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Twin reverb loud hum when volume at 0.

Discussion in 'Amp Tech Center' started by BlacknWhite, Nov 28, 2020.

  1. BlacknWhite

    BlacknWhite TDPRI Member

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    Hey guy's. Recently picked up a 1975 Fender twin reverb. Guitar signal sounds great through it but it has a strong hum. Mostly at zero or max volume. Has a sweet spot at 3.5 where it sounds amazing. I've re capped all the electrolytics and coupling caps. Touched up solders and swapped tubes like mad. I have VI pulled as I never use the 1st channel. Loud hum at zero volume. Any ideas? All plate and bypass resistors replaced as well.
     
  2. Dacious

    Dacious Poster Extraordinaire

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    Bad solder joint in the doghouse. Did you clean up behind the controls earthing plate? All knobs right? MV model, right?
     
  3. Telecastoff1

    Telecastoff1 Tele-Afflicted

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    Check all of the solder joints carefully along the brass strip under the pots. That's notorious for what you're experiencing.
     
  4. BlacknWhite

    BlacknWhite TDPRI Member

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    I've checked and re flowed most solder joints. Cleaned all the pots. It was a master volume which I removed. Sound is still the same even after I removed the master. The control plate looked pretty Clean. I'll pull the pots and try and clean it up a bit.
     
  5. BlacknWhite

    BlacknWhite TDPRI Member

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    They all look good. I reflowed one of the. But no change.
     
  6. Lucius Paisley

    Lucius Paisley Tele-Holic

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    Does that zero have a 1 in front of it?
     
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  7. tubelectron

    tubelectron Tele-Afflicted

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    IMHO, loud hum - I think about :

    - a problem in the PSU.
    - a problem in the Phase Inverter or Output Stage.

    ---> Check all plate voltages and bias voltages with a DMM. Tolerance = +/-10%. It may help you to locate the issue.

    Be careful and ready to turn off : your amp may be in danger... ;)

    -tbln.
     
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  8. 24 track

    24 track Doctor of Teleocity Ad Free Member

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    pull the tubes one by one , clean the contacts on each tube with DeOxit and a Q tip reseat the tube rince and repeat for each tube one at a time see if that might help to clear it up , take a chopstick tap on the tubes to see if a tube is the culprit ,( microphoning) if no tap, on the components is side to see if something comes to light (one hand rule always)

    before you go balls deep into rewiring
     
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  9. BlacknWhite

    BlacknWhite TDPRI Member

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    I was just checking the wiring at the PI. I pulled the pre amp tubes no change. Pulled the PI and it was dead quiet. Still white noise but dead quiet. This is the first twin I've had to work on. I can't see any issues around the PI though every thing looks good. Haven't found any wonky voltages yet.
     
  10. Dacious

    Dacious Poster Extraordinaire

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    Try putting another tube in the PI position. Any tube will do, the tube you pulled from the normal channel. All you're doing is seeing if the tube's gone bad.
     
  11. BlacknWhite

    BlacknWhite TDPRI Member

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    I've tried a bunch of tubes from my stash and same with each one unfortunately
     
  12. Telecastoff1

    Telecastoff1 Tele-Afflicted

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    Are all of your jacks and pots down tight? Nothing loose?
     
  13. Milspec

    Milspec Poster Extraordinaire

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    Bad pot...what brand did you use, seen some cheaper ones do that very thing new out of the box
     
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  14. mojek

    mojek Tele-Meister

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    Agree. Had this issue with 77 AC30. It’s bad volume pot. Tha shaft works, but at 0 position it’s lifted and not grounded as it should be. Chamged the 1M/A pot and works perfectly now.
     
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  15. smuc

    smuc TDPRI Member

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    Since the twin has two channels, I would doubt that both of the pots are bad... Generally, @BlacknWhite please give a bit more clarity of how the hum behaves. Is it consistent or random? How does it change with volume or other controls? Is the reverb on or not? Does it start immediately or when the amp gets warm...

    And another stupid question - is your amp earthed? Have you tried plugging it into an another outlet?
     
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  16. Asmith

    Asmith Friend of Leo's

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    I've seen this as an issue before, if you're using an isolation transformer (maybe some variacs) then it may be something you've over looked.
     
  17. BlacknWhite

    BlacknWhite TDPRI Member

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    OK sorry guys I'll be a little more descriptive. Last night my brain was fried after a day of trouble shooting haha. The hum is kinda more of a buzz/hum. Almost like a bad ground but I've yet to find one bad ground. Also when you move the volume pots on either channel near the middle position the amp goes dead quiet and it sounds great. I've tried multiple outlets around my house. I've currently got it on my bench with the reverb tank hooked up but off. I pulled V1, V2 and V3 with no change in hum/buzz. When I pull V4 hum disappears. Haven't found anything yet though. As I said before I replaced all caps and out of spec resistors. Touched up all solders and grounds. Here's a video of me sweeping the volume pot. The initial hum is at 0 or 1 haha and that's what you hear until you pull V4.

     
  18. Whatizitman

    Whatizitman Friend of Leo's

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    Did it hum before you modded the MV? The MV was stock when you got it? That can at least narrow it down to driver error ;)
     
  19. BlacknWhite

    BlacknWhite TDPRI Member

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    Yeah the hum was there before I removed the master. This one is driving me insane lol. I don't get a chance to work on many Fender amps because there don't seem to be many of them around this area. That's why I grabbed this one up. The few I have worked on were all head scratchers though haha.
     
  20. Wally

    Wally Telefied Ad Free Member

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    The hum does not go away when you pull V2, which is the input gain stage for the Vib ch. When you pull V4, which has one triode as the reverb return and one triode as a gain stage in the Vib ch, the hum goes away. Good info. So, when V4 is out of the circuit, does the Normal channel operate with no buzz? If so, then you can isolate your search to that V4 tube and the associated circuitry and components.
     
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