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Thread: What did I do to my PDP-8 today.

  1. #61

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    Hi Doug,

    Quote Originally Posted by DougIngraham View Post
    I have the M840K revision also. And that cap is C65 on the diagram which goes across the 4 diodes in series. It is definitely installed backwards.
    Okay, good to know. You can check it better on that board.

    Quote Originally Posted by DougIngraham View Post
    How much could the 8881 do? I guess if you needed more drive you could sub a 74S38 since it can drive 60 ma. It doesn't look like the higher speed should be an issue on the application on this board.
    The 8881 = 7439 can do 60mA at the output. But the bus is only pulled up for max 15mA.
    (1K pull up at 15V limited by diode to limit on 3V. This is done to get a higher slew rate on the bus)
    The higher current output chips have the strength to pull that 15mA to a really low level.

    Since the 7401 is pin compatible with the 8881 and 7439 I would place one of these for testing.
    If you want I can sent you a 7439 or an 8881 for shipping costs. But I don't know how things
    are now regulated with shipping to the US...

    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  2. #62

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    Doug, I do see the cut near BV2, though yours is further from BV2 than mine.

    Trash got picked up today (yay!). The air quality index is better today (230+ vs 380+), so I guess it was a win for the Waste Management employees. (Jeanne says she can see the difference, though I hadn't noticed it.)

    I think the 7438 is the better choice for new work, but for replacing the old part I'd go with 7439 if I had it, and 7401 in the meantime if not.

    Vince

  3. #63
    Join Date
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    Vince, put out those fires. Although the air quality here is not bad, the night skies are rather hazy. We had a blood red sun set last night. AND the other night I could only see two stars. Seems the jet stream is running over Oregon and Washington, looping into Canada and then dropping over MInnesota and Wisconsin carrying the smoke high up.

  4. #64

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    Quote Originally Posted by vrs42 View Post
    Doug, I do see the cut near BV2, though yours is further from BV2 than mine.

    I think the 7438 is the better choice for new work, but for replacing the old part I'd go with 7439 if I had it, and 7401 in the meantime if not.
    BV2 is Power Ok on the Omnibus I think. What did they connect it to since that wasn't what they wanted?

    I see now. The 7438 is what Roland was recommending for new or rather replacement designs. 7439 has the correct pinout. I have quite a lot of TTL. Not sure if I have a 7439 though. But no hurry as I have no plans to bring that reader into service at this time. I can get a new (in bag) one off of Ebay for a little over $5. Warren had collected a lot of parts, I have not inventoried them, it is possible I have tubes of them. One of these days. I suppose I can inventory them after I paint the house while watching the paint dry or something.
    Doug Ingraham
    2nd owner of Straight 8 SN1173
    5 other PDP-8's including an 8/i and a DECSet 8000
    SOL-20

  5. #65

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    Quote Originally Posted by DougIngraham View Post
    BV2 is Power Ok on the Omnibus I think. What did they connect it to since that wasn't what they wanted?
    After the rework, it's connected to the logic that detects the "out of tape" condition on the reader.
    BTW, there's another tiny bit of rework that connects R28 to 5V, presumably disabling whatever that used to do.

    I see now. The 7438 is what Roland was recommending for new or rather replacement designs. 7439 has the correct pinout. I have quite a lot of TTL. Not sure if I have a 7439 though. But no hurry as I have no plans to bring that reader into service at this time. I can get a new (in bag) one off of Ebay for a little over $5.
    I had a look at the "ICs in tubes" tub. It seems I have several hundred 7439 here, in two bags (of tubes) that used to be 500 chips each.

    Vince

  6. #66

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    I did finish Eagle drawings of the DEC M840K. I haven't decided what to do about a "modern" version yet.

    I have a number of questions about the bit of schematic attached. From the maintenance manual I gather that the circuit around Q2 models the ramp up of reader motor speed, affecting the clock rate to approximate the ramping motor speed.

    Is there a discussion somewhere of what W1 and W2 do, and when I would install or remove them? I find the whole thing quite funky and I'm having trouble getting clear on what it does and how it works.

    Both copies of the DEC drawings that I have are pretty crappy. I made a PDF from my drawing here:
    http://svn.so-much-stuff.com/svn/tru...0/M840KSCH.pdf

    Vince
    Attached Images Attached Images

  7. #67

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    Quote Originally Posted by vrs42 View Post
    I have a number of questions about the bit of schematic attached. From the maintenance manual I gather that the circuit around Q2 models the ramp up of reader motor speed, affecting the clock rate to approximate the ramping motor speed.

    Is there a discussion somewhere of what W1 and W2 do, and when I would install or remove them? I find the whole thing quite funky and I'm having trouble getting clear on what it does and how it works.

    Both copies of the DEC drawings that I have are pretty crappy.
    I may have found another crappy scan. http://www.vandermark.ch/pdp8/upload....PC8E/pc8e.pdf

    I didn't see anything specifying what W1 and W2 are supposed to do. Only that 22AWG wire is to be used for them. Best guess is the support of different models of the reader. I will keep an eye out for more...
    Doug Ingraham
    2nd owner of Straight 8 SN1173
    5 other PDP-8's including an 8/i and a DECSet 8000
    SOL-20

  8. #68

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    Quote Originally Posted by Mike_Z View Post
    Vince, put out those fires.
    It seems our hapless experiment with extensive cloud seeding has finally managed to cause rain in mid-September. It rained here the other day, and it's cleared up nicely since. Of course, fires are still out there, and the clearing weather suggests a resumption of the regular wildfire season. Only about 6 weeks to go until the rain resumes in earnest (for fall, winter, and spring).

    Vince

  9. #69

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    Quote Originally Posted by DougIngraham View Post
    I may have found another crappy scan. http://www.vandermark.ch/pdp8/upload....PC8E/pc8e.pdf
    Um, yeah, that's about on par with what I had. You are meant to enjoy the PDF of the Eagle drawing, which is much easier to read .

    I didn't see anything specifying what W1 and W2 are supposed to do. Only that 22AWG wire is to be used for them. Best guess is the support of different models of the reader. I will keep an eye out for more...
    Near as I can guess, cutting them would lengthen one phase or the other of the clock signal. Perhaps cutting both works with some slower reader? I should hunt up the ECO document, and see if there is any insight there.

    Vince

  10. #70

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    Quote Originally Posted by Roland Huisman View Post
    For the 314 I used different approaches like a 74HCT05 (connect the open collector or open drain outputs
    together and pull them up) or a 74HCT32 in combination with a 7427. (Check the RX8E clone that I made)
    Very often there are some not used ports which can be very useful to solve a problem like the nasty 314.
    I started down that path, but the possibilities for 380 and 384 replacement mangled the layout enough that it seemed the routing would have to be totally different.

    When I got to the 314, I was thinking to replace the IOT decoding with something like the attached, which replaces 4 chips with two, I think.

    Of course, the more of that I do, the less it looks like the original PC8E.

    Vince
    Attached Images Attached Images

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