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Thread: TV Typewriter replica project back on track

  1. #311
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    Quote Originally Posted by falter View Post
    A new problem -- sometimes when I drill through pads, the pad on the opposite side lifts! I hope age hasn't weakened the copper's adhesion..
    Don't drill all the way though from both sides. Drill though the top layer of copper on one side, flip over and drill through on the other.

  2. #312
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    When you drill, back your work up with a sacrificial piece of PCB material--clamp tightly--use a sharp carbide drill and a high-speed drill press (>3000 RPM).

  3. #313
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    Thanks for the tip.

    Well, I managed to pull the two epoxied boards apart. I will attempt a redo once I have a better process. Now I just need to figure out how to remove the cured epoxy, which is advertised as solvent proof once cured.

  4. #314
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    Try scraping the epoxy off. If that fails, try sanding.

  5. #315
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    I'm thinking that too.. but worried about scratching the substrate.

  6. #316
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    Well, if you use a fine grit, it'll give the epoxy something to grab onto, so not a bad thing.

  7. #317
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    Well, here it is.. the first new Signal 24-1A transformer in probably eons:

    20170721_172238.jpg

    The part itself cost $105US. There was a setup charge of $44. And then Signal had to use UPS, the courier every Canadian hates, because UPS *has* to charge a brokerage fee equivalent, in this case, to half the value of the part, for each and every shipment coming up from the US. Ugh. But at least now I have exactly the right piece for the job.

    Now some difficult decisions.

    As you'll recall, the TVT I have built was built using artwork from the SWTPC site. Because there were no dimensions on the original artwork (because it was printed at full size), I ended up with boards that are about 1/4 inch too small all around. That means the transformer is uncomfortably close to the timing, memory and cursor boards as things sit. It also means things like the mounts for the case are too close to traces/switches/etc.

    You'll also recall I recently got my hands on some vintage 1973 board stock. However, that stock is 0.031" thick, way too thin to take the weight of this transformer. My idea was to reinforce by epoxying a second piece of substrate to the first, but epoxy is proving much more difficult to work with than anticipated. Despite my best efforts, I always end up with these unsightly gaps. They don't affect performance, but they're ugly.

    The other problem is, I built my prototype TVT case to the wrong dimensions also. It is correct width wise but not length. And of course, the correct size board will not fit in the compartment at the back. And I cannot increase the depth of that because the wood was cut too short and there's not enough room to stretch it out. Aaaaah!

    So my choice now is to try and jury rig this, or start over. And then from there, whether to insist on using these vintage boards somehow (if I can find a better adhesive) or use modern stuff. Sigh.
    Last edited by falter; July 21st, 2017 at 05:40 PM.

  8. #318
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    So yeah, this idea of bonding two PCBs together is not working out so great. I got myself a press, but it makes no difference.. the epoxy always forms pockets somewhere. I mean, functionally it does its job, but it looks like crap. And the whole point of using vintage stock was to look convincing.

    So I'm trying a different tack - epoxy around the edges and then where I have double sided, like my Mark-8 boards, epoxy under ICs, etc.. where it won't be visible.

    Best solution I can find. Unfortunately too late for some of the boards I already did.. I've got them apart but epoxy does not come off without major scraping!

  9. #319
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    Here's an attempt bonding around the edges:

    20170727_095058.jpg

    I figure.. things will be reinforced because you will have parts that are mounted on one side and fastened on the other. For the Mark8 boards, everything will be soldered on both sides.. and I can put a bit of epoxy under socket pads, where it'll never be seen, to reinforce. Thoughts? The TVT mainboard is the one I worry about most.. it has to support the weight of the transformer, upside down.

  10. #320
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    I'm kinda surprised that you'd use epoxy; seems to me that'd be pretty hard to apply evenly and not leave voids.

    Did you try any spray-on adhesives? Even if the bond isn't as strong as epoxy I'd expect that two sheets glued and pressed together would still be pretty hard to separate. Might be worth a test with some scrap pieces...

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