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  • Fender 75

    Good morning everyone.

    I have a Fender 75 amp that I recently purchased. This amp worked when I got it but sounded funny (ugly distortion on bass notes or hard chording)

    I opened it up and saw that three of the cathode bypass caps appeared to be leaking. I replaced all the cathode bypass and filter caps as a precautionary measure as they all were dated 1979.

    It worked and sounded much better (It's been a week but only sporadically playing through it) until last night when it developed a loud hum.

    Symptoms:

    No hum in standby
    Loud hum with all knobs at 1 (or zero).
    Very minor volume change when rotating master volume control but only responds (and very subtle response) to this control.
    Hum is still present with power tubes removed although much quieter.
    Hum is still present with all tubes removed.

    Thanks in advance for any help.

    Added SchematicClick image for larger version

Name:	75 schematic.jpg
Views:	1
Size:	2.76 MB
ID:	873303

    GordB
    Last edited by GordB; 02-12-2018, 11:38 AM.

  • #2
    Is this a mechanical transformer hum, or one that's coming through the speakers?

    Comment


    • #3
      Thanks Mick.

      The hum is coming through the speaker.

      Comment


      • #4
        It's a holiday here, so I have some time for this.

        I have the amp out of the cabinet and am going to check some voltages.

        I have only a DMM which has limited capacitor testing capability.

        Comment


        • #5
          Does turning the "output tubes matching" control or "hum balance" control affect the hum?

          Comment


          • #6
            Thanks PF.

            Yes both of these controls work.

            I would like say that I like the idea of both of these controls but also I think an adjustable bias would be nice.

            Comment


            • #7
              SOLVED

              I went back over my work and found a poor solder joint in the mains filter caps.

              As soon as I pulled the can off there it was.

              Soldered it back up then touched up everything I did in the amp.

              Now it's quiet again. (Well, quiet as it ever was)

              I can't play til a little later so not 100% sure but I think it's fixed.

              Thanks a bunch.

              Comment


              • #8
                Well it is definitely quieter but I'm still thinking that there is something not quite right with it.

                I believe I've gone as far as I can and I'll have to take it to a tech.

                Comment


                • #9
                  There are probably multiple issues, regarding the hum. If you had three bad cathode bypass caps leaking, and a bad solder joint on the main cap board, it's probably a safe bet that every electrolytic cap in the amp is bad. Not every bad electrolytic cap leaks...

                  The fact that various knobs change the noise to SOME extent shows that there are hums in at least two different stages, both in the power amp and the preamp before the Volume control. I'd go ahead and replace all of the filter caps on the main board, and then DON'T FPRGET THE BIAS SUPPLY CAPS! Then go through and finish the ones in the preamp one by one, then tweak any Hum Balance or Tube Matching controls or what have you. Also, check to make sure the heater center tap (whether it's a transformer wire or 2 100ohm resistors, I'm not sure) is still good, and I bet you'll be on your way.

                  Since this is a factory-built amp, it's pretty safe to say it didn't leave the factory noisy. So it's a safe bet that something went bad, and you CAN figure it out. (Thanks to Jack Darr for that idea!) You can do this. Check out R.G. Keen's Geofex.com page for more info.

                  I'm not telling you you can't take it to a tech; I just think that if you can change the bypass caps, you can tackle the rest!

                  Justin
                  "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                  "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                  "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                  Comment


                  • #10
                    Originally posted by GordB View Post
                    ... I think an adjustable bias would be nice.
                    Yes, to add one replace the 33k resistor on the 'output tube matching' control with a 25k trimmer pot (wired as a variable resistor) in series with a 27k 1/2 watt resistor.
                    My band:- http://www.youtube.com/user/RedwingBand

                    Comment


                    • #11
                      Thanks Justin. I'll do just that. After all it is what I bought the amp for, developing some level of skill in fixing these things.

                      Comment


                      • #12
                        Thanks a whole bunch for that pdf64. I've seen this mentioned on a couple of sites. I also want to add bias test points and have picked up the parts for it but am working my way up to that. I'll bring my findings with pictures when I get to it.

                        Comment


                        • #13
                          There are 2 ceramic disc caps on the bias balance pot. It looks like they used this pot as ground for these two caps in the heater circuit.

                          Comment


                          • #14
                            Originally posted by GordB View Post
                            There are 2 ceramic disc caps on the bias balance pot. It looks like they used this pot as ground for these two caps in the heater circuit.
                            Are you sure that those caps aren't on the hum balance control?
                            My band:- http://www.youtube.com/user/RedwingBand

                            Comment


                            • #15
                              100% sure. I have a picture of it but it's very large so when I can I will post some pics.

                              Comment

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