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

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  • Perfect and reasonable order/sequence of steps

    Regarding "6.- Faraday Cage add Aluminum foil to the enclosure", you may tack some household aluminum foil to a large enough cardboard and place it on the chassis (making sure it makes contact) to test its effect outside the cabinet.
    The additional shielding may actually reduce oscillation and unwanted coupling issues, so also check with scope at SPK OUT.
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    • Aluminum coil stock is available at any home center, sold as flashing.

      I understand the thicker/heavier foil is called flashing, right?

      What would you call the alu sheet metal used in Marshall tops?
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      • Originally posted by Helmholtz View Post
        I understand the thicker/heavier foil is called flashing, right?

        What would you call the alu sheet metal used in Marshall tops?
        I don't think the heavy foil is flashing. At least here, flashing is a building material, most often seen at roof-edge, so more like the marshall stuff.
        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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        • TE=Helmholtz;529871]I understand the thicker/heavier foil is called flashing, right?

          What would you call the alu sheet metal used in Marshall tops?[/QUOTE]

          In the States aluminum foil is thin (easily torn by hand) rolls used to cover a casserole or a potato for baking.

          Flashing is a mm or two thick with some strength and rigidity to it.

          I could imagine aluminum foil torn/ unglued hanging down by the high voltage parts. That was my concern.

          Sorry for any confusion.

          Comment


          • Originally posted by Helmholtz View Post
            Regarding "6.- Faraday Cage add Aluminum foil to the enclosure", you may tack some household aluminum foil to a large enough cardboard and place it on the chassis (making sure it makes contact) to test its effect outside the cabinet.
            The additional shielding may actually reduce oscillation and unwanted coupling issues, so also check with scope at SPK OUT.
            Great, will definitely try it like that unmonted. I Did before but have not seen a change in the oscillation, now that the 560 pF is doing part of the job flattening it out I will give it another try.

            Originally posted by ric View Post
            You might want to avoid aluminum foil. Torn or loose foil could be a problem. Aluminum coil stock is available at any home center, sold as flashing.
            thanks Ric for jumping into the conversation. Any particular (affordable!) one you might know of? A quick search on Home Depot shown results of $80 or more (and I don't really need that much Aluminum)

            https://www.homedepot.com/p/Spectra-...W187/205009266

            Comment


            • alright, people. First reports back from the front:

              1.- Add 20 uF / 500 V Cap
              I got the cap soldered to point "A" ('+' side) and with an alligator clip to the ground side of the V3 grid resistor for a quick A/B comparison



              Note that since post # 85 the 470 Grid resistor has been replaced by a 25 uF/25V cap and a 2.2K resistor, so it looks like this:

              (meaning that the alligator clip, or '-' cap side is attached to the ground shared by the 2.2K resistor and the 25 uF cap)

              I believe that maybe there's one test that I forgot to do after post #240 in which I placed the scope at V3 pin 6 when there was still no 560 pF cap (added on post #262).
              I think should have placed the scope BEFORE and AFTER the 560 pF, to see what is the effect of the cap. If I do that now, I get the following:



              POINT 1: before the 560 pF cap, I get the same oscillation we've seen on post #240



              this does not change when connecting the newly introduced 20 uF / 500 V cap

              POINT 2: after the 560 uF cap, point "A". Completely flat line, either with the 20 uF / 500 V cap connected or not



              So, given the fact that there was no signal after the 560 pF I moved the scope to the SPKR OUT to see if there was any benefit in introducing the 20 uF / 500 V cap. The answer is: no. The same bump place where the oscillation used to be before introducing the 560 pF cap is still there.



              Additionally tried disconnecting the .0022 uF cap from the REVERB INPUT, leaving it disconnected and connecting it to the REVERB OUTPUT but have seen no change.
              Last edited by TelRay; 05-20-2019, 11:04 PM.

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              • POINT 1: before the 560 pF cap, I get the same oscillation we've seen on post #240
                What do mean with before and after the 560p cap?

                POINT 2: after the 560 uF cap, point "A". Completely flat line, either with the 20 uF / 500 V cap connected or not
                Confused. Didn't you report and even show a scope picture of point A having signal voltage of 90mV? (Scope input should be AC coupling and mV range.) What changed?
                Do (did) you always have the reverb control at zero?
                Last edited by Helmholtz; 05-20-2019, 09:59 PM.
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                • [QUOTE=Helmholtz;529969]What do mean with before and after the 560p cap?

                  I edited the post for clarity, there was a paragraph placed in the wrong section. Sorry.

                  What I mean is that: yes, I have placed the sope on Point "A" before and seen the oscillation. At that point in time, placing the scope on point "A" or at V3 pin 6 meant the same thing as there was no component in between.

                  Now it's different with the 560 pF cap so there is a difference in placing the scope in point 1 (between V3 pin 6 and the cap) or point 2 (between the cap and point "A").

                  When I introduced the 560 pF cap into the circuit (as per the schematic) I have measured the effectiveness by placing the scope at the SPKR OUT (not at point "A" anymore) and seen how the oscillation got flattened into a bump.

                  Now, if I place the scope before and after the 560 pF cap I see signal + oscillation on the V3 pin 6 side but a flat line on the point "A" side (which is something I have not done, unless I am wrong, after the introduction of the 560 pF cap).

                  Hope is clearer now with this image



                  NOTE: need to check on the AC coupling setting and REVERB at 0 (in any case the two photos here were done with the same setting s on the scope and the amp)
                  Last edited by TelRay; 05-20-2019, 11:21 PM.

                  Comment


                  • Originally posted by TelRay View Post


                    thanks Ric for jumping into the conversation. Any particular (affordable!) one you might know of? A quick search on Home Depot shown results of $80 or more (and I don't really need that much Aluminum)

                    https://www.homedepot.com/p/Spectra-...W187/205009266
                    Looks like Lowe's is selling unpainted mill flashing rolls @ 6"x12' for $13 and change/ 6"x50' for $17 and change here in mid Michigan. 24" is considerably more.

                    Also saw it advertised as being at Wal-Mart, Ace Hardware and around here it's a fair price @ Menards, if you have one of those out there.

                    Comment


                    • Originally posted by ric View Post
                      Looks like Lowe's is selling unpainted mill flashing rolls @ 6"x12' for $13 and change/ 6"x50' for $17 and change here in mid Michigan. 24" is considerably more.

                      Also saw it advertised as being at Wal-Mart, Ace Hardware and around here it's a fair price @ Menards, if you have one of those out there.
                      we do have an Ace store a couple of miles away... great tip, man!!!

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                      • At that point in time, placing the scope on point "A" or at V3 pin 6 meant the same thing as there was no component in between.
                        Definitely no! V3 pin 6 is the reverb driver output (must have signal voltage) and point A is connected to a power supply filter cap (should show no signal as this is an AC ground) - no matter if the 560p cap is connected or not. Now I wonder what the vid in post #255 actually shows.

                        Now it's different with the 560 pF cap so there is a difference in placing the scope in point 1 (between V3 pin 6 and the cap) or point 2 (between the cap and point "A").
                        A tiny 560p cap influences HF but could never eliminate signal voltage from the power supply (point A).

                        (Saying "before" and "after" the cap is confusing. Please always specify the circuit point you mean.)

                        BTW, 1µ=1000000 (1million) times 1p.
                        Last edited by Helmholtz; 05-21-2019, 03:23 PM.
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                        • thank you guys for being patient (my learning capabilities allow me to barely keep pace with the overwhelming support you are providing).
                          I deep dived into the scope measurements around point "A", V3 pin 6 with and without the 560 pF cap and with and without the 20 uF 500V ecap.
                          I believe that my statement before about a "flat line after 560 pF cap" is wrong due to not changing the SCOPE's settings to actually "amplify" and see what that flat line really was (vertical scale needed to change from 5V/div to 10 mV/div as Helmholz correctly pointed)
                          This is what I would see if I leave the settings on the SCOPE at 5V/div and 0.5 ms/div



                          in the following video you can see:
                          - scope placed at V3 pin 6 with REVER INPUT RCA connected
                          - scope placed at V3 pin 6 with REVER INPUT RCA disconnected
                          - flat line on SCOPE when measuring POINT "A"
                          - change in SCOPE settings from 5V/div to 10 mV/div



                          - keeping the same scope settings at 10 mV/div added the 20 uF 500 V e cap




                          Originally posted by Helmholtz View Post
                          Definitely no! V3 pin 6 is the reverb driver output (must have signal voltage) and point A is connected to a power supply filter cap (should show no signal as this is an AC ground) - no matter if the 560p cap is connected or not. Now I wonder what the vid in post #255 actually shows.
                          totally right, I incorrectly mentally deleted the 560 pF from the circuit but kept V3 pin 6 and POINT "A" connected by an imaginary wire
                          Last edited by TelRay; 05-21-2019, 06:51 PM.

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                          • Sorry, this is too much and confusing.

                            - There is no sense measuring at the open end of the 560p.
                            - With 560p connected at both ends, do you see a difference when connecting the 20µ (I would actually prefer the 20µ soldered at both ends, as this ensures lowest contact resistance). Please read the peak-to-peak voltages without changing controls' settings or signal level.
                            - Whenever posting a scope picture, specify the scope settings.
                            - All comparisons must be done at exactly the same controls' / scope settings and signal level.
                            - Own Opinions Only -

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                            • Originally posted by Helmholtz View Post
                              Sorry, this is too much and confusing.
                              Sorry about that
                              The intention was to clarify that my mistake was to keep the same settings on the SCOPE when measuring V3 pin 6 and POINT A

                              Originally posted by Helmholtz View Post
                              With 560p connected at both ends, do you see a difference when connecting the 20µ (I would actually prefer the 20µ soldered at both ends, as this ensures lowest contact resistance). Please read the peak-to-peak voltages without changing controls' settings or signal level.
                              That's a good question, the video shows a very unstable and jumpy signal. I will try to take a photo so I can freeze it and count. Is that instability normal or I am doing something wrong?


                              Originally posted by Helmholtz View Post
                              Whenever posting a scope picture, specify the scope settings.
                              Good point. I think I did by saying the first batch of photos were at 5V/div and 0.5 ms/div and then (only in the video) there is a gradual change to 10 mV/div (will make sure is clear every time)

                              again (and again) thx!

                              Comment


                              • That's a good question, the video shows a very unstable and jumpy signal. I will try to take a photo so I can freeze it and count. Is that instability normal or I am doing something wrong?
                                The "jumpiness" is normal and caused by the underlying 120Hz ripple voltage. Only look for a significant difference.

                                I think I did by saying the first batch of photos were at 5V/div and 0.5 ms/div and then (only in the video) there is a gradual change to 10 mV/div (will make sure is clear every time)
                                Too much guesswork for me. I prefer big scope pictures with precise settings information, because we need to use the scope as a measuring instrument.
                                Last edited by Helmholtz; 05-22-2019, 06:53 PM.
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