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  1. Thread: Swtpc 8212

    by falter
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    Swtpc 8212

    While waiting for some stuff to finish up the homebrew terminal I got, I've been cracking open this SWTPC 8212 I've got.

    I put some pictures here: ...
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    I'd love to see the motherboard in this one. I'm...

    I'd love to see the motherboard in this one. I'm curious if it has the oddball board mine has. They are close enough in serial number.
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    I haven't much time tonight but I hooked up the...

    I haven't much time tonight but I hooked up the recorder just to see what would happen. If you aren't punching anything and just working with the computer, it produces a continuous leader tone. ...
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    Maybe I should just go ahead and build one of...

    Maybe I should just go ahead and build one of those 1702 EPROM reader/burners suggested to me here and elsewhere. Might be an interesting project, and useful from time to time.
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    For sure. I'd like to find out how the tape...

    For sure.

    I'd like to find out how the tape encoding system works. I'm curious how it is initiated/stopped. On an AC-30, you have to toggle a switch on and off to record what it coming off the...
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    It actually kind of seems like the more stuff you...

    It actually kind of seems like the more stuff you have onscreen, the more likely the horizontal sync changes. Hrmm.
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    Here is a video of it operating more or less as...

    Here is a video of it operating more or less as it should:

    https://youtu.be/oOEAWxSb1PQ

    I found a bad 74193 in the video board that wasn't counting across columns properly. I also (re)learned...
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    I have been working on documenting the circuit. ...

    I have been working on documenting the circuit. It is tedious work, and a bit mind bending because of how hard it is to distinguish the wires from one another.

    In terms of troubleshooting I've...
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    Another question... can you have all those side...

    Another question... can you have all those side cars *and* a Racore expansion?
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    Ok. I'm going to need a picture of this...

    Ok. I'm going to need a picture of this arrangement all setup. :)
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    The chargen is connected to a 74165. There are...

    The chargen is connected to a 74165. There are also two 7495s on the same board that seem to interact with it somehow. I'm trying to sort through the tangle of wires but it's tricky. I'm wondering...
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    Okay. Let me work on undoing my mistake here and...

    Okay. Let me work on undoing my mistake here and then I'll hook up and we can see where this goes.
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    I'm game for using this Rigol if someone wants to...

    I'm game for using this Rigol if someone wants to give me a primer. I have been (slowly) working through the manual, some videos, etc. I find it a bit over my head. This stuff is sort of math...
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    If you've seen the pictures, the EPROM I'm...

    If you've seen the pictures, the EPROM I'm referring to is marked '3' and is installed on the board furthest to the right. I call it the cursor board because when it is removed the cursor is gone. ...
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    Ok.. did I do the wrong thing? I removed the...

    Ok.. did I do the wrong thing? I removed the 390ohm resistors from the cursor board, and replaced them with 1k. This caused the entire TVT to essentially freeze up - I can see the characters...
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    I think the problem is when I look at the...

    I think the problem is when I look at the 'orange' on that band I see gold or light brown.
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    It'll take me a while to wrap that around my...

    It'll take me a while to wrap that around my brain.

    In the meantime, I looked at the resistors on the other EPROMs. In fact, only the EPROM on the UART board also has resistors attached. The one...
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    Ok so it's not about the EPROM trying to drive...

    Ok so it's not about the EPROM trying to drive things up to "1" per se, it's about trying to hold back the TTL logic and prevent them from dragging the EPROM up to that level?
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    Thanks Dwight. Interesting. I'm guessing this...

    Thanks Dwight. Interesting. I'm guessing this thing must have worked at some point though?

    I'm still trying to wrap my head around this resistance stuff. :) I know it makes sense but in my...
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    So you're saying in order for the bypass of the...

    So you're saying in order for the bypass of the 1702 to work, you'd need stronger resistors?
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    Ok. I sort of assumed the 'reset' one was the...

    Ok. I sort of assumed the 'reset' one was the one ww wanted to look at, since it seemed to react to the logic probe most directly with a change in onscreen output. I should try it on the UART EPROM...
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    Homebrew Xerox 820

    Have this machine with zero power, so I was looking at the power supply (made by Borschert) and saw this rusty wire connected to a rectifier diode. I'm going to take this apart tomorrow and test...
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    The end not connected to the 1702 data out pins...

    The end not connected to the 1702 data out pins is connected to ground.

    Here is a pic of them... I've always had trouble reading these. My DMM says 470ohm but I don't know how accurate that is.
    ...
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    I just tried replacing the EPROM with wires again...

    I just tried replacing the EPROM with wires again and typing.. basically nothing coming out... just cursor moving as I'm hitting keys. I can occasionally see bits changing on the address lines. But...
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    I could not make out what the output was with the...

    I could not make out what the output was with the wires in place. It basically confined itself to the very first column and produced junk that overwrote itself so quickly I had no time to make it...
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