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Screaming Bright Switch Cap??? - 1974 Fender SF Twin Reverb Master Volume Push Pull Switch

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  • Can you try disconnecting the bright cap at the pot, and the wire from the bright switch at the pot?
    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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    • Originally posted by g1 View Post
      Can you try disconnecting the bright cap at the pot, and the wire from the bright switch at the pot?
      And retest on the scope for the parasitic.
      "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


      • thx G1 + Chuck,
        i haven’t noticed a difference in the scope’s readings after removing the Bright Switch (you can see it on top of the scope in the video)
        that small “bump” in the wave is there and gone when unplugging the REVERB IN

        Comment


        • What are you doing in that vid? It looks like you're turning something? Are you somehow disconnecting the reverb there?

          When you say "unplugging the REVERB IN" do you mean you are unplugging the tank from the recovery stage?
          "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


          • I think that 'bump' may be a different artifact. I'm not seeing the parasitic from the video of post #208 (20 second mark).
            Suggest you try listening test.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • Originally posted by g1 View Post
              I think that 'bump' may be a different artifact. I'm not seeing the parasitic from the video of post #208 (20 second mark).
              Suggest you try listening test.
              I think that bump may be the parasitic and he's just not pushing the amp in that vid to where the bump takes the jagged shape?
              "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


              • the difference between that and this video is that on the #208 one the scope is attached to the SPKR Jack and in this one to V2 Pin 7 (that oscillation is not "that" evident earlier in the circuit).

                to tell you the truth, it was 5:30 AM and I was trying to reproduce the same thing shown on the #208 video with the sope attached to the SPKR out before removing the bright switch (to make sure the settings were right and establish a baseline) but couldn't (I saw two sharp spikes the the left of the top of the wave but not the ripple under the zero crossing).

                and... "unplugging the REVERB IN" means I am disconnecting the RCA Plug from the back of the amp (the one that reads REVERB INPUT)

                Comment


                • 1) Oscillation
                  It might make sense to experiment with different placements of the 2nF cap again (rev in/out).

                  2) Gain
                  The power amp open loop (no NFB) voltage gain (Go) from the PI input (V6A, pin 2) to the 4 Ohm output is the product of the PI gain and the power stage gain. The PI gain using this calculator (from Richard Kuehnel's excellent ampbooks site): https://www.ampbooks.com/mobile/ampl...ir/calculator/ is 22. The theoretical power stage gain is 0.58. Thus Go is roughly 13.

                  The closed loop gain (820 Ohm connected) Gcl is given by the global NFB formula: Gcl = Go/(1+HGo). H is the NFB voltage divider ratio 100/(820+100) = 0.11. It follows Go/Gcl = 1+HGo = 2.43 (and Gcl = 5.35). This means that the gain should increase by a factor of 2.43 when disconnecting the 820R resistor.

                  Can you measure Go and Gcl (speaker output amplitude/PI input amplitude)? A DMM should do at 400Hz.
                  Last edited by Helmholtz; 05-06-2019, 05:40 PM.
                  - Own Opinions Only -

                  Comment


                  • Originally posted by Helmholtz View Post
                    1) Oscillation: It might make sense to experiment with different placements of the 2nF cap again (rev in/out).
                    Vielen Dank!
                    It sounds totally logic and I've done that, having the oscillation reproduced in the SCOPE I've tried the same as described in post #143
                    .0022 uF cap placed according to the 3 options shown on the schematics AB763, A1172 and AA1069/270 (no cap, rev in to ground, rev out to ground). There was no change (which is not at all what I expected)


                    Originally posted by Helmholtz View Post
                    2) Gain: The power amp open loop (no NFB) voltage gain (Go) from the PI input (V6A, pin 2) to the 4 Ohm outp
                    about the second part, I need to print that and re-read it... but sounds like fun! thx man!

                    Comment


                    • Originally posted by TelRay View Post
                      I've never heard a change in sound when lifting the 820 Ohm resistor going to the speaker jack (marked on schematic below). I think there's something wrong in this area. All the components are new (resistors, capacitors)
                      It should be louder or more distorted with the 820R disconnected. Have you checked the 0.1u capacitor between V6 pin 7 and the junction of the 820R and 100R resistors? To test the cap measure the AC voltage on both sides of it with the scope AC coupled. The readings should be the same.

                      Comment


                      • Originally posted by g1 View Post
                        I think that 'bump' may be a different artifact. I'm not seeing the parasitic from the video of post #208 (20 second mark).
                        Suggest you try listening test.
                        i believe that on that very early morning the SINE WAVE I was feeding the amp with was set at a low volume (I’m generating it with a PC)
                        I am able to reproduce the same parasitic oscillation now with the BRIGHT SWITCH still out

                        I can also re-confirm that that parasitic oscillation shows up on V2 pin 7 and it’s gone if I disconnect the REVERB INPUT (that’s what the label on the back of the amp reads) RCA Plug
                        Last edited by TelRay; 05-08-2019, 12:29 AM.

                        Comment


                        • Originally posted by Dave H View Post
                          Have you checked the 0.1u capacitor between V6 pin 7 and the junction of the 820R and 100R resistors?

                          No, sir. I haven’t. Nevertheless it’s a brand new orange drop.

                          Originally posted by Dave H View Post
                          To test the cap measure the AC voltage on both sides of it with the scope AC coupled. The readings should be the same.
                          That sounds like an interesting test. Sorry about my basic skills, could you detail how is it done?
                          Thx!!!

                          Comment


                          • Originally posted by TelRay View Post
                            That sounds like an interesting test. Sorry about my basic skills, could you detail how is it done?
                            Disconnecting the 820R feedback resistor should increase the output voltage a lot more than you are seeing on the scope. It would be good if you could test it as below so we check if the power amp and feedback is working correctly.

                            Connect the amp to a dummy load and apply a signal to drive it to about half way to clipping so the output is a nice clean sinewave then measure the input voltage (MV pot wiper), feedback voltage (junction of 820R and 100R)) and output voltage (dummy load). If you are measuring with a scope use a x10 probe so as not to load down the MV pot.

                            Comment


                            • REVERB TANK
                              - Removed the tank from the amp's cabinet
                              - Moved cables (from tank to amp) around
                              - the oscillation is present also using other cables
                              - the oscillation disappears either when disconnecting the INPUT cable at the tank or at the amp
                              - the tone generated with the tone generator is heard on the INPUT transducer of the tank (this is probably normal?) - you can hear this in the video below
                              - the oscillation show up on the scope between (more or less) 300-900 Hz

                              Comment


                              • Originally posted by Dave H View Post
                                Disconnecting the 820R feedback resistor should increase the output voltage a lot more than you are seeing on the scope
                                thanks for that. the difference was also not audible when I've tried connecting the SPEAKER, run the SINE WAVE and connect / disconnect the 820R

                                Originally posted by Dave H View Post
                                Connect the amp to a dummy load and apply a signal to drive it to about half way to clipping so the output is a nice clean sinewave then measure the input voltage (MV pot wiper), feedback voltage (junction of 820R and 100R)) and output voltage (dummy load). If you are measuring with a scope use a x10 probe so as not to load down the MV pot.
                                Got a nice SINE WAVE (scope at the SPKR OUT) with VIBRATO CHAN VOL @ 3 and MV @ 4

                                I've done this in different ways, measuring both DC and AC, multimeter and the scope hoping any of these results will make sense to you guys

                                DC MEASUREMENTS
                                MV Pot Wiper = 360 mV DC (w/multimeter, ground connected to chassis)
                                FBK V = 393 mV DC (w/multimeter, ground connected to chassis)
                                DUMMY = 101 mV DC (w/multimeter, connected to the output SPKR jack)

                                AC MEASUREMENTS
                                MV Pot Wiper = 1.64 V AC (w/multimeter, ground connected to chassis)
                                FBK V = 0.75 V AC (w/multimeter, ground connected to chassis)
                                DUMMY = 6.9 V AC (w/multimeter, connected to the output SPKR jack)

                                Additionally, with the scope set at 0.2 V/DIV I counted 8.25 DIV between the top of the WAVE and the bottom, so it should be 1.65 V (AC? DC?)
                                Last edited by TelRay; 05-08-2019, 04:28 PM.

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