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C.Itoh CIT 100 Maintenance Manual

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    #16
    Al:
    Yessir! I did dump the EPROMs. The contents look good. There are ASCII strings in the
    first three, the forth one has nothing readable, I hope that's because its all code. Hard to know.
    Let me know where you would like them sent.
    When my attention turns to the non-prime board, which is very different from my prime board,
    I'll get those ROMs too.

    Chuck:
    The original caps were in fact not clearly marked, but the board was.
    You can see an orignal board (above) compared with my recap, lower.
    See my dwarf caps in the upper left of the lower board.
    https://i.imgur.com/FRlVP8R.jpg
    There were + marks on the board.

    I made some progress. The flyback is located on its own module.
    On both machines, the flyback diode was practically dust. The cap there was
    broken on the parts machine. I replace the diode with a beefy one I had handy-
    may not be the best/right specs but I figured it would be OK for starters.
    https://i.imgur.com/wo6NvkL.jpg
    I am looking for similar flyback design to get a diode part number off of.
    (Cause I don't have the maintenance manual to know the right one!)
    The module on the parts machine also had a capacitor with one leg detached.
    Clearly these components get some abuse.

    So after recapping the video board, the flyback module and replacing that diode,
    I now have high voltage generation. I could hear it. But I heard two SNAPS that I could
    not locate the source of. Unfortunately I was not right over the unit to get a sense of it.
    I *think* it was from the video board.
    I powered down, looked around, and next power up, no snaps.
    I guess I blew something but noting looks fried.
    There was never any image or beam on the CRT.

    I discharged the tube to continue diagnosing, and it was definitely charged- SNAP!
    That's progress but I really hoped it was going to work this time, now I have to dig
    deeper into measurements I don't know how to do. yet.

    When I got the board out, I noticed the transistor with the heatsink in the lower left was warm,
    the similar one on the upper right was cold. Cold that might mean that part of the circuit isn't doing
    anything?

    QUESTION: If I have HV being generated, what can I assume in the path is good?
    I have to admit, out of the transistors on the board, I can't say I definitely know which
    one is the HOT.
    Last edited by dmemphis; March 12, 2018, 06:46 PM.
    http://abitoutofplace.wordpress.com/

    Comment


      #17
      Well, you're getting HV, so it means that the FBT and HOT are operating.

      But those itty-bitty replacement caps look very wrong. Got any shots of the markings on the original? It may be time to do some research.
      Reach me: vcfblackhole _at_ protonmail dot com.

      Comment


        #18
        The Caps

        That's good news! Now why no trace on the screen. Tube bad? Video input isn't any good?
        Always blanked? These caps?
        Here they are.
        https://i.imgur.com/CWfc4Oj.jpg
        https://i.imgur.com/qftdGa4.jpg

        Is it essentially a composite signal that interfaces with the video section?
        http://abitoutofplace.wordpress.com/

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          #19
          Originally posted by dmemphis View Post
          Al:
          Yessir! I did dump the EPROMs. The contents look good. There are ASCII strings in the
          first three, the forth one has nothing readable, I hope that's because its all code. Hard to know.
          Let me know where you would like them sent.
          You should also try to dump the 2332 character generator rom.
          Emailing them would be easiest (will PM the adr)

          Comment


            #20
            OK will get that too.
            http://abitoutofplace.wordpress.com/

            Comment


              #21
              The troubling thing about the caps is that there's no polarity marking on them. To me that says, irrespective of the PCB markings, that they must be nonpolar, which would account for their size. Here's what a 50V 4.7uF nonpolar Elna looks like--note the absence of polarity markings--and that it's marked with the CE(BP) notation--you might want to see what CE(BP) means to Elna. (To the best pf my determination, CE=cerafine series and BP=bipolar. These were bipolar caps treasured for their use in audio crossovers. They're no longer made, but Elna has an equivalent bipolar audio series, as do other manufacturers).



              Those capacitor markings aren't put there for decoration!
              Last edited by Chuck(G); March 13, 2018, 08:05 AM.
              Reach me: vcfblackhole _at_ protonmail dot com.

              Comment


                #22
                That's excellent sleuthing. Probably the two SNAPS I heard were the two offending caps I
                installed. I'll further research, and obtain some like caps. I do have 4.7uF non polar-
                but they are just as small as the polarized ones I used.
                I removed both pairs of ELNA caps from the two video boards and measured them,
                and reinstalled the best back into the prime board. (which were the two that came off of it).
                There was also a 33uf 250v cap that was close to 20% high, I didn't have a direct replacement
                and put in a 47uF 250v cap.
                With that all done, still no difference in the display.
                If I get to it, I'm going to scope the incoming signals and see if they are reasonable,
                and I guess follow the path inward from there.
                http://abitoutofplace.wordpress.com/

                Comment


                  #23
                  >Those capacitor markings aren't put there for decoration!

                  So true. Should pay closer attention to markings!
                  http://abitoutofplace.wordpress.com/

                  Comment


                    #24
                    Ordered some ELNA CE BP audio caps form hong kong.
                    http://abitoutofplace.wordpress.com/

                    Comment


                      #25
                      As far as not getting an image, you might troll some of the documents on bitsavers wtih regard to terminals in general. At the CRT level, they all operate pretty much the same, so that voltage and waveform specs should be in the same ballpark.
                      Reach me: vcfblackhole _at_ protonmail dot com.

                      Comment

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