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Thread: Lear Siegler ADM-3A working

  1. #11
    Join Date
    Dec 2017
    Location
    Oklahoma, USA
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    13

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    Here's a great video on "curing CRT cataracts" on an ADM-3A if you haven't seen it:

    https://www.youtube.com/watch?v=iTcZbBp5yGY&t=5s

  2. #12
    Join Date
    Jan 2010
    Location
    San Antonio, TX
    Posts
    20

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    Well I did it... and I didn't even use the hot air gun! I was kind of bored and curious (always a dangerous combination) and started picking away at it with this:

    https://www.matcotools.com/catalog/product/H1/90-pick/

    And it also helped me get the anode cap off earlier, there was an annoying piece of metal that made it hard to reach the pins. Mostly I held the tool perpendicular to whatever edge I was working from, and started digging. Once I got about an inch or so out of the short sides it was all from the long sides. After a while I got good at scooping out a lot at one time. The hook let me pull the dust out. Once there was about a 2 inch band left in the center, another good long scrape through the gelatin from both sides and it popped off easily.

    The sunny day probably didn't hurt. The glass was noticeably warm from being in the sun for two hours on a hot day. I wouldn't recommend doing this for anything much bigger than a 12" tube, but who knows?

    Now I gotta clean off all that rubbery dust. And I discovered a little protip: put something heavy in the bottom of your bucket (like an old lead-acid gel-cell battery) to keep it counter-weighted. A 40 lbs Scoop Away cat litter bucket was the perfect size.

  3. #13
    Join Date
    Oct 2017
    Location
    Kentucky USA
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    121

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    I did a lower case converter with a 2716, wire-wrapped it a long time ago, and it worked OK, plugging into the lower case socket. I didn't have these problems. There really was a 2513-LC chip and I don't think that it was custom for LSI (one of mine came with it). I was a little worried about access times, but I suspect that later UVEPROM's had higher access times than were spec'ed.

    On my ADM-3A's, I didn't have cataract problems, they had burned-in characters (I HATE this). I replaced the CRT's with new ones (B/W, of course). Are they getting hard to find? I did this maybe 10-15 years ago. The keyboards are OK (so far). The models that I have were probably not heavily used.

    ONE OF THESE DAYS, I intend to do a "screen saver" circuit for the ADM-3A.
    Last edited by Slob; October 14th, 2020 at 11:26 AM.

  4. #14
    Join Date
    Jan 2010
    Location
    San Antonio, TX
    Posts
    20

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    Okay, I finally have it put back together now. Glass cleaner really does work to get that stuff off, at worst you just need to scrape a few spots with your fingernail. Do it outside in the sun and spots on the CRT glass will be very obvious. And in good light I was able to see the burn-in. It looked like the word LEVEL repeatedly along the left, but there was a 30 or so character line burned in, so it may have been left on 24/7 without being used for days or weeks at a time.

    That video circuitry is a total pain in the ass to work on. Now that I can see the screen better, I decided it needed a little bit of focus adjustment. All I could do was turn it off, poke a tiny screwdriver onto the edge of the FOCUS pot and turn it a little bit, power on, find it's worse, turn it off, then repeat in the other direction until it's good enough that you don't want to do that crap anymore.

    The videos I watched a few weeks ago helped me get the lens spacing right the first time. I figured out that two layers of my foam was right, and I only uncovered the sticky stuff on the four corners, then a bunch of mylar tape around the edges to keep dust out. I had to shift it around a bit when putting the CRT back into its notch, but it's snug to the front now.

    As for screen savers, I recently came up with a stupid simple trick. I don't like it when I forget to unplug a soldering iron, especially one that doesn't have a display. I discovered Christmas light power things that can turn on for two, four, six, or eight hours after sunset. All I have to do is cover the photocell with opaque tape. To reset turn it off and back to 2hr, or reveal the photocell for a moment. I can hear a relay click, so there's no worry if it's some kind of electronic switch circuit that only works with incandescent loads, etc. So plug your terminal into one of those and it'll never stay on long enough to burn in.

    The thing about the lowercase chip is that it worked with inverted address inputs. It's so weird that they would intentionally do it that way, so I suppose it's possible that someone else ordered a chip with negative logic character (but not row!) inputs, and it was easier to buy a bunch of those than getting their own mask run. The only other thing I can think of is if the 2513 inputs had really bad fan-in and they didn't want both of them on the same latch output.

  5. #15
    Join Date
    Jan 2010
    Location
    San Antonio, TX
    Posts
    20

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    Now I've got another problem. I wanted a nice screen of text for a picture, so I tried to set the "auto NL" mode and found out that the DIP switch had partly collapsed and I couldn't turn it on. Not that I was happy with those recessed rocker switches, but I'm annoyed that an original part got broken like that. And sheesh, I just looked at the manual, there was actually an official "DIP switch actuator" tool, and "use of other tools (pen, pencil, etc.) may damage DIP switch". Brilliant. Bleep the original switches.

    My current plan is to desolder both of those 7-pin switches, solder in a pair of 14-pin sockets, put in a pair of new switches, and live with it being slightly proud of the hole. There weren't many 7-position options on Amazon, so I'm not sure if the red ones I got will fit in the frame, which is pretty snug for a typical blue switch.

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