EDIT ROM help Giovi's jitter? Do they actually run at a 50 Hz Vertical or merely correct the jiffy clock?

    -Dave

  5. #75
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    Quote Originally Posted by giobbi View Post

    I'm waiting for the composite video saga produce some results (I think I will wait for Dave to build the board, to debug it, and to post here the working circuit, LOL).
    Ha! OK, but I was thinking of trying a Composite-to-VGA adapter (I just ordered) to avoid buying an old TV. I might run into the same issues you did with the CGA adapter. Now that will be funny!
    -dave_m

  6. #76
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    Quote Originally Posted by dave_m View Post
    Good work.
    But you may be reading an old schematic. On the latest schematic (Universal 2), both jumpers are tied to separate pull up resistors. By grounding jumper 3, the HSYNC is inverted to positive going which is what the MDA wants. The MDA Horizontal frequency of 18.43 KHz is closest to the 20 KHZ of the standard 8032 EDIT ROM. So that is why it may be working with the standard ROM. But the MDA Vertical frequency is 50 Hz, so there may be some incompatibility.
    See the chart HERE for MDA parameters.
    Really I was looking at first release of 8032 schematics.

    But... the difference from early release: http://www.zimmers.net/anonftp/pub/c...8032029-10.gif

    universal release: http://www.zimmers.net/anonftp/pub/c...8032081-10.gif

    and latest universal release: http://www.zimmers.net/anonftp/pub/c...8032087-10.gif

    is only the way the J2 and J3 are set.

    In my board, UC2 pin 2 and 5 were connected together to pull up trace. I had to cut a trace on the board before to put the jumpers.

  7. #77
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    [QUOTE=dave_m;619467]...Would a European 50 Hz EDIT ROM help Giovi's jitter? Do they actually run at a 50 Hz Vertical or merely correct the jiffy clock? [quOTE]

    Quite possibly and worth a try; the PET itself is strictly DC operated and doesn't know about line frequency but I think the issue is interference between the power transformer and the monitor and/or monitor signals getting back into the PET logic. Just out of curiosity I once tried the opposite, a 50Hz ROM in my 60Hz Canada location and there was definitely some noticeable jitter.

    I recall Radio Shack and a few others had sort-of-similar interference issues back in the day when you would have mysterious diskette read errors that disappeared when you changed the relative orientation of the drive vs. the display and power supply.

    As mentioned there is no actual line frequency on the PET board anywhere so yes, the point of adjusting the jiffy clock divisor is to compensate when you change the frame rate from which it's derived

    Let us know how the composite-VGA adapter works out; the world is always looking for more data points since there is quite a variation in the quality of those units.

    I'm going to have to try the PET>Monochrome (MDA) monitor experiment; seems to me it should be much crisper than a composite or CGA display.

    BTW, if you look at Steve's colour PET you'll see that aside from the colour aspect it drives a CGA monitor with no other mods aside from the 20kHz>15kHz change:

    http://www.6502.org/users/sjgray/pro...pet/index.html

  8. #78
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    Quote Originally Posted by giobbi View Post
    Really I was looking at first release of 8032 schematics.
    OK, I see that the early version of the 8032 did not invert the VSYNC. Perhaps it used a different video board?

    Try the 50 Hz EDIT ROM and see if that helps the jitter you are seeing. Use a 2716 EPROM.

    edit-4-80-b-50Hz.901474-04.bin

  9. #79
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    Quote Originally Posted by dave_m View Post
    OK, I see that the early version of the 8032 did not invert the VSYNC. Perhaps it used a different video board?
    Which schematic are you looking at?

  10. #80
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    Quote Originally Posted by MikeS View Post
    Which schematic are you looking at?
    Never mind

    I misread that as there not being a jumper on the old board. Interesting; I wonder what the default was on the -081 board.

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