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Repairing and sealing burned PCB

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  • Repairing and sealing burned PCB

    I have a Super Reverb reissue that appears to be a victim of a previous repair. The R60, R61,
    grids, the R62, R63 screens, and R70, R71 heater false center tap 100R resistors are all fried. I believe that high voltage arced on burned carbon that was left and not cleaned and sealed during the previous repair.

    Once I remove all the burned material, what product do I need to seal this area to prevent further damage?
    Attached Files

  • #2
    Sealant, like conformal coating, is to prevent oxidation and to prevent arcing from component to component...but it will not help or prevent conduction WITHIN the fibreglass material.

    Your PCB needs careful investigation, both sides, to see if it's even a candidate for repair.
    --
    I build and repair guitar amps
    http://amps.monkeymatic.com

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    • #3
      The burn is on the surface only. I believe I can remove all the bad material and preserve the solder paths. I think.

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      • #4
        In that first photo, that bad circular burn is deep. Flip the board and look at the side with the traces. If any go near any browned-out parts of the PCB, there will be trouble.
        --
        I build and repair guitar amps
        http://amps.monkeymatic.com

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        • #5
          It looks repairable to me (without seeing the underside). Just cut out the burnt board. It appears that the burnt part will be between traces.
          "Yeah, well, you know, that's just, like, your opinion, man."

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          • #6
            Originally posted by The Dude View Post
            It looks repairable to me (without seeing the underside). Just cut out the burnt board. It appears that the burnt part will be between traces.
            It doesn't go all the way through, and luckily it is between the traces and solder pads. I've been up since 3 am so I'm gonna wait till morning to break out the Dremel.

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            • #7
              My original question was, do I need a specific sealer or "paint" to seal the affected area, or is removing the charred material enough?

              I thought I remember a discussion awhile back about a particular product made for just that.

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              • #8
                If you grind away all of the carbonized PCB material then there is no need to apply any sealant. It also does not even matter if you end up with a hole in the PC board.

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                • #9
                  Originally posted by Tone Meister View Post
                  My original question was, do I need a specific sealer or "paint" to seal the affected area, or is removing the charred material enough?

                  I thought I remember a discussion awhile back about a particular product made for just that.
                  Super Corona Dope
                  The source for the MG Chemicals Super Corona Dope P/N 4226-55ML is Mouser Electronics, their P/N 590-4226-55ML @ $11.06 55mL bottle. thank nevetslab for this info

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                  • #10
                    It can affect the integrity of the board. In the past I have used a piece of scrap board, even perf board, and 5 minute epoxy to patch an area.

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                    • #11
                      Sometimes I'll leave a neat hole if it's between pads - say, where a resistor body burns through the board. Just like how Mesa does the resistor cutouts on power amp PCBs. If warranted I'll epoxy a piece of bare board over the area. In some cases I'll let a piece in so it's flush. It all comes down to how it's burnt, what's affected and how the components need to be reinstalled. I have some tiny eyelets (about 1mm bore when fitted) that I use to mount parts on the new piece of board.

                      I always take pictures of the original board on both sides as a reference.

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                      • #12
                        All very good information folks. Thank you.

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                        • #13
                          I'm going to back what Tom said because it's important and warrants repeating. Grind away any PCB material that shows ANY discoloration. You may yet end up with a hole.
                          "Take two placebos, works twice as well." Enzo

                          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                          "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                          • #14
                            It's also worth noting that, when you grind away PCB material, you are generating fine dust composed of epoxy and glass particles so use appropriate protection. For a small job I wear a dust mask and hold the shop vac hose next to the spot being worked.

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                            • #15
                              That also reduces the risk of carbon contamination elsewhere on the board from the dust. I always do a cleanup of the area first with acetone - sometimes the residue is slightly oily and a good deal of surface contamination can be removed to get to the actual burnt fabric of the board. Where there surface is burnt and grinds off down to good substrate I just flow epoxy in to level the crater, though over the past few years I've been using UV cure epoxy for this type of work as it gives near instant results.

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