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Thread: PCJR mobo so close to being working!

  1. #1
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    Default PCJR mobo so close to being working!

    So I decided to take a crack at a PCJR board that was not working I had on my shelf for a while. What it is doing is not displaying text graphics of any kind. Technically the machine is booting and beeping as intended but there is no text or graphics but blocks where there should be graphics and nothing where there should be text.

    Alas today using a good board, I determined the fault is the XE 1503723 IC just behind the connector for the Infrared PCB on the board. When I swapped chips from the good board the machine had perfect graphics, and when I tried the faulty IC in the good board is displayed the same bad onscreen graphics.

    So apparently the 1503723 is an Analog Multiplexer / Demultiplxer made by Motorola, and IBM used it in other machines like the 5140 portable. But I haven't been able to find a source to buy one. Anyone out there familiar with this IC?
    214332.jpg

  2. #2
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    According to the schematic that chip you're pointing to is the character ROM, not a multiplexer. (Does the PCB have chip positions silkscreened under the sockets? That looks like position ZM64, which is the character generator.)
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  3. #3
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    I was just going by the information I was finding on the web. The sockets are actually really good turned pin sockets that are solid with no view of what is under them.

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    I'm pretty sure that page that claims it's a multiplexer is wrong. Pretty sure that there was a recent thread about that same chip going bad; said thread was kind of catastrophic to follow because the poster kept switching between this forum and Mbrutman's PCjr forum, but, yeah, *pretty sure* it's a character ROM. Supposedly it's pin compatible with some reasonably common flavor of EPROM?
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  5. #5
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    Actually it was 3 different sites that claimed it was what I mentioned. But regardless, character ROM makes sense considering what its doing. Well IF it does take an EPROM, knowing which is supported and where the BIN file can be found would be a start.

    I was thinking of posting on the Mbrutman forums, but there was a bunch of PCjr posts here lately so I figured strike while its hot.

  6. #6
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    The PCjr's tech manual says it's compatible with 2716 or 2732 EPROMs. I'm sure the PCjr folk probably know where the BIN for it is, although I imagine you might be able to find it in a MAME ROM package for the PCjr as well.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  7. #7
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    Well I tried the Brutman PCJR forums, No one chimed in so I am guessing this is either not a documented or backed up rom or noone has a replacement method in place. Which stinks because I just need one stupid IC!

  8. #8
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    Here is a dump from a working PCJR character ROM:
    Anyone wants to try writing it and verifying please let me know. I don't have any 2716 or 2732, but I have some on order: https://www.brutman.com/forums/download/file.php?id=288

  9. #9

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    I had a problem where if I pushed on that chip it would work properly. Dunno if you think that's worth checking.

  10. #10
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    Why does that sound like a bad socket (wipers losing their springiness or a bad solder joint)?

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