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

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  • Did someone replace the external speaker jack with a shorting jack? That would prevent NFB from having an effect.
    But that would also ground/kill the output from the main speaker jack. The picture in post#46 shows the correct jack types and wiring.
    Last edited by Helmholtz; 04-25-2019, 01:46 PM.
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    • Another symptom is that I've never heard a change in sound when lifting the 820 Ohm resistor going to the speaker jack
      Looks as if the 820R resistor is/was either open or not properly connected.
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      • about 60 or 120 Hz hum, should I be hearing that hum?
        Not necessarily.

        The superimposed low frequency makes your signal tops move periodically up and down.
        What do you see if you change the horizontal setting (time deflection coefficient) to 10ms?
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        • thx guys for the posts, I have just checked the wiring making sure there was continuity from one point to the next.
          additionally made the following schematic to clarify the connections (hope it is clear enough):

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          • Originally posted by TelRay View Post
            thx guys for the posts, I have just checked the wiring making sure there was continuity from one point to the next.
            additionally made the following schematic to clarify the connections (hope it is clear enough):

            I guess the outer circles of the jacks in your drawing connect to the tip of the plug and the center dots mean ground contacts?

            If the 820R actually measures 820R and its other end connects to the 100R node, it MUST make a difference if you lift it. Your output signal should be noticeably higher without the 820R.
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            • Caveat: In your model/version of the SF Twin none of the master volume lugs must be grounded. Rather the MV sits on top of the 100R (NFB shunt resistor in the PI tail).
              If you grounded the MV, this would short out the 100R and kill NFB.
              Last edited by Helmholtz; 04-25-2019, 07:59 PM.
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              • Originally posted by Helmholtz View Post
                I guess the outer circles of the jacks in your drawing connect to the tip of the plug and the center dots mean ground contacts?
                yes, I was trying to match what is seen in the photo (where the BLACK, WHITE, GREEN wires go)

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                • conventionally, the jack is represented something like this (click to zoom in):
                  Click image for larger version

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                  With the sleeve part of the jack, which contacts the larger oval, shown in a type of 'shield' symbol
                  and the tip contact shown as the dot.
                  Note that if you look at it on-axis, the dot is in the center of the larger circle.
                  Does that help?

                  edit: of course, there's the 'other' way to represent a spring-contact phone jack, but that's not what we're going for here.
                  If it still won't get loud enough, it's probably broken. - Steve Conner
                  If the thing works, stop fixing it. - Enzo
                  We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                  MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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                  • Originally posted by eschertron View Post
                    conventionally, the jack is represented with the sleeve part of the jack, which contacts the larger oval, shown in a type of 'shield' symbol and the tip contact shown as the dot.
                    you are right! thx... in my defense... it was 5:30AM

                    WHITE and GREEN wires connected to the TIP of EXT and SPKR jacks
                    BLACK wire connected to SLEEVE of EXT jack

                    sorry guys for the unintended confusion

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                    • Originally posted by TelRay View Post
                      you are right! thx... in my defense... it was 5:30AM

                      WHITE and GREEN wires connected to the TIP of EXT and SPKR jacks
                      BLACK wire connected to SLEEVE of EXT jack

                      sorry guys for the unintended confusion
                      No problem.
                      We are still left wondering why the NFB does not seem to be working. Helmholtz has made a good point to verify that the 100R resistor is indeed lifting the tail of the PI from ground.
                      If it still won't get loud enough, it's probably broken. - Steve Conner
                      If the thing works, stop fixing it. - Enzo
                      We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                      MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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                      • Originally posted by Helmholtz View Post
                        If the 820R actually measures 820R and its other end connects to the 100R node, it MUST make a difference if you lift it. Your output signal should be noticeably higher without the 820R.
                        the connections tested good:
                        SPKR tip to EXT SPKR tip to 820R to 100R to GND

                        the 820R is dead on and a new resistor (as the other ones you see in the photo)





                        Originally posted by Helmholtz View Post
                        In your model/version of the SF Twin none of the master volume lugs must be grounded. Rather the MV sits on top of the 100R (NFB shunt resisistor in the PI tail)
                        yes, that actually puzzled me as I did not understand why were not grounded and went through the 100R that is connected to ground.
                        checked that with the schematic when I was putting the new coax cables on the potentiometer a couple of days ago. here's the schematic:

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                        • the connections tested good:
                          SPKR tip to EXT SPKR tip to 820R to 100R to GND
                          just to verify:
                          You did test the 100R node to ground and measured 100R with your meter? Actually, with the 820R lifted it will measure 100R. With the 820R in circuit it should read about 90R. If you haven't, please do these tests with one clip of the meter on a ground reference. I usually use the chassis to verify that there is a common ground that runs through the circuit, and I don't have to move that lead around - it just stays there for all my checks.

                          The way you've drawn your schem, it looks like the COAX shield is carrying signal. That's all I'm going to say about that.
                          If it still won't get loud enough, it's probably broken. - Steve Conner
                          If the thing works, stop fixing it. - Enzo
                          We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                          MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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                          • Originally posted by TelRay View Post
                            the connections tested good:
                            SPKR tip to EXT SPKR tip to 820R to 100R to GND

                            the 820R is dead on and a new resistor (as the other ones you see in the photo)







                            yes, that actually puzzled me as I did not understand why were not grounded and went through the 100R that is connected to ground.
                            checked that with the schematic when I was putting the new coax cables on the potentiometer a couple of days ago. here's the schematic:

                            What is the DC voltage at the 820R/100R node?
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                            • Originally posted by eschertron View Post
                              just to verify:
                              You did test the 100R node to ground and measured 100R with your meter? Actually, with the 820R lifted it will measure 100R. With the 820R in circuit it should read about 90R. If you haven't, please do these tests with one clip of the meter on a ground reference. I usually use the chassis to verify that there is a common ground that runs through the circuit, and I don't have to move that lead around - it just stays there for all my checks.
                              yessir, the values are:

                              100R to ground with 820R connected = 89 Ohm
                              100R to ground with 820R lifted = 100 Ohm
                              820R lifted = 817 Ohm
                              820R to ground (with 820R lifted) = 917R (logical)

                              Originally posted by Helmholtz View Post
                              What is the DC voltage at the 820R/100R node?
                              It is, 405 mV

                              Actually measured all 3 nodes wired to the MASTER VOLUME:

                              Last edited by TelRay; 04-26-2019, 01:08 PM.

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                              • It is, 405 mV
                                Around 0.4VDC across the 100R is O.K.
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