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Pro Junior Recap Question... Excessive Hum

Discussion in 'Amp Tech Center' started by Huddy, Jan 11, 2019.

  1. Huddy

    Huddy Tele-Meister

    Age:
    35
    479
    Nov 5, 2016
    Newport News, VA
    I recently replaced the capacitors in a newly acquired USA pro junior from 97. I replaced the speaker because the original must've had a broken voice coil - no resistance. While waiting on the speaker I decided to replace the filter caps - nothing seemed too wrong with them but being over 20 years old and the original Illinois caps, I figured what's an hour?

    I don't remember what it was like before I changed the caps but now there is an excessive hum when not touching the strings - compared with a different amp (Mesa Rocket 44 on dirty channel) in the same location of my home, same guitar.

    Is this more than likely just a characteristic of the amp that I didn't notice before where the interference picked up by the guitar is more prominent?

    I've recapped one for a buddy before because it had a leaky B+ filter cap. I don't remember his being this loud. Again it's only RF interference when not touching the strings - but I feel like it's excessive. I could also be being hyper sensitive to it.

    Thoughts and advice aside from the regular "burn it" and "get a real amp" that are sure to come?
     
  2. jrblue

    jrblue Tele-Holic

    777
    Nov 14, 2010
    Santa Barbara
    First of all, screw the "burn it/real amp" nimrods. If Jeff Beck plays 'em, they're OK and mine is freaking great. I can't diagnose your problem, as there are multiple possibilities. The old PJs can be pretty noisy and the web is full of remedies. Mine comes from the original run and I did various mods and wire re-routing to get the hum under control, and my amp is now quite quiet. A rare few just defy any and all efforts and remain noisy, but most can be tamed. I also use a 12AU7 as the first preamp tube because I don't really want aggressive early breakup, and the lower power tube makes my amp less fizzy and less noisy. It still gets plenty loud! I'm sorry I can't guess what's up with yours, but I am really pleased that I chose to clean mine up, electronically, make some other mods (better OT, for one -- really transformed it), and turn it into an absolute keeper. I get compliments on the sound all the time.
     
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  3. Andy B

    Andy B Tele-Afflicted Gold Supporter

    Mar 16, 2003
    Colorado
    Check your grounds. Especially thr input jack. Also see the Bill M pages for how he arranges the ribbon connectors.
     
    Last edited: Jan 12, 2019
  4. Huddy

    Huddy Tele-Meister

    Age:
    35
    479
    Nov 5, 2016
    Newport News, VA
    I trust you mean the input jack of the amp and not the output jack of the guitar - I'll definitely double check that but it looked fine on the board and doesn't seem to give any trouble.

    I've also found that if I roll the tone all the way down - be it on the guitar, the amp, or both - the RFI hum subsides significantly. So I'm thinking it may just be the nature of the beast.

    Regardless I'll do some reading on the Bill M site for some bedtime stories. Thanks!
     
  5. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Sep 13, 2008
    Lansing, MI
    Could be a preamp tube too. If you can control the hum with the frequency of the guitar then it's likely not an issue with the capacitor change, although you can poke around with an insulated tool (aka chopsticking) and see if moving the wires around changes the behavior at all.
     
    Andy B and Huddy like this.
  6. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Any chance you burned a trace somewhere? Do you have a service manual? Should include a schematic with test points and suggested voltages. Its somewhere to start.
     
  7. Huddy

    Huddy Tele-Meister

    Age:
    35
    479
    Nov 5, 2016
    Newport News, VA
    I've got the diagram with the test points and voltages that I'll check this afternoon. As far as burning the trace - I'd say not likely. A slight maybe only if it's possible to damage the trace without showing any visible damage to the board.
     
  8. dougstrum

    dougstrum Tele-Holic

    590
    Oct 6, 2015
    blu ridge mtn cabin
    I did filter caps in mine abt a month ago.
    Used double 47uf for c15. The new caps a definite improvement.

    The traces are thin and easy to damage.
     
    JuneauMike likes this.
  9. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Another reason to dislike PCB amps. Nothing wrong with well laid out PCB boards as long as everything is operating perfectly.
     
  10. Huddy

    Huddy Tele-Meister

    Age:
    35
    479
    Nov 5, 2016
    Newport News, VA
    Improvement in tone? or Improvement in reducing hum? Keep in mind I'm talking about RFI from the guitar not the typical hiss/white noise. When I did the 100 µF mod on my friends and it seemed to tighten up the low end but nothing other than that.

    Not gonna lie... I've been contemplating doing a complete rewire with something. What? No clue but the gears are turning.
     
  11. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Yep, I know the feeling. I'd bet the Pro Jr would be easy.
     
  12. dougstrum

    dougstrum Tele-Holic

    590
    Oct 6, 2015
    blu ridge mtn cabin
    The new caps should reduce hum, by filtering out ripple current.

    Hiss seems to be the main complaint with the ProJr. I've had mine since '93 and have done mods to it over the yrs.
    Noise level is very acceptable.

    The internet has plenty of info for quieting down the Jr. Try different pre amp tubes. Heater wires, and the ribbon cables can cause problems. The el84's are biased pretty hot, may help to cool them down a bit...
     
  13. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Unplug the guitar.....is that noise gone? If so, then I would suggest doing what most guitar players do.....do NOT remove your hand/s from contact with the strings/bridge of the guitar unless the instrument’s volume control pot is at zero.
    If the noise is there with nothing plugged in, then the problem is in the amp. Tech time....open it back up and go to work. Review what you did. Make a voltage chart. Pull tubes and see where the hum is coming from....start with V1.
     
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