Pro Junior Recap Question... Excessive Hum

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

  1. Huddy

    Huddy Tele-Meister

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    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-Afflicted

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    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

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    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

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    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

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    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.
     
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  6. JuneauMike

    JuneauMike Friend of Leo's

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    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

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    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

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    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.
     
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  9. JuneauMike

    JuneauMike Friend of Leo's

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    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

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    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

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

    dougstrum Tele-Holic

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    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

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    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.
     
  14. Huddy

    Huddy Tele-Meister

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    Alright so it's been 4 months and I'm back to work on this thing ;-) So I don't know what's going on with it. I plugged it in 2-3 days ago... I get a smidge of guitar noise, but now what I notice more is hissing hum that everybody complains about... At first I though - "This isn't that bad"... Then last night I recapped an early 90s Classic 30 and it's quiet as a mouse so the bar has been raised.

    I checked all my connections that I made with the power supply caps and reflowed the only one that was halfway questionable. No improvement. I pulled the tube board... everything looked okay until I saw the last EL84 on the left... the board was more brown than the rest from excessive heat and almost all of the connections from the socket were cracked. I was hoping this was the cause and not a symptom... but after cleaning up those connections no change.

    So I've been reading a lot (and you mentioned it was well) regarding the hot bias of the output tubes... So tonight I'm going to follow @Wally 's advice and make a voltage chart. I checked all my B+ B+1 and B+2 and they're all spot on within a few volts but there's a lot that can vary. Wish me luck
     
  15. dougstrum

    dougstrum Tele-Holic

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    If board is discolored from heat and there was cracking around socket connections; make sure traces are good.

    It's easy to cool bias down, but I don't have access to my circuit diagrams right now~ if I remember correctly change R29 from 15k to 20k? I'll check it later in the week if you want.
     
  16. Huddy

    Huddy Tele-Meister

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    I appreciate that. That'd be great. Making my voltage chart now... I gotta get a refresher on my terminology/jargon. The only thing that's off so far from my schematic is the AC voltage between either end of the output transformer and output tube plates. They both measure ZERO. my DC Voltages have been absolutely spot on.

    I do believe I have one microphonic 12AX7 (JJ ECC83S) You can hear it clanking through the speaker with the volume down when you tap it.

    More to follow shortly...
     
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