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  • #91
    Yes it dries out the board. I guess you could try it, and if it improves you could let us know how long the effect lasts.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #92
      Originally posted by Helmholtz View Post
      I think a conductive board is the result of hygroscopic (water attracting) fibers and poor impregnation treatment in combination with a humid environment.
      I could be wrong, but I thought the idea (at least with the wax coated like vintage Fender) was that the heating could help the wax impregnate when it had done so poorly. Thus displacing the moisture and helping prevent it from coming back.

      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


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      • #93
        Originally posted by g1 View Post
        I could be wrong, but I thought the idea (at least with the wax coated like vintage Fender) was that the heating could help the wax impregnate when it had done so poorly. Thus displacing the moisture and helping prevent it from coming back.
        Oiling, waxing or laquering wood won't prevent it from attaining its natural equilibrium moisture content - just takes a little longer to equalize.

        Nevertheless I like experiments, so I support trying the hair dryer method. Just be careful with the temperature of film caps.
        Last edited by Helmholtz; 09-23-2020, 11:38 PM.
        - Own Opinions Only -

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        • #94
          I'm not qualified to argue the physics of the matter. I'll just say this. I've used the hair dryer method in the past with good results. I've also used it with no results. It can't hurt to try it. There's nothing to be lost but a little time.
          "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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          • #95
            There is a similar effect with cloth insulated shielded PU wire as used by Gibson and others. This kind of wire has extremely high capacitance per length (around 400pF/m at low humidity and over 1nF/m at higher humidity) and very low Q (high losses).
            After baking it in an oven at 85°C for some hours capacitance was lower by 50% (did the experiment in a dry period) and Q had improved by a factor 6. So I wax potted it immediately after oven drying, which didn't change much.
            But after a few weeks values were as bad as before the treatment.

            In his book Prof. Zollner reports an even more impressive capacitance change: The PU cable (0.5m) of a Gibson P-90 measured 700pF, having a leakage resistance of only 500k. After drying at 75°C for 5 hours capacitance had reduced to 160pF.

            Such drastic changes can only be explained by varying water content, having a dielectric constant of 80. Common plastic dielectrics are around 3.
            Last edited by Helmholtz; 09-24-2020, 01:49 PM.
            - Own Opinions Only -

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            • #96
              I used a heat gun rather than a hair dryer. Hotter. On conductive Fender boards I basically blew it on the board area by area watching the waxy coating evaporate, or at least change appearance. Like watching frost disappear from my windshield as the vent blows on it. it cured the problem for me. I have yet to have one come back on me. If the board rehydrates (or whatever it does) five years later, I never knew about it, and I serviced in this town over 30 years.
              Education is what you're left with after you have forgotten what you have learned.

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              • #97
                Wow this is crazy stuff! I have a heat gun, my wife has a hair dryer. I'll give it a shot and see what happens.

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                • #98
                  Well, now...

                  This isn't a vintage amp. Most of the problems with permeable boards have been minimized in modern production products (though I still would never build with a black paper board). A different scenario I've encountered is that repeated soldering and resoldering can also make a board conductive. I can't say if it's something like flux contaminate or repeated heating aligning carbon or whatever, but I know it happens. When it does no amount of moisture removal will help. Because the problem isn't moisture in the board. The board has become permanently conductive. The only fix at that point is to replace the board or, if you're really lucky, lift the components from the conductive area and float them. I've replaced three permanently conductive boards and firmly believe that all black paper boards would do well at the bottom of the sea. And...

                  Since this isn't a vintage amp I don't suspect moisture permeating the board. Maybe it's conductive if you've worked it really hard.?. I do think steps should be taken to rule out the possibility, but I wouldn't put a wager on it, personally.
                  "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

                  "If you're not interested in opinions and the experience of others, why even start a thread?
                  You can't just expect consent." Helmholtz

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                  • #99
                    Like anything else, we try a technique, and it pays off or it doesn't and we move on to the next thing.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • Originally posted by Chuck H View Post
                      Well, now...

                      This isn't a vintage amp. Most of the problems with permeable boards have been minimized in modern production products (though I still would never build with a black paper board). A different scenario I've encountered is that repeated soldering and resoldering can also make a board conductive. I can't say if it's something like flux contaminate or repeated heating aligning carbon or whatever, but I know it happens. When it does no amount of moisture removal will help. Because the problem isn't moisture in the board. The board has become permanently conductive. The only fix at that point is to replace the board or, if you're really lucky, lift the components from the conductive area and float them. I've replaced three permanently conductive boards and firmly believe that all black paper boards would do well at the bottom of the sea. And...

                      Since this isn't a vintage amp I don't suspect moisture permeating the board. Maybe it's conductive if you've worked it really hard.?. I do think steps should be taken to rule out the possibility, but I wouldn't put a wager on it, personally.
                      This is a brand new board and the only action it's seen was the one-time installation of components and being wired into the amp. So I personally don't think I've worked it too much IMO.

                      The board seems to be conductive though. I don't think the scope lies. If you can explain some other reason why I'm picking up signal in what is basically a piece of heavy cardstock I'd truly love to hear it.

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                      • For example....

                        This is the cropped out normal channel from the layout. The layout is easier to show where I'm getting the signal on the board.

                        Click image for larger version

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                        Probing only the board in the green circle between the two plate wire connection points picks up a little bit signal from the input. Just off the top of my head, it's around 50mv pk-pk. Even with V1b pin 7 grid grounded, a little signal is passing from V1a's plate wire through the board into V1b's plate wire which then goes through the .047 cap and to point X which goes straight to the PI input. There is leaked signal at point X with the volume pot on zero, vol pot grounded, or V1b grid grounded. It doesn't matter. Signal is getting to point X, and it sure seems like it's shmoozing it's way through the board between those two plate wire connection points...the green circle. If I hover the scope probe over the connections or along the plate wires I pick up no signal. As soon as I touch the board in that green circle I pick up input signal.

                        The blue dots represent areas I've probed and there is no signal. Actually, I've probed and stabbed all over this board. The space between the plate resistors is the only place where I've found the signal taking a little detour.

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                        • Your scope probe could be picking up nearby signal. Put a piece of tape on the board in the sensitive area and put your scope probe on that. That way the probe is in the same place but not touching the board.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • The area in the green circle would have a maximum AC voltage differential when the preamp is conducting since there are opposing plate pin connections there. I like Enzo's idea of adding an insulator to the board surface and retesting. Though I don't doubt I could be wrong, one shouldn't devise reasons to NOT test something. Since many mass produced products aren't as QC as they should be I suppose something like an abundance of pigment could have been used in a run of black board material. The black is just carbon used as pigment. Too much carbon soaked into the fiber can absolutely make it a semiconductor.

                            I've never tested a board for AC conduction. Whenever I've detected a conductive board what I saw was DC voltage bleed near high voltage nodes dissipating randomly with distance from the test point, sometimes forming vague conductive paths. I'm not saying small signal AC conduction can't happen though. As I said, I've never tested for it. But in my own experience it took some whopping DC potentials to create the problems.

                            All that said... I have such distain for black fiber boards that "I" would probably already be considering replacing it anyway. Which is a real PITA of course. I've done it several times. Even going so far as to build the new board myself when a non black fiber option wasn't available. So I'm not blithely relegating anyone to the hours of effort it takes to rebuild and replace a board.
                            "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

                            "If you're not interested in opinions and the experience of others, why even start a thread?
                            You can't just expect consent." Helmholtz

                            Comment


                            • Originally posted by Enzo View Post
                              Your scope probe could be picking up nearby signal. Put a piece of tape on the board in the sensitive area and put your scope probe on that. That way the probe is in the same place but not touching the board.
                              Ok I'll try that. Thanks.

                              Comment


                              • Originally posted by Chuck H View Post
                                The area in the green circle would have a maximum AC voltage differential when the preamp is conducting since there are opposing plate pin connections there. I like Enzo's idea of adding an insulator to the board surface and retesting. Though I don't doubt I could be wrong, one shouldn't devise reasons to NOT test something. Since many mass produced products aren't as QC as they should be I suppose something like an abundance of pigment could have been used in a run of black board material. The black is just carbon used as pigment. Too much carbon soaked into the fiber can absolutely make it a semiconductor.

                                I've never tested a board for AC conduction. Whenever I've detected a conductive board what I saw was DC voltage bleed near high voltage nodes dissipating randomly with distance from the test point, sometimes forming vague conductive paths. I'm not saying small signal AC conduction can't happen though. As I said, I've never tested for it. But in my own experience it took some whopping DC potentials to create the problems.

                                All that said... I have such distain for black fiber boards that "I" would probably already be considering replacing it anyway. Which is a real PITA of course. I've done it several times. Even going so far as to build the new board myself when a non black fiber option wasn't available. So I'm not blithely relegating anyone to the hours of effort it takes to rebuild and replace a board.
                                Other high voltage areas of the board aren't having this problem. I have not looked for DC voltage leaking anywhere. Funny you mention the black pigment. Handling this board did make my fingers a little dirty, sort of like when you thumb through a newspaper. At this point, as I was instructed to much earlier, I'm content to just live with it because there's no way I'm replacing the board. LOL. The amp functions perfectly and sounds wonderful. Honestly I'm just relieved that it's not a mistake on my part and it just is what it is. This last little detail just ate at my brain and I wanted to use it to learn some troubleshooting. In the future I will probably not use these types of eyelet boards anymore.

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