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Thread: PDP 11/05 does not complete current loop in teletype

  1. #1

    Default PDP 11/05 does not complete current loop in teletype

    I am working to narrow down why one of my pdp 11/05's does not complete the current loop connection with a Teletype ASR33. Assuming it's a power supply issue.

    Teletype works fine with other systems, controller cards work fine in another PDP 11/05. Cable is good. So it must be something in the power supply. Before I disassemble, any ideas? BTW the system worked fine until I powered it off one day, and when I turned it back on after a brief period (maybe a minute) the system would not longer cause the teletype to stop chattering like it normally would when power engaged.

    My next move will be to partially take apart the power supply enough to measure the voltages coming off the regulators.

    BIll
    @ BillDeg:
    Web: vintagecomputer.net
    Twitter: @billdeg
    Youtube: @billdeg
    Unauthorized Bio

  2. #2

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    Could be a PSU issue as you write. Current loop is using -15V so check that your -15V i OK. Not much else are using -15 unless you have core memory in the system.

    It is the current loop UART on the data path board of the CPU, right? In theroy it could also be a problem with the backplane or the M9970 board if your backplane requires such a card. But this should be more unlikely.

    Or do you have a separate controller card? M7800? Is this card the console UART at 177560? Have you disabled the UART on the data path board if this is the case?

    You could measure the voltages in the backplane. What box is it? BA11-D or BA11-K or the single height box?

  3. #3

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    It's in a BA11K chassis. I am.using the M9970, no serial card. I will check on my h745 and measure the -15v see how that's doing. I do have core memory, it appears stable. I am unsure if a marginal h745 will fail first with the current loop but still be ok enough for the core memory.

    Here are some notes I made in 2015

    http://www.vintagecomputer.net/brows...m?id=622&tid=7
    and the entire thread is here
    http://www.vintagecomputer.net/browse_thread.cfm?id=622
    Last edited by billdeg; August 8th, 2018 at 05:28 AM.

  4. #4

    Default

    Quote Originally Posted by billdeg View Post
    It's in a BA11K chassis. I am.using the M9970, no serial card. I will check on my h745 and measure the -15v see how that's doing. I do have core memory, it appears stable. I am unsure if a marginal h745 will fail first with the current loop but still be ok enough for the core memory.

    Here are some notes I made in 2015

    http://www.vintagecomputer.net/brows...m?id=622&tid=7
    and the entire thread is here
    http://www.vintagecomputer.net/browse_thread.cfm?id=622
    That kind of core memory uses -15 V compared to the newer in the MUD slots which need the h754 (-5V and +20V). So your -15 should be ok.

    You said that it works fine in another chassis. So have you checked continuity in the backplane from the serial connector to the relevant pins in the backplane?

  5. #5

    Default

    I opened everything up and measured the backplane. -15V was toast. I replaced the h745 -15V regulator and restored current loop service with the teletype. Problem solved.

    bill
    @ BillDeg:
    Web: vintagecomputer.net
    Twitter: @billdeg
    Youtube: @billdeg
    Unauthorized Bio

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