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

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  • #31
    Originally posted by Chuck H View Post
    ...stab the board in random places with the red probe. Especially near HV eyelets.
    Hey, I am guessing that the “doghouse” eyelets should be a target for this check as well, right?

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    • #32
      I've never seen a board conduct enough voltage for it to cause more than a little hum or buzz from leakage on the doghouse board. More of an issue on the signal board. Stabbing near plate resistor junctions and then near grid and cathode associated eyelets. Not that the grids and cathodes are high voltage, but this is where leaky voltage can cause the biggest problems, so we check.
      "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

      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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      • #33
        well, I found a couple of hours to do some measurements.

        Initially I wanted to gather reference values as I have no idea what voltage range was acceptable so I kind of probed the whole board. I've found values ranging from 10 to 160 mV DC. Which I am guessing that since I've measured my guitar output at 150 mV it might not be a good thing. The problem is that these higher values are more or less all over the place. Here's a mapping:

        Last edited by TelRay; 02-26-2019, 06:35 AM.

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        • #34
          AC or DC?
          Education is what you're left with after you have forgotten what you have learned.

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          • #35
            all voltages DC

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            • #36
              That's about what I typically see on a "normal" black paper circuit board. Well, I've never ranged it like that (nice work BTW), but yeah, low DC readings randomly present all over. When you consider that even a meager preamp tube biases in one to two volt DC range .016VDC isn't going to make a whole lot of difference. I wouldn't worry about it for now. If you get to a point where you plan to rebuild the circuit I would replace the board with something that is:

              1) Not paper
              2) Not filled with carbon

              "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

              "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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              • #37
                Yeah, I won't worry about 100mv. I see boards where I can measure 150v near eyelets for plate resistors.
                Education is what you're left with after you have forgotten what you have learned.

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                • #38
                  Well, thank you guys.

                  on the POSITIVE side I am happy I do not need to change that complete board (it would have been a real pain in the a** and would have created potential wrong wiring)

                  on the other hand, the mystery still remains.

                  I've found 2 things:

                  1) I have remeasured the grids on V1 and V2 using the multimeter in the mV position
                  a) I could confirm 0 mV on pin #2 for both tubes (grid section 2, the one attached to the INPUT JACK)
                  b) however, pin #7 (grid section 1, attached to the CHANNEL VOLUME) was 0 mV with all the knobs turned to minimum... but... the values went up when turning the controls up (on V1 to 2 mV and on V2 to 6 mV)
                  NOTE: the 6 mV were a bit jumpy on the reading (from 2 to 6mV)

                  2) measuring voltages on V6 (phase inverter) there are some noises that range from scratchy, to a loud pop and to a super loud pop (with all knobs including the master volume at minimum).
                  V6-1: 309 V (scratchy noise, see video)
                  V6-2: extremely loud POP (near speaker damaging, did not dare to leave the probe there for a reading again but in the past was near 110 V)
                  V6-3: 141 V (scratchy noise)
                  V6-6: 311 V (scratchy noise)
                  V6-7: 110 V (loud POP - see video - , but 50% of the loud pop on pin #2)

                  NOTE: I purposely measured on the board and not at the tube to discard the noise coming from the socket



                  I have on the list:
                  - recheck all 0V returns
                  - lift the 820 Ohm NFB resistor

                  keep tuned in!!!

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                  • #39
                    The PI remains pretty much signal hot to the power tubes on all pins even with all the controls down. Though I have to say that it's a strange static sounding tick/pop that I just can't remember hearing. Though I may have and just don't remember. Tapping the meter on the PI circuits isn't really something I do. I just stick the probe there and leave it. And if I am measuring something there I'm usually using a dummy load so there's no audio.

                    But checking all 0V returns seems like a good idea. I've tried to trace some looking at the pics, but a vein in my head started bulging so I had to stop
                    "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

                    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                    Comment


                    • #40
                      Originally posted by Chuck H View Post
                      checking all 0V returns seems like a good idea. I've tried to trace some looking at the pics, but a vein in my head started bulging so I had to stop
                      I did check (almost) all the 0V returns (not the ones coming from the doghouse capacitors). I did it by measuring there was actually 0V and checking continuity between the component and chassis. All was OK.

                      however... don't ask why... but I placed the multimeter probe at the eyelet where the vibrato pedal cable is connected to the 2.2 MOhm + the 2x 1 MOhm resistors and read -37V. Turned the vibrato ON and read 0 V. So I was curious and followed that to the BALANCE control wiper and found there is a missing capacitor (80 uF 75V).



                      Why the hell is this capacitor missing... who knows... 45 years of history lost in the sands of time.

                      After installing a 100 uF 100V cap no change in the sound (still high freq distortion when playing the guitar).

                      Last edited by TelRay; 02-27-2019, 05:24 AM.

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                      • #41
                        That cap is part of the bias circuit. Since you changed the bias circuit it may no longer be desirable in that position.?.

                        The 0V doghouse cap connections are actually the most suspect with this problem, so do check those. Also, do try lifting the FB circuit. If there's still no change I think you should pull the reverb driver tube V3 and see if that changes anything.
                        "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

                        "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                        • #42
                          I always use a resistor in series to the bright cap,100 to 330k,that solves all problems for me

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                          • #43
                            all 0 V returns checked (now including the doghouse)
                            mulimeter reading 0V and chassis continuity connection checked - all OK

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                            • #44
                              Originally posted by alexradium View Post
                              I always use a resistor in series to the bright cap,100 to 330k,that solves all problems for me
                              thank you for jumping in, man
                              as the problem analysis progressed it seems that the BRIGHT SW capacitor is not the problem (I have taken it out of the circuit) but something else (still trying to figure out what)
                              thx!

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                              • #45
                                Originally posted by Chuck H View Post
                                That cap is part of the bias circuit. Since you changed the bias circuit it may no longer be desirable in that position.?
                                Mr Chuck, sir... here I will need to, respectfully, disagree. As I understand the mod I introduced (and as I said I am trying to back the amp back to spec so I do not like introducing any substantial changes) was just transforming the fix 15 KOhm resistor value into a 10 KOhm one + a potentiometer
                                so I understand the electrolytic capacitor should have been kept in the circuit (it wasn't there when I bought the amp, of that I am sure)

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