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Ampeg Portaflex POPS

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  • #1

    Ampeg Portaflex POPS

    Hi folks, I have a 70's SB-12 on the bench with constant popping sound. It is volume dependent, suggesting an issue in the first half of V1. It occurs every 5 seconds or so.
    Though it sounds like it, I cannot spot any arcing.
    Voltages are correct. Tried a new tube.
    The socket was cruddy and worn, I replaced that. Reflowed every solder joint in the stage.
    Replaced R5, 6, 7, 28 and C1.
    PSU caps are new, also lifted the V1 B+ and used a bench power supply. No change.
    Humdinger pot is new.
    I've scraped and cleaned the PCB thinking there might be a flux or crud path, to no avail.

    I've hit a wall, can anyone suggest a next step to troubleshoot this?
    Schematic:
    SB-12 Schematics.pdf
    Thanks!!



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  • #2
    Thanks for posting a schematic. Have you tried just grounding the grid of V1 ?
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      I would suspect R4 as this circuit is half grounded grid and half cathode biased.
      (Great for extra distortion).
      All high value carbon composition. I call them, as do most, 'Carbon Drifters', because they do.
      Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
      If you can't fix it, I probably can.

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      • #4
        Originally posted by loudthud View Post
        Thanks for posting a schematic. Have you tried just grounding the grid of V1 ?
        Yes, and the popping persists. This also rules out R4, etc. This amp must be a later one, the resistors are all carbon film.

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        • #5
          The popping is volume dependent, but that doesn't necessarily mean it's not happening elsewhere in the amp. Perhaps it is but it's not noticed so much because only V1 is amplifying it significantly. It could be earlier in the power supply than the node feeding that triode. Have you tried a different rectifier tube?
          "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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          • #6
            Originally posted by Chuck H View Post
            The popping is volume dependent, but that doesn't necessarily mean it's not happening elsewhere in the amp. Perhaps it is but it's not noticed so much because only V1 is amplifying it significantly. It could be earlier in the power supply than the node feeding that triode. Have you tried a different rectifier tube?
            Yes I have, and replaced the standby switch as well.
            If what you suggest is the case, I'm not sure how to troubleshoot it from here.

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            • #7
              What happens if you lift one end of C1?
              - Own Opinions Only -
              • Likes 1

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              • #8
                Originally posted by Helmholtz View Post
                What happens if you lift one end of C1?
                Good call, but the pop continues.

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                • #9
                  Originally posted by hylaphone View Post

                  Good call, but the pop continues.
                  Does it still pop if you pull V1?
                  - Own Opinions Only -

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                  • #10
                    Originally posted by Helmholtz View Post

                    Does it still pop if you pull V1?
                    Yes but much quieter, same with V2. ChuckH was right

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                    • #11
                      Originally posted by hylaphone View Post

                      Yes but much quieter, same with V2. ChuckH was right
                      Not sure a smiley is in order.?. This means the power supply needs to be touch soldered with good flow and then, if not solved, re diagnosed.
                      "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

                      • #12
                        V2 is the phase splitter ... the final drive before the output valves. I also would expect no noise if removed.
                        You say the volume control reduces the noise. At zero, has the noise stopped completely? If it has your fault is V1 associated; connect the wiper of VR1 to ground. Has it stopped ....
                        If not and removing V1 still gives noise, look at the coupling capacitor.
                        Link J3.That stops the signal path if it comes from V1 and work backwards towards the input.
                        There are only a few components and it won't take many minutes to isolate the fault.
                        Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
                        If you can't fix it, I probably can.

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                        • #13
                          Regarding the "cap job". A new electrolytic capacitor isn't always a good electrolytic capacitor. If, say, it's been stored for a decade or it's some kind of NOS thing. So if there's any reason to suspect the power supply caps are anything but new and fresh the possibility of a fault here still exists.

                          Can we safely assume that the bias supply filter caps were replaced with the PSU caps. Not that I suspect this is the problem, but it's important.

                          "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

                          • #14
                            Also, it's not entirely clear if the pop is just "every five seconds or so" at somewhat random intervals or if it's steady like a clock.
                            "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
                            • Likes 1

                            Comment

                            • #15
                              Well, I re-flowed a few of the B+ joints on the PCB, and the amp is nice and quiet now. *Slightly* disappointed I couldn't find the specific culprit, but all's well that ends well.
                              This was eye opening though, I was hung up on volume pot and prior while the gremlin was lurking on the other side of the chassis.
                              So thank you, gents.
                              • Likes 2

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