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Completely bizarre and frustrating problem

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  • Completely bizarre and frustrating problem

    I'm doing the Presidential treatment on a 1979 Deluxe Reverb Boost model. It needs a bunch of things done, and while I am at it, it develops a loud, intermittant crackling noise that almost sounds like a bad ground or even a guitar cord not plugged into a guitar. I pull tubes and find it does it with only the reverb recovery tube, PI and of course output tubes. I concentrate around the secons stage of V4 and find the noise stops if I ground the grid. I disconnect, one at a time everything I see that is connected to that part of the circuit, even replacing it's plate resistor, to no avail.

    Curiously it is aggravated by slightly moving the power tubes, so I resolder all pins as well as on V4, and when I fire it back up it is silent, so I think I have remedied it. But, not so fast, it starts again when I wiggled V8, and now it is worse, and far more sensitive. Here is where it gets weird, I can now stop it by pressing down slightly on the chassis, so I go looking for grounds, all seem good. I can also stop it by grabbing the chassis and pressing down onto my bench, AND it gets worse if I lift it off my bench???

    Eventually I touch a heater wire as it passes by the PT, and I can make it better or worse with the slightest little touch. That's whan I discover not one, but two blackened 100 ohm virtual center tap resistors hiding beneath the wires at the pilot lamp. I unsolder everything, and both of them came out in two pieces. I got those replaced, and now all is good. I don't exactly understand what was going on there, especially why it effected V4, but it's on the the other issues.

    What a strange issue to hunt down that was. Not to mention time consuming.

    https://cdn.shopify.com/s/files/1/23...97210715640943
    It's weird, because it WAS working fine.....

  • #2
    Originally posted by Randall View Post
    I discover not one, but two blackened 100 ohm virtual center tap resistors hiding beneath the wires at the pilot lamp. I unsolder everything, and both of them came out in two pieces. I got those replaced, and now all is good.
    So, it's working now without annoying crackles n pops? Good on ya for persevering, and finding those cooked resistors. I wouldn't have expected crackles & pops but rather annoying hums & buzzes, nonetheless it shows us that everything counts when it comes to fixing these beauties. Yesterday I was working on a rare 1985 Vox Concert 501, yes annoying intermittent hums & buzzes. At some point I found the power transformer felt a little wobbly - all 4 mounting bolts were loose and wouldn't ya know it, Vox used one of them to hold a ground wire to chassis. Tighten those nuts & bolts = prob-a-lum feex!

    This isn't the future I signed up for.

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    • #3
      The reverb return amp can be very sensitive to grounding problems on late 70's Fenders I have found.

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      • #4
        Originally posted by drewl View Post
        The reverb return amp can be very sensitive to grounding problems on late 70's Fenders I have found.
        +1
        High gain and long leads. That's one reason many reverb amps with this or similar circuits hum like turning up the reverb knob makes them forget the words

        It may also be that the reverb recovery cathode bypass cap is higher impedance than it should be, allowing artifacts on the heater supply to bleed onto the cathode without being shunted to ground. I would check the relative goodness of that cap and it's connections in light of the circumstances.
        "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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        • #5
          "It may also be that the reverb recovery cathode bypass cap is higher impedance than it should be, allowing artifacts on the heater supply to bleed onto the cathode without being shunted to ground. I would check the relative goodness of that cap and it's connections in light of the circumstances."

          The cathode cap is a fresh Sprague that I measured before installing. As I mentioned, replacing the 100 ohmers cured it.It's nice and quiet now.
          It's weird, because it WAS working fine.....

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