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Thread: Fender 75

  1. #1
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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 Schematic75-schematic.jpg

    GordB
    Last edited by GordB; 02-12-2018 at 01:38 PM.

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

  3. #3
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    Thanks Mick.

    The hum is coming through the speaker.

  4. #4
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    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.

  5. #5
    Senior Member potatofarmer's Avatar
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    Does turning the "output tubes matching" control or "hum balance" control affect the hum?

  6. #6
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    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.

  7. #7
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    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.

  8. #8
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    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.

  9. #9
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    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
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  10. #10
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    Quote 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.

  11. #11
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    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.
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  12. #12
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    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.
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  13. #13
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    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.

  14. #14
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    Quote 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?

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

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    OK here's the picture. The bias balance is on your right and the hum balance is on the left. These caps are in the heater circuit, just in a funny place. It complicates the adjustable bias business a little.75-bias-balance-pot.jpg

  17. #17
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    That's ok, they're just using the casing (ground), not to the actual pot lugs. They are connected to the heater wires as on the schematic.
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    Exactly.

  19. #19
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    I think pdf64 and myself both had the impression they were going to the pot lugs as well as the casing.
    If they are in the way I'm sure another ground spot would suffice.
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  20. #20
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    Quote Originally Posted by g1 View Post

    I think pdf64 and myself both had the impression they were going to the pot lugs as well as the casing.
    If they are in the way I'm sure another ground spot would suffice.

    We are talking about the same thing, but maybe I didn't communicate it accurately enough.

    I'd like to abandon the bias balance/bias adjust control idea, and instead install test points and a separate pot for adjustment.

    It looks like the ideal spot would be where the ground switch is located, because it is located close to the output tubes. I have a three conductor cord which is factory installed, so I can't see any issues there that need to be addressed.

    Please feel free to let me know if this is a bad idea.

  21. #21
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    You need to keep the bias balance control or you'll always have hum issues from not being able to cancel out the power supply ripple that gets to the screen node. On my 135W Twin I removed the "line out" jack and put a standard Fender-style bias pot in that location, though it's a pretty tight fit.
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  22. #22
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    I like having a balance AND adjust in my Fenders! Matched tubes? I don't need no stinking matched tubes!

    Justin
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    "If you're using one of those Digiyuk module type things..." - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  23. #23
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    Quote Originally Posted by potatofarmer View Post
    You need to keep the bias balance control or you'll always have hum issues from not being able to cancel out the power supply ripple that gets to the screen node. On my 135W Twin I removed the "line out" jack and put a standard Fender-style bias pot in that location, though it's a pretty tight fit.
    Oh yeah definitely.. Like on a Twin amp or The twin, that kind of set up.

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