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Snapped screws - Welded standoffs - DPO

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  • Snapped screws - Welded standoffs - DPO

    Working on an Ampeg PF-500, and it appears the DPO put RED loctite on 3mm screws. Nine of the of ten screws holding the main board snapped at the top of the boss. To make things worse, the standoffs seem to be welded/brazed - although I suppose they could be self-clinching blind standoffs and the fillet I see is just the powder coat. I'd like to avoid snapping any more off at this time, so I'm reluctant to remove any of the other boards -which sort of limits the amount of grinding and heat I can apply here.

    3mm is smaller than any screw extractors I have, and if they are red-loctited then I don't have a lot of hopes for that going according to plan either. If these are welded, I suppose I could cut them flush and drill for a hollow threaded standoff and use a flat head from the bottom to hold them on. I'm not certain I could locate them accurately enough to drill the screw out if I drill to the minor diameter and retap - not without some sort of self-centering jig (the bosses are round). Additionally - I've not had good luck trying to drill snapped screws without first shaving the top level - the drill wants to wander.

    And it's a $300 amp, so taking this to a machine shop is probably going to total it. Suggestions before I further goober this thing up?

    The prince and the count always insist on tubes being healthy before they're broken

  • #2
    Originally posted by NateS View Post
    Working on an Ampeg PF-500, and it appears the DPO put RED loctite on 3mm screws. Nine of the of ten screws holding the main board snapped at the top of the boss. To make things worse, the standoffs seem to be welded/brazed - although I suppose they could be self-clinching blind standoffs and the fillet I see is just the powder coat. I'd like to avoid snapping any more off at this time, so I'm reluctant to remove any of the other boards -which sort of limits the amount of grinding and heat I can apply here.

    3mm is smaller than any screw extractors I have, and if they are red-loctited then I don't have a lot of hopes for that going according to plan either. If these are welded, I suppose I could cut them flush and drill for a hollow threaded standoff and use a flat head from the bottom to hold them on. I'm not certain I could locate them accurately enough to drill the screw out if I drill to the minor diameter and retap - not without some sort of self-centering jig (the bosses are round). Additionally - I've not had good luck trying to drill snapped screws without first shaving the top level - the drill wants to wander.

    And it's a $300 amp, so taking this to a machine shop is probably going to total it. Suggestions before I further goober this thing up?

    Oh, MAN! 9 of the 10 snapped off! I think the best you can do at this point is to remove the pressed-in or Weld-nut standoffs. What does the standoff look like on the opposite side? If they are Pressed in, they can be snapped out from the inside. If they're weld-nut style, as you suggest, you could slice them off, if you have a Dremel with a disc cutoff mandril, allowing you to carefully slice them off at the base. You've already mentioned having no luck (in the past) in attempt to drill out the broken screw shank. You can still try that method. Overdrilling to enlarge the screw size, so you could re-tap the new clearance hole for #6-32, which would typically use a number 35 drill.

    If unable to get success in drilling them out from the opposite side, the hole size you end up with would probably allow you do counter-sink the hole from the opposite side, then using a longer #6-32 FHMS, and threaded standoff at/near the same height of the failed standoffs, that would lock the replacement screw/standoff in place (ITL washer between chassis and standoff), leaving threaded studs to pass thru the PCB instead of thru-holes to accept the replacement screws.

    It will be a tedious process, but, that's what you're facing. I've also had to make use of my smaller Foredom hand piece fitted with small Carbide grinding burrs, and worked from the bottom (if thru-hole present), or from the top, and just take the time req'd to grind thru the locked/failed screw shank. Then, retap the standoff. The procedure does require cursing and swearing, and shaking ones' fist at Ampeg's production line leaders setting this ultimate failure in servicing!
    Logic is an organized way of going wrong with confidence

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    • #3
      I don't have a mill but I do have a good sized drill press. One idea I had was to look for a left hand stubby drill bit. They're pricier than normal but not horrible. I did find a left handed center drill from McMaster Carr. It occurs to me that I might have better luck if I kiss the top of each one with an endmill to leave a nice flat starting point. The other side is ... hallelujah I'm glad you asked. They're cinch style - now see the hex outline I missed last time. That changes the equation quite a bit. Can they be pressed out or do they need to be drilled or cut first? Is it a rapid punch, or a slow steady push, or try to rock them enough to break the staking? Or should I try to cut the standoff part way through, high enough to save enough hole to locate from, and then unscrew the stuck screw/standoff top? If there's thread left I could possible thread a standoff into that.


      I suppose I should swab them for DNA first and see if at some point in the future, the technology exists to prosecute whoever put red loctite on a 3mm screw.
      The prince and the count always insist on tubes being healthy before they're broken

      Comment


      • #4
        Originally posted by NateS View Post
        I don't have a mill but I do have a good sized drill press. One idea I had was to look for a left hand stubby drill bit. They're pricier than normal but not horrible. I did find a left handed center drill from McMaster Carr. It occurs to me that I might have better luck if I kiss the top of each one with an endmill to leave a nice flat starting point. The other side is ... hallelujah I'm glad you asked. They're cinch style - now see the hex outline I missed last time. That changes the equation quite a bit. Can they be pressed out or do they need to be drilled or cut first? Is it a rapid punch, or a slow steady push, or try to rock them enough to break the staking? Or should I try to cut the standoff part way through, high enough to save enough hole to locate from, and then unscrew the stuck screw/standoff top? If there's thread left I could possible thread a standoff into that.


        I suppose I should swab them for DNA first and see if at some point in the future, the technology exists to prosecute whoever put red loctite on a 3mm screw.
        With the base of the standoff inserted being HEX, is the diameter of the standoff body larger or smaller than the width of the hex pattern any flat-to-flat? I'll have to go to the PEM website to have a look at that form factor to see it's basis of installation.

        PEM Standoffs sodata.pdf.pdf

        These appear to be installed from the outside of the chassis, not the inside. If the inserts have thru-hole in them, you may be able to drill them out from the back side. Snapping them out will deform the metal, which would then have to be smashed flat again on an anvil or equiv, to drill out the back side with a countersink. As the chassis wall thickness is probably 0.060" thick, 100 deg Flat Head screws and countersink would be in order. You need thicker chassis material to use 90 deg heads.

        To knock them thru from inside to outside, you need a hole a bit larger than the size of the hex pattern tip-tip, so registration of the hole to the standoff is req'd, and a bit tricky. Larger the hole to provide clearance, the more the steel will deform as you drive them thru the chassis with a suitable punch. I'd try as you're suggesting, trying to loosen them in their holes. You could try whacking them from the side, or if you have a tool that can collet onto the standoff body, gripping them tight, then try rocking them until they loosen and if lucky, extract them. That might be less destructive than hammering. I've never tried driving any of these out thru the chassis. You could try driving them from the outside, though that's bound to create a dome before it releases.

        Stupid locktite!
        Last edited by nevetslab; 04-30-2020, 03:21 AM.
        Logic is an organized way of going wrong with confidence

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        • #5
          I have nothing to offer in the way of help. I just have to say that it's interesting seeing an Ampeg amp containing screws you can't get out. It's usually the opposite with Ampeg- no locktite or lockwashers and screws loose or rolling around in the chassis.
          "Yeah, well, you know, that's just, like, your opinion, man."

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          • #6
            This actually had lock washers - something I don't usually see on pcb mounts. The loctite was not on the first screw, or I would have never kept at it. Though they broke at first movement. It wasn't coming out without damage
            The prince and the count always insist on tubes being healthy before they're broken

            Comment


            • #7
              In this case, I would probably just cut off the standoffs. Then I would find the appropriate sized non conductive filler/spacer to jam under the board to secure it. How necessary is it to be screwed down anyway, if there are no grounds involved? If it's a $300 piece.
              It's weird, because it WAS working fine.....

              Comment


              • #8
                If you have a decent drill.. I would simply try to drill them out and tap them to the next standard size screw. A single tap is inexpensive... but a quality one is preferable. You should be able to center punch and drill the screws out. If it gets a little sloppy you can back fill with J.B. Weld and tap that.

                Comment


                • #9
                  Originally posted by olddawg View Post
                  If you have a decent drill.. I would simply try to drill them out and tap them to the next standard size screw. A single tap is inexpensive... but a quality one is preferable. You should be able to center punch and drill the screws out. If it gets a little sloppy you can back fill with J.B. Weld and tap that.
                  Drilling through and retreated is the first thing I.ll try aso. There are big chances the drill to self center inside the standoff at a point if usualy the screw is more light material as bronze , for instance
                  "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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                  • #10
                    How necessary is it to be screwed down anyway,
                    There is quite a bit of mass in the inductors heat sinks and transformers. This definitely should not float.
                    Last edited by NateS; 04-30-2020, 02:53 PM.
                    The prince and the count always insist on tubes being healthy before they're broken

                    Comment


                    • #11
                      Originally posted by catalin gramada View Post
                      Drilling through and retreated is the first thing I.ll try aso. There are big chances the drill to self center inside the standoff at a point if usualy the screw is more light material as bronze , for instance
                      That's true - thanks for mentioning that. I'm going to try to pick up some left handed drill bits. I was hoping to find some left handed stub/screw machine length, but that would be special order and I can pick up some jobbers at HF next door to work.
                      The prince and the count always insist on tubes being healthy before they're broken

                      Comment


                      • #12
                        Before drilling them out I would try this:

                        Cut a slot in the end with a Dermel wheel cutter suitable for a small screwdriver, heat the post with a mini butane torch, unscrew (hopefully)

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                        • #13
                          Agree.
                          The heat will soften up the loctite. (it will bubble)
                          Plus the difference in thermal expansion of the PEM & the screw should give you a chance to remove the broken piece.

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                          • #14
                            Loctite easily breaks down with heat. It's possible to dremel a slot across the standoff and screw using a cutting disk and then you have a screwdriver slot in the broken screw. Heat up the standoff and apply penetrating oil and use as large a screwdriver as possible to see if the screw will budge.

                            In a situation where I need to drill out a screw like this I turn up a bush in the lathe to firmly fit over the standoff and which has a hole to take the drill bit. If I need to drill out few broken screws I use silver steel (drill rod in the USA) and harden it right out in oil. With small screws I find that LH drills or extractors don't work too well. With an accurately drilled hole of the core diameter of the screw I can get the thread to wind out like a spring and leave a decent thread which can be cleaned up with a tap.

                            EDIT; dmeek beat me to it!

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                            • #15
                              Found out 1) my drill press won't reverse. 2) Pretty sure it was the previous owner and not the factory - I decided to go ahead and remove the front and rear panel boards because I needed access to them, and it was really limiting where I could use heat. The Ampeg logo lights up using an LED and some clear plastic light pipe held on with a really long hex standoff - that was fine. But the two screws holding the rear board in also snapped at the head. It looks like they also snapped some of the fingers holding the IEC connector in place, and put some epoxy goop - looks like a putty - wasn't hard to remove.

                              I have just two PEM nuts remaining - the front panel chassis ground wires - they also seem to be recalcitrant. But once they're out I can pull all electrical, plastic and front panel finish stuff off. All the stuff I don't want to burn up. The two back panel PEM nuts are really really long and at an angle that's going to make them hard to drill - they open towards the front panel. But I might be able to come up with something there. Perhaps cut them and epoxy a tube over it.
                              The prince and the count always insist on tubes being healthy before they're broken

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