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33944RE: [midatlanticretro] LA120 Destination^Wfaults unknown

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  • Cory Smelosky
    Jan 14, 2014
    • 0 Attachment
      On Tue, 14 Jan 2014, Brian Schenkenberger, VAXman- wrote:

      > Cory Smelosky <b4@...> writes:
      >
      >> On Tue, 14 Jan 2014, Brian Schenkenberger, VAXman- wrote:
      >>
      >>> Cory Smelosky <b4@...> writes:
      >>>
      >>>> On Tue, 14 Jan 2014, Brian Schenkenberger, VAXman- wrote:
      >>>>
      >>>>> Cory Smelosky <b4@...> writes:
      >>>>>
      >>>>>> Afternoon,
      >>>>>>
      >>>>>> I have the PSU operational again.
      >>>>>>
      >>>>>> 2 +12, 1 -12, and -5V are all present.
      >>>>>>
      >>>>>> These voltages could be wrong: (pin-wise) on J2.
      >>>>>> Pin 6: 4.97VDC
      >>>>>> Pin 5: GND
      >>>>>> Pin 4: GND
      >>>>>> Pin 3: -12.28VDC
      >>>>>> Pin 2: about 12VDC
      >>>>>> Pin 1: about 12VDC
      >>>>>>
      >>>>>> +/-24VDC are both present on J4 where they should be.
      >>>>>> The motors do not run and there is no voltage present at fuse F2 (the fuse
      >>>>>> OTHER than the line feed one. There is +24VDC present at the line feed
      >>>>>> one.) There is continuity between the -24V pin and the non-line-feed
      >>>>>> motor fuse. No fuses blow.
      >>>>>>
      >>>>>> With J2 disconnected, power draw drops from ~65W to ~42W. Several random
      >>>>>> resistors checked (in-circuit) that should measure 540Ohms measure about
      >>>>>> 588Ohms. No LEDs light up and there is no bell. No components are
      >>>>>> particularly warm to the touch with power removed.
      >>>>>>
      >>>>>> Anything with this taste wrong to you? I'm completely stumped. The motor
      >>>>>> ran before I started working on the PSU. When I was doing repair work on
      >>>>>> the PSU (caused by my own mistakes -- oops) I had it disconnected from
      >>>>>> the LA120 entirely save for a couple tests with test resistors. There was
      >>>>>> no -12V present when these tests were performed and +5 would've measure
      >>>>>> +4.77VDC.
      >>>>>
      >>>>> Are you aware of and or have copied of these?
      >>>>>
      >>>>> http://vt100.net/docs/la120-tm/la120tm1.pdf
      >>>>> http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/terminal/la120/MP00663_LA120_Oct79.pdf
      >>>>>
      >>>>
      >>>> Yup. I have been browsing through them.
      >>>>
      >>>>> I have a fully functional LA120 if you need me to check voltages for you.
      >>>>> However, I believe you have the appropriate voltages within tolerances.
      >>>>>
      >>>>
      >>>> Yes please. Do check them in relation to which pin is which.
      >>>>
      >>>>> Are both fuses (carriage return and line feed) good? Are you sure that
      >>>>> the ribbon cable from the logic board is connected to the keyboard logic
      >>>>> board? Make sure pin 1 and pin 1 on each are correct. The cable has a
      >>>>> marker to coincide with pin 1 on the main board. Check that whatever
      >>>>> color coincides with pin 1 there is closest on the connection on the
      >>>>> keyboard logic to the keys themselves. Do you have the carriage return
      >>>>> motors sense wires connector properly oriented on the main logic board?
      >>>>> The paper out (pins 1 & 4) and cover open (2 & 3) (note I may have them
      >>>>> backward but it doesn't matter) won't matter if you connect ti upside
      >>>>> down/backward.
      >>>>
      >>>> I'll need to look more closely at those pins. I could have any number of
      >>>> them orinted wrong. Both fuses pass a basic continuity test.
      >>>
      >>
      >> My J4:
      >>
      >>> +-----+-----+-----+-----+
      >> (1) | +24 | 0 | -24 | UNK | J4 (all the below ones NC are same)
      >> +-----+-----+-----+-----+
      >> | NC | 0 | NC | UNK |
      >> +-----+-----+-----+-----+
      >> | NC | 0 | NC | UNK |
      >> +-----+-----+-----+-----+
      >>>
      >>>
      >>> +-----+-----+
      >> (1) | +12 | 0 | J2
      >> +-----+-----+
      >> | +12 | 0 |
      >> +-----+-----+
      >> | -12 | +5 |
      >> +-----+-----+
      >>>
      >>
      >> Looking at this, I see a +12 in mine that is not in yours. I can make
      >> that go away, however.
      >
      > I'd reconcile things with the schematics before you go making changes that
      > might cause you problems. Also, I believe that there are fusable links or
      > resistors on the power supply board. Have you checked that they're intact?
      >
      >

      Yes. However, in my previous repairs I made some mistakes that I
      attempted to work around. I mostly reconcilled them with the
      schematic...but there's still that +12V that shouldn't be there.

      --
      Cory Smelosky
      http://gewt.net Personal stuff
      http://gimme-sympathy.org Projects
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