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Thread: PDP-9 at the RICM

  1. #21

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    Quote Originally Posted by anders_bzn View Post
    "Someone" has given me a new PDP-8 project to finish first, I should get the RK8E and the RK05 drive going.
    I really should stop forwarding stuff to you Anders But it happens to be so that you are so damn good to get it working. And that is what important... Much better than have it sitting on a shelf.

    Besides, I am about to start working with two RK05 drives for the 11/45 and then it would be great to discuss with someone.

  2. #22
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    Quote Originally Posted by MattisLind View Post
    Besides, I am about to start working with two RK05 drives for the 11/45 and then it would be great to discuss with someone.
    I'd be happy join that thread, too! I did one RK05 last year for my 11/45, and am just starting in on the second one now.

  3. #23

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    Quote Originally Posted by MattisLind View Post
    I really should stop forwarding stuff to you Anders But it happens to be so that you are so damn good to get it working. And that is what important... Much better than have it sitting on a shelf.

    Besides, I am about to start working with two RK05 drives for the 11/45 and then it would be great to discuss with someone.
    Thanks for those nice words. I appreciate the things I get, its fun to get them going! And YES we should start the RK05 repair thread. My first issue is to remove all foam and find something to replace it with.

  4. #24
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    Quote Originally Posted by anders_bzn View Post
    My first issue is to remove all foam and find something to replace it with.
    I used self adhesive foam weather strip that I bought at the local hardware store. Two layers of 3/4" x 7/16" worked for the blower. One layer of 3/16" x 5/16" worked for the disk pack seal.

    I have some experience with RK05 drives from fixing the one on my PDP-8/e and the one on the RICM's PDP-12.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  5. #25
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    Mmm ... maybe time for a new thread?

    Or this old one - http://www.vcfed.org/forum/showthrea...8E-Restoration
    Last edited by jackrubin; March 1st, 2019 at 04:47 AM.

  6. #26
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    Whoops - missed the edit window - and also here:

    http://www.computerarium.org/doku/do...05_disk_drives

  7. #27
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    Jul 2014
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    We powered up the DEC PDP-9 on Saturday and tried some simple functions. We can write/read core, and executed back-to-back JMP instructions. Since this is a microcoded machine a large amount of the processor need to be functional to perform these simple functions.

    While it was running the JMP instructions bit-9 stuck on, so likely there is an issue with a bus multiplexer. It is a hard fault, so we should be able to localize it by swapping multiplexer and register flipchips.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  8. #28
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    The attached image shows the organization of the PDP-9 processor. Since the stuck bit-9 shows up in all of the registers the problem was most likely to be in the Adder or one the multiplexers. We swapped the B131 Adder modules in for bits 8 & 9 that are in slots A21 and A23. The stuck bit moved from bit-9 to bit-8 so we now knew that the problem was on the B131 module that came from slot A23. At this point we don't know if the issue is the output or an input on the B131 module. It should only take a few minutes to determine that. We don't have any spare B131 modules, so we will trouble shoot and repair the B131 module.

    pdp9mc_Page_01.jpg
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  9. #29
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    We measured the input and output signals of the bad B131 module and the adjacent good modules. On the bad B131 module the SUM output on pin V was a logic 1 (-3.46V) and the A BUS input on pin S was a logic 0 (-0.11V). This explains the stuck bit-9. We installed the bad B131 module in a socket and connected it to a lab power supply. We then compared the behavior of a good and the bad B131. After studying the schematic and making lots of tests we determined that Q4, a 2N3669 transistor wad bad, so we replaced it with a NOS Motorola part. The B131 works OK now and the stuck bit-9 is gone. We also found that the CARRY INSERT signal on the V131 is used to invert the adder contents. Maybe for complimenting the AC or for 1s compliment math?

    The Adder in the PDP-9 is mostly analog with four possible voltage levels on the board. Alex is making an LT Spice model of the B131 so we can better understand how it works. Then we can string 19x B131 modules together and see how the whole adder works.

    We did some more debugging on Sunday and found a stuck bit-17 in the AC register. We swapped the B213 module that holds that AC register bit, the B169 that gates the output to the A BUS, and the B169 that gates the input, but did not see any changes. Next week we will do some more debugging and fix this issue.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  10. #30
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    Nice progress - thanks for sharing!
    Torfinn

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