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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
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
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!
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