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Help Troubleshoot - working amp suddenly at 10% volume

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Nov 27, 2020.

  1. kleydejong

    kleydejong Tele-Holic

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    Background

    I was recently giving an old amp some TLC / servicing to get it back into tip top shape. I cleaned up a few odds and ends including putting an IEC power cord, moving the input jack to a more convenient location, and adding some heat shrink to exposed wires. I took voltages and rebiased. Here is the amp in question for background:

    https://www.tdpri.com/threads/hammond-ao-29-organ-conversion-2-to-ab763-and-beyond.763327/

    One issue I had noticed was intermittently low volume. I wondered if it was tube related so I rolled a few and filmed this demo where everything was performing perfectly.



    Problem

    I tried the amp out twice and the low volume issue how now become all the time. The output level is extremely low. Maybe 5-10% of what is normal. The amp seems to be functioning as normal. The tone controls all work. The amp overdrives nicely as you crank the volume. All switches are functioning correctly, just at SUPER low volume.

    What I've Tried

    I've tried some troubleshooting. I took voltages and they all appear to be fine. I did some chop sticking through the signal path and took a snippers on some leads that were sticking out and could maybe be causing some kind of short.

    During my testing I was powering the amp on and off several times. Twice it seemed to perk back up early upon startup, but it would not hold as a fix. Almost like as the amp warms up it settles into that low volume state.

    What Could I Try?

    Here's where I could use some input on troubleshooting. Given my voltages are good and the controls seem to be working correctly, I am thinking the power supply and preamp are working correctly and it must be a problem with the phase inverter or output section.

    I'm thinking about trying to venture into signal tracing. I think it would be a good learning experience. Specifically could I send the preamp out to another power amp to see if that works? Or can you just walk your way from the input through the circuit and confirm normal activity easy enough?

    Any other things I should be considering to troubleshoot? Thanks.
     
  2. Gunny

    Gunny Tele-Holic

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    don't overlook the speaker. connect this amp to a different speaker so you can eliminate that. Do you have an oscilloscope or signal tracer?
     
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  3. dan40

    dan40 Friend of Leo's

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    Does the amp still begin to overdrive at the same point on the volume dial as it did when working properly? If so, I would be looking for an issue in the output jack, speaker cable or speaker as Gunny mentioned above. If everything functions normally and the amp sounds normal other than the low volume, the problem would seem to be after the output section.
     
  4. kleydejong

    kleydejong Tele-Holic

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    Bingo...

    I started with a different speaker cable. Same problem. Tried a different speaker cable into a different cab. Same problem.

    Next I tried using some aligator clips hooked onto the leads from the Output Transformer / Output Jack straight onto a spare speaker I have lying around. I got NO SOUND. Hmmm. That's weird. So I took a close look at that output jack. SHEESH. That has to be the cheapest, crappiest, 1/4" jack in the world. It must have come from a 90's Epiphone and stuck in my parts drawer for 20 years. The sleeve connection spun freely from its base. It could wiggle up and down a bit too. Replaced it with a new shunting jack.

    I have never been so confident in my life that a repair would work. Sure enough, loud beautiful guitar tones.

    I don't know if I am over the moon relieved that resolved this pesky problem, or if I am deeply disturbed at my willingness to put this thing as an output jack on a tube amp 3 years ago.

    Thanks for the input guys, sometimes you need a sounding board.
     
  5. kleydejong

    kleydejong Tele-Holic

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  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Kley, I knew you would find the problem quickly and that this would be a short thread. Congrats on the fix, these intermittent problems can be maddening.
     
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  7. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Yes, good fix. Maybe the motto is 'cherchez le jack' -- dirty, loose, bent prongs, unbent prongs, no switch, faulty switch, no spring left, wired loose, wired wrong, grounded wrong....

    The only culprits higher on the list of usual suspects at my house are:
    #3: "Knob set wrong;"
    #2: "Bad cable;" and
    #1: "I forgot to plug something in somewhere."
     
    Last edited: Nov 28, 2020
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  8. dan40

    dan40 Friend of Leo's

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    I have had brand new Cliff jacks give me problems in a new build and it took me several hours to track down the culprit. Glad you found the problem quickly and got it taken care of.
     
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  9. kleydejong

    kleydejong Tele-Holic

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    Thanks a lot for chiming in guys. This amp is one that has been in need of some quality of life improvements. It was probably a bit ambitious to go after a Trainwreck style circuit at that time in my progression as a builder. So it felt very rewarding to get the tones I had been seeking today after getting it all dialed in.

    This amp is insanely loud and gainy from the get go. It is extremely touch responsive from clean to mean. A real mustang. When playing I feel like my technique and chops need to be on point or it just gets away from me. But if I'm in control it sings like none other.

    I have some shielded cable on order that I may install in the preamp when it arrives to tame some noise. But overall I'm quite pleased.

    I'm experimenting with some longer form videos. My nature is more scientific to try and 'document' my adventures. I perceive that good youtubers tend to lean more into telling a story. Taking you on an adventure.

    I meander through the service, do some preamp tube rolling, and the proper demo starts at 16:05.

     
  10. dan40

    dan40 Friend of Leo's

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    Kley...your video is marked as "private" and won't play.
     
  11. kleydejong

    kleydejong Tele-Holic

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    Ah, I'm a dunce. Should be fixed now.

    I got a big line of shielded cable that I would like to put in this amp to quiet her down a touch, but overall I'm pretty thrilled with how she is playing. I have a gig on Saturday and would like to see how she performs in that environment.
     
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  12. dan40

    dan40 Friend of Leo's

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    It sounds great! I need to build me one of these in the very near future!
     
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