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Thread: Frustrating Problem Part 2

  1. #1
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    Default Frustrating Problem Part 2

    In an earlier post I asked for help with an intermittent problem on my CP/M 2.2 machine. It is a home made 8080 machine based on an old IMSAI mother board. I pretty well determined that the old mother board sockets were most of the problem. Over the last week or so, I removed all 11 sockets. There were a mix of original sockets and a few newer ones someone else must have installed. I had to crush the socket plastic to isolate each pin and then could remove one pin at a time. It took a lot of time. I purchased some new sockets, but now am having a hard time installing the new sockets. The holes in the motherboard, although they are open, must have some solder in them that makes each hole just small enough that the new socket will not penetrate. I used a solder sucker, solder wick, an old IC socket pin as a probe to open the holes. I just about have one socket inserted, but it has taken two days. I'm to the point of wanting to drill out the offending holes with a drill bit. I am little worried about removing the thru hole plating, but each hole is connected on both sides of the circuit board. I figure a 0.025" drill bit may do the job. Has anyone else had any experience replacing the S-100 edge connectors? Thanks Mike

  2. #2
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    Not S-100 specifically, but those cheap Chinese hollow stainless needles for desoldering clean holes pretty well.

  3. #3
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    Thanks Chuck. I never seen such a thing. Watched a couple of youtube and they look pretty neat. Just purchased some and hopefully they work as shown. Thanks again. Mike

  4. #4

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    If there are only a few pins that don't fit, and you can generally sense where they are (by pressing on the socket), you can heat the offending holes while applying a little pressure on the top of the socket. At this point, though, I have to wonder how much more the PCB can take. I have tried the drill procedure before, with mixed results. I'm not a fan of it - damage to the through-hole plating is pretty irreversible.
    - Doug

  5. #5

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    For cleaning out stubborn solder holes, my SMD workstation came with a hand drill bit that works remarkably

    well for this task. It is not quick at all; but will clear out the solder from the plated holes. Circuit

    Specialist in Tempe Arizona sells these as replacement parts with holder for about $6.00 #20178.

    >>> Charles

    20178-0.jpg

  6. #6
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    I've used the small carbide PCB bits in a Foredom handpiece to do the same thing. Problem is that they often damage the via if you're not careful. The Chinese needles really work well in my experience.

  7. #7

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    As noted, a dill bit can work (the non fluted part of it) while heating the pad, but a bit awkward.

    For this problem I use stainless steel wire, 4 or 5 inches long folded at one end into a small hoop to hold onto it and the working end filed to a small taper or rounded off a little. So if you can measure the size of the new IC socket pins with a micrometer, and buy some stainless wire on ebay the same or just a tiny bit bigger. Then also, for thermal coupling to the plated through hole you may have to apply a small amount of fresh solder to the solder side of the board, which might partially re-fill the hole, but it won't matter at all, then heat that with the iron on the solder side while pushing the wire through from the component side. That will clear the hole/s so the IC socket can be fitted.Its important to use the stainless wire so the solder cannot stick to it. Also, don't push on the wire, until the solder is definitely melted.

    The Chinese needles sound good, I have not used them.

  8. #8
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    The nice thing about the needles is that they're hollow, so they can slip right over the end of the obstruction (wire or contact tail) and go between the obstruction and the walls of the via. Very slick.

  9. #9

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    Mike

    Maybe a longshot but do you have any 74LS245's on your cards? I've just tracked down a problem with my EPROM programmer interface. Like yours my system is homebrew and sat in the loft for 35years. My system is based on eurocards and a backplane all the cards are buffered by 74LS245's on the databus and 74LS240's on the address bus. The eprom programmer is connected by a ribbon cable to a port on one of the eurocards, that is also buffered in a similar way.

    What I discovered one gate of a 74LS245 on the eurocard was playing up. 7 of the gates '0' state were at about 0.5-1v by the other was about 3v. The gates were switching OK but the gate at 3v the '1' state of the bad gate was barely above the 3v level and not passing data. Maybe you've got a similar problem and one of your old chips has degraded and you have a marginal level that is sometimes working and sometimes not. Quick way to test is a bit of substitution. If you've got a card that isn't core it could lend you a chip for testing.

    BTW the only time I ever attempted a fix on a PC motherboard it was a bad 74245. The only other chip failures I've experienced in the past have been self induced by me.

    Pete
    Last edited by Doubletop; February 18th, 2020 at 07:12 PM.

  10. #10

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    Quote Originally Posted by durgadas311 View Post
    damage to the through-hole plating is pretty irreversible.
    I have used Copperset for many years to restore plated through holes.

    These are fine sleeves filled with solder, you fit them to a hole, and use a small punch to expand them, de-solder the hole and you have a new plated through hole:

    https://uk.rs-online.com/webdocs/003...6b800318fb.pdf

    I found though, that while these are great if the pads are intact (or for some of my prototype pcbs) they don't help as much if the pads are lifted, though its still better than nothing. For that reason, when that is the case I use sub-miniature brass eyelets, for which I had to make the staking tools myself. There is an article here showing a pcb I repaired with brass eyelets (somebody in the past had brutally replaced an IC damaging the pcb)

    http://worldphaco.com/uploads/PCB-EYELET_REPAIRS.pdf

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