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

  1. #51

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    Hi Vince,

    Quote Originally Posted by vrs42 View Post
    I have taken a hint and created an Eagle CAD project for the M840
    Then it might be a good idea to take the same approach which I did with several cloned controllers.
    Use 'modern' replacements for several chips. I've checked the M840 schematic... A few suggestions:

    8271 = 74179
    8251 > 7442
    8881 > 7438
    380 > 74HCT02
    384 > 74HCT32
    NTE9601 > 74122

    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.

    Good for you that the 8235 is not on the board. That is the hardest chip I had to replace on the clones.
    For several boards I used different solutions, just depending on other free ports. But fun to do!
    And I think it is very nice that there are more clone controllers made. This makes it affordable and
    easy to expand your PDP8.

    But I'm just still wondering what the prices are for these original M840 boards... The standard boards
    on Epay are sometimes sold for ridiculous prices. But controllers are just not offered very often...

    Besides that, the most boards are offered in the US. So shipping and customs make them
    extra expensive for me to buy and get them to Europe... So I'm very happy with my TTL clones

    Regards, Roland
    Last edited by Roland Huisman; September 11th, 2020 at 01:16 PM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

  2. #52

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    Quote Originally Posted by Roland Huisman View Post
    Then it might be a good idea to take the same approach which I did with several cloned controllers. Use 'modern' replacements for several chips. I've checked the M840 schematic... A few suggestions:
    Cool suggestions, thanks! I'll start by drawing the DEC board, then think about tweaking it for more modern times.

    Speaking of which, it appears that C65 (39uF, installed horizontally up top) on the DEC boards is installed backwards! The marking ("+") matches the way it is installed, but the "+" end is at GND, and the "-" end is at +3V. (Maybe that's why the pull-up for the 3V supply is only 47 ohms, which is much lower than on other boards?)

    Vince

  3. #53

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    Quote Originally Posted by vrs42 View Post
    Speaking of which, it appears that C65 (39uF, installed horizontally up top) on the DEC boards is installed backwards! The marking ("+") matches the way it is installed, but the "+" end is at GND, and the "-" end is at +3V. (Maybe that's why the pull-up for the 3V supply is only 47 ohms, which is much lower than on other boards?)
    Hi Vince,

    I checked both of my M840 boards and you are right. That the capacitor is placed in the opposite
    way as the PCB marking shows. But the + marking is placed wrong...

    Left is - and right is + So the capacitors are placed in the right way...

    DSC05129.JPG

    Regards, Roland
    Last edited by Roland Huisman; September 17th, 2020 at 12:59 AM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

  4. #54

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    Quote Originally Posted by Roland Huisman View Post
    I checked both of my M840 boards and you are right. That the capacitor is placed in the opposite
    way as the PCB marking shows. But the + marking is placed wrong...

    Left is - and right is + So the capacitors are placed in the right way...
    A look at
    http://svn.so-much-stuff.com/svn/tru...M840Kfront.jpg
    will show that on my M840K, the capacitor is installed to match the markings (which as you say are wrong).

    I also don't have that capacitor from 3V to the collector of Q3 (installed as rework on yours). What revision is that? (Mine is stamped "X S" in the plastic handles.)

    Thanks,

    Vince

  5. #55

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    Hi Vince,

    It looks like you have that rework capacitor as well but integrated in your PCB already...
    That rework cap seems to be connected to the -15V.
    compare.jpg

    The other capacitor C65 seems to be 90 degrees rotated on your board. Quite confusing I have to say

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

  6. #56

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    I located my M840 and the cap appears to be installed + to + as Vince indicates his is. I ohmed the + side of the cap does appear to go to the omnibus ground. Whats the other side of the cap go to? In circuit my Fluke tells me that cap is 95.7 uf which is greater than the 39 uf listed on the cap. Must be paralleled with some other caps. It is a wonder that this cap has not blown up if connected reversed.

    Attached are photos of my board. The smoke from the fires in California, Oregon, and Washington has finally reached South Dakota and we don't have bright sunlight out or I would take the photos outside to get better images. What is the deal with the forum only accepting 256k JPEG files?

    My board has some high quality sockets on some of the chips in the lower left corner area. These are clearly factory as the sockets are the original wave soldered parts. One of the sockets is empty. What is supposed to go there or is that unpopulated on your boards?
    Attached Images Attached Images
    Doug Ingraham
    2nd owner of Straight 8 SN1173
    5 other PDP-8's including an 8/i and a DECSet 8000
    SOL-20

  7. #57

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    Hi Doug,

    Quote Originally Posted by DougIngraham View Post
    I located my M840 and the cap appears to be installed + to + as Vince indicates his is. I ohmed the + side of the cap does appear to go to the omnibus ground. Whats the other side of the cap go to?
    Probably to the bus on AB2 = -15V. Then the +cap to GND and the -cap to the -15V is completely normal.
    Please check my earlier posting. I think we are a bit confused by board revisions. Vince has the
    M840K and I have the M840F. That C65 has been moved over the different board revisions...

    Quote Originally Posted by DougIngraham View Post
    The smoke from the fires in California, Oregon, and Washington has finally reached South Dakota and we don't have bright sunlight out or I would take the photos outside to get better images.
    Must be horrible! Any protections available to not breath in that smoke?

    Quote Originally Posted by DougIngraham View Post
    What is the deal with the forum only accepting 256k JPEG files?

    My board has some high quality sockets on some of the chips in the lower left corner area. These are clearly factory as the sockets are the original wave soldered parts. One of the sockets is empty. What is supposed to go there or is that unpopulated on your boards?

    Quote Originally Posted by DougIngraham View Post
    My board has some high quality sockets on some of the chips in the lower left corner area. These are clearly factory as the sockets are the original wave soldered parts.
    Are you sure? These look like they have been soldered by hand to me. But hey we have 256K pictures so maybe I'm wrong

    Quote Originally Posted by DougIngraham View Post
    One of the sockets is empty. What is supposed to go there or is that unpopulated on your boards?
    On my board is this a DEC97401 (selected 7401) but on Vince his board this is the 8881. They are pin compatible but the 8881 has
    a much higher current capability.

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

  8. #58

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    My missing chip is E33 (DEC 8881) which is the Omnibus buffer for the upper 4 bits of the reader port. Have to find a 7438 to replace it with. I don't know how close a 7438 is to the 8881 but it should probably be close enough for one chassis setups.

    The cap appears to be C65 and is the bulk filter cap for a 2.5 to 3 volt supply they made on the board. Regulation is 4 diodes in series and so the drop is between 0.6 and 0.75 volts each. I guess 2.5 volts is the reason the cap didn't explode.
    Doug Ingraham
    2nd owner of Straight 8 SN1173
    5 other PDP-8's including an 8/i and a DECSet 8000
    SOL-20

  9. #59

    Default

    Quote Originally Posted by DougIngraham View Post
    I located my M840 and the cap appears to be installed + to + as Vince indicates his is. I ohmed the + side of the cap does appear to go to the omnibus ground. Whats the other side of the cap go to? In circuit my Fluke tells me that cap is 95.7 uf which is greater than the 39 uf listed on the cap. Must be paralleled with some other caps. It is a wonder that this cap has not blown up if connected reversed.
    Your board looks a lot like mine, except for the sockets and the missing 8881. I see the blue wire near E29; do you also have the cut near BV2?

    Attached are photos of my board. The smoke from the fires in California, Oregon, and Washington has finally reached South Dakota and we don't have bright sunlight out or I would take the photos outside to get better images.
    It is *still* murky here in Oregon. The smoke seems to be trapped low to the ground by an inversion layer. Even more restrictive than the Covid stuff, as no-one wants to go out in it. (Trash pick-up was missed yesterday.) Was able to at least get to the grocery the other day, so at least that's open.

    Vince

  10. #60

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    Quote Originally Posted by Roland Huisman View Post
    Probably to the bus on AB2 = -15V. Then the +cap to GND and the -cap to the -15V is completely normal.
    Please check my earlier posting. I think we are a bit confused by board revisions. Vince has the
    M840K and I have the M840F. That C65 has been moved over the different board revisions...
    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.

    Quote Originally Posted by Roland Huisman View Post
    Must be horrible! Any protections available to not breath in that smoke?
    It is not what I would call horrible. What we have outside right now is more like a really clear day in Los Angeles in the 1970's. I can see the sun now.
    You can't even smell any smoke (yet). I was in Oregon for part of the fires and that is pretty horrible. Has it cleared up any Vince?

    Quote Originally Posted by Roland Huisman View Post
    Are you sure? These look like they have been soldered by hand to me. But hey we have 256K pictures so maybe I'm wrong
    Grrr, Grumble. You are not wrong. Looked at it without my readers before I posted. Under the magnifier I can see that they were replaced. It is a pretty good job. They cleaned up all the flux and didn't overflow the solder.

    Quote Originally Posted by Roland Huisman View Post
    On my board is this a DEC97401 (selected 7401) but on Vince his board this is the 8881. They are pin compatible but the 8881 has a much higher current capability.
    Looks like the 7438 can sink 48 ma. That is quite a lot actually. 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.
    Doug Ingraham
    2nd owner of Straight 8 SN1173
    5 other PDP-8's including an 8/i and a DECSet 8000
    SOL-20

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