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Vox super beatle V114 tremolo thump

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  • Vox super beatle V114 tremolo thump

    This amp's tremolo is thumping so hard it's more like a knocking sound. Anyway I'm using the schematic to the royal guardsman, it appears to have an identical tremolo circuit. So far I changed out the oscillator caps C216, C217 and C218 which did nothing. Might put those back after when I'm done. There's 23v on R234 and about 16vdc fluctuating on the collector of Q206. Not getting much of a reading on the emitter.
    The 'MOD BAL' R230 doesn't seem to be doing anything.
    Does anyone have an idea what could be causing this?
    Thanks
    Attached Files

  • #2
    What about the mixer section, like if C224 is leaky?

    Comment


    • #3
      Is the thumping in the power supply or the audio circuit? Does it stop when the tremolo control is turned off or is it always there as long as the oscillator is running?

      Comment


      • #4
        I can turn off the thump with the depth knob. It starts in at almost half rotation and loudest when cranked

        Paralleled a cap to C236, no effect on thump

        Comment


        • #5
          Originally posted by pontiacpete View Post
          I can turn off the thump with the depth knob. It starts in at almost half rotation and loudest when cranked

          Paralleled a cap to C236, no effect on thump
          Did you do the recap on this amp?

          My guess is that the problem lies in either the Q205 circuit or the modulator circuit. Have you tested all of the diodes in the ring and oscillator? The fact that the MOD control does little is also something to check. Are the wires all still connected?

          Comment


          • #6
            The Thomas Vox tremolo circuit relies on equal + and - side injections of current into the four-diode modulator to reject the control voltage. And it works in an odd fashion - the dry signal is buffered, and the modulated signal is amplitude modulated and then amplified and inverted, then added back to the dry signal to cancel it.

            The modulator balance adjusts the size of one side of the modulator signal so that the two perfectly offset in the diode modulator and no LFO signal comes through.

            Any modulator feedthrough would of course only be on the diode-modulated side of the tremolo depth pot, so it only becomes audible when you get a significant amount of the tremo'd signal with the depth control.

            How to fix it? Something is hosed in the LFO/modulated signal side. The usual processes apply: take DC/no signal readings where you can on every active device in the section, then reason about what that says about the operation of that section.

            About here I have to fall back on my standard advice to Thomas Vox owners: never do any significant debugging on one of these until you have replaced every stinking one of the electro caps in the amp. The caps are 60 years old now. It's amazing that any of them are good. Even if you fix one thing, another cap may fail any day, and you're back at fixing it again. Replace all the electros, then if that hasn't fixed today's problem, you won't have a soft-failing electro sabotaging your work time.

            I'm a little bit of nut about Thomas Vox amps, as witness my web site: http://www.thomas-vox-repair.com/, where there are tech tips and resources, advice, as well as out-and-out repair manuals for many of the amps. Shoot, I even laid out replacement boards for them. As I say, a monstrous mind is a toy forever.
            Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

            Oh, wait! That sounds familiar, somehow.

            Comment


            • #7
              Originally posted by 52 Bill View Post
              Did you do the recap on this amp?

              My guess is that the problem lies in either the Q205 circuit or the modulator circuit. Have you tested all of the diodes in the ring and oscillator? The fact that the MOD control does little is also something to check. Are the wires all still connected?
              I didn't do the re-cap that you see. The non polarized cap is replaced with two back to back(neg connected) 2.2uf polarized caps.
              The diodes are not shorted. The Mod Bal pot is connected.

              Q205
              C 4.9vdc
              E .2v
              B .7v

              Q204
              C 4vdc
              E .05v
              B .64v

              Q206
              All voltages are fluctuating, the highest voltages noted
              C 19v
              E .3v
              B .9v

              Comment


              • #8
                Originally posted by pontiacpete View Post
                Q206
                All voltages are fluctuating, the highest voltages noted
                C 19v
                E .3v
                B .9v
                There's one issue, and maybe all of it. The emitter of 206 has to be up at several volts to be able to make and equal-and-opposite signal for the diode bridge. The schemo says it's supposed to be 7.5V, not 0.9.

                Disable the LFO from oscillating by shorting C217. Now see what the DC voltages on Q206 are, and track down why the emitter is so low.

                I'll go dig out the simulator runs I did on the tremo to get the diagnostic stuff and see what it ought to be with the tremo disabled.
                Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                Oh, wait! That sounds familiar, somehow.

                Comment


                • #9
                  With C217 shorted
                  Q206
                  E .21v
                  C 10v
                  B .8v

                  Comment


                  • #10
                    Originally posted by pontiacpete View Post
                    With C217 shorted
                    Q206
                    E .21v
                    C 10v
                    B .8v
                    DC conditions are hosed in the LFO transistor. I couldn't find the original model, had to re-enter it. With 24Vdc where it says "24V" on the schemo, and a switch across C217, the thing reliably oscillates when the switch is open, and reliably stops when the switch is closed, so at least I didn't tell you wrong about C217. That's always possible, so I check.

                    With C217 shorted, the oscillation stops and the voltages on Q206 settle down to 14.8V on the collector, 10.1V on the emitter. It's not too suggestive that these don't match the schematic's 17V and 7.5V because a collector feedback stage like that is sensitive to all kinds of things for bias, like the exact value of hfe, and so on. But it is significant that the emitter has to be several volts above ground and the collector several volts above that to give good, equal-and-opposite LFO signals as the diode bridge requires.

                    Now as to guesses for why. I think something is pulling the emitter down. Can you measure the resistance of the emitter of Q206 to ground with power off? It ought to be 10K to 15K ohms, the sum of the 10K and 5K balance pot. If it's much different from that, something is erroneously connected in.

                    This is a guess. I puttered around, shorting caps and making them leaky, putting in likely failures and didn't come up with anything that makes the circuit itself show close to your voltages. So I think it's an extraneous connection at this point.
                    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                    Oh, wait! That sounds familiar, somehow.

                    Comment


                    • #11
                      214 ohms to ground. Far from the expected 10k to 15k.
                      I have some sorting out to do. I’ll be back

                      Thanks!!

                      Comment


                      • #12
                        So where does the voltage on the emitter of Q206 come from? Does it come from collector of Q206? Somehow through it? There's nothing after the diode modulator and that 1meg resistor R225.
                        R234 is not 2K in this amp, it is 12K. Is this a schematic error or is this the wrong resistor?
                        I've checked the connections to Q206 over and over and it is connected to the 'Bal Mod' pot the way it is drawn and the readings no matter if I sub in a resistor for the pot or not I'm getting the same resistance readings and voltages. Are we sure the schematic is correct about the emitter voltage?

                        Here it looks like R234 is 12K
                        http://www.voxshowroom.com/us/amp/v1...eamp_hood.html
                        Last edited by pontiacpete; 03-10-2019, 01:08 PM.

                        Comment


                        • #13
                          Originally posted by pontiacpete View Post
                          R234 is not 2K in this amp, it is 12K. Is this a schematic error or is this the wrong resistor?
                          [...]
                          Here it looks like R234 is 12K
                          http://www.voxshowroom.com/us/amp/v1...eamp_hood.html
                          It's 12K. I used my own schematics, taken from the original service literature and checked against actual amps when I did the simulation model, which both said 12K. That linked schemo you show is incorrect, at least about that resistor value. Perhaps the creator of that schematic missed it in the scanning/rework that's obviously been done.

                          But 12K is correct.

                          So where does the voltage on the emitter of Q206 come from? Does it come from collector of Q206? Somehow through it? There's nothing after the diode modulator and that 1meg resistor R225.
                          R234 is not 2K in this amp, it is 12K. Is this a schematic error or is this the wrong resistor?
                          I've checked the connections to Q206 over and over and it is connected to the 'Bal Mod' pot the way it is drawn and the readings no matter if I sub in a resistor for the pot or not I'm getting the same resistance readings and voltages. Are we sure the schematic is correct about the emitter voltage?
                          There are the odd mistakes in the factory schematic, especially about the noted voltages. I found and worked through a number of these in re-generating the service literature for the repair books. But in this case, the schematic does produce a working circuit (aside from that 2K/12K thing) as checked by both the replacement PCBs and the simulation model.

                          Here's how the circuit works.

                          First, ignore R235, R236, C228, SD 206, C220, R231, and R 232. Those are only relevant to turning off the LFO with the footswitch.

                          Next, mentally erase C216, C217, and C218. they are all AC-only parts, and block DC. They don't affect the DC-only operation of the circuit.

                          What you're left with is a 24V supply feeding R234 (12K) to the collector of Q206. Current flows through R234 to the collector, and through R233 to the base of Q206. R233 feeds a trickle of current to the base of Q206 to bias it on, and provides some feedback for DC stability. There is no other DC path to the base of Q206, so all of the DC conditions for the base come through R233.

                          Any current through Q206 has to follow a DC path to ground, which is on the bottom of the modulator bias pot. That's the only place DC current can get back to ground from the +24V supply; every other path is DC-blocked by a capacitor. The emitter leads to R229 and the balance pot.

                          There is one other DC path - the modulator bridge. R224 and 225, and the four diodes form a 2M+2xdiode path from collector to base of Q206. Even if Q206 is not working, a little DC leaks through this 2M. The 2M is about 100 times as much resistance as the other DC resistances, R234, R229 and the balance, pot, so for a first order evaluation, they can be ignored.

                          The DC conditions are entirely determined by current from the 24V supply, through R234, through R233, Q206 and the 2M diode bridge, then into R229 and through the balance pot to ground. That's why I said to short C217 - this kills the ability of the oscillator to oscillate, and lets the underlying DC come through. It's also why I said to measure DC from the emitter to ground. If your circuit was wired up properly, that ought to be just the sum of R229 and the balance pot, with some trivial leakage back through the diode modulator and MAYBE a little through R232, SD206, and R236 if the footswitch unit is attached and shorting to ground to inactivate tremolo.

                          Does that make sense?
                          Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                          Oh, wait! That sounds familiar, somehow.

                          Comment


                          • #14
                            RG I really appreciate the above info you gave me here. But I bailed on this amp after hours of checking connections and comparing to the schematic and swapping in parts with no results. It's not easy to admit defeat but time doesn't allow me to go further
                            Thanks again.
                            pete

                            Comment


                            • #15
                              No problem.

                              The owner contacted me about the amp, and I had to turn him down. I just don't have any time to fix amps right now. I'll try to find him someone who can take it on.
                              Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                              Oh, wait! That sounds familiar, somehow.

                              Comment

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