What's the weirdest amp fault you have found and fixed?

Discussion in 'Amp Tech Center' started by mgreene, Dec 7, 2017.

  1. mgreene

    mgreene Tele-Holic

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    The Champ thread below got me thinking about a Champ that had me flummoxed once.

    Amps dont get any simpler than a Champ so I felt a little dumb, not being able to fix it lickedy split.

    This was a early 70's Silverface Vibro Champ. The symptom was that the amp would power up and play normally for a few minutes and the sound would get scratchy (distorted like a poorly tuned radio) and start to diminish in volume until it was almost silent.

    A bad tube, right? Nope. A burnt resistor that stopped conducting? Nope. A cold solder joint? NOPE! I could literally find nothing wrong with this amp.

    So, we all know about the rca speaker connector on a champ - I had unplugged and plugged it in to break up any corrosion and there was no change.

    I find when I am at wits end over electronics (and I aint no expert) walking away for a day or two gives the "ol brane" time to think.

    The one thing I didnt do was to actually clean the speaker RCA connector - even though it was not visibly corroded. I cleaned it with some contact cleaner and brushed it (I keep old tooth brushes for this operation) and then used a q-tip.

    To my amazement, this was the fix. Still works fine today.
     
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  2. archiemax

    archiemax Tele-Meister

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    Bought a circa-1965 Magnatone 431 tube amp for about $100 from somebody in Canada via ebay. This amp had reverb, tremolo AND pitch-shift vibrato. Somebody had removed one of the capacitors from the vibrato circuit rendering it inoperable. They'd also tried to bypass the preamp tube. Why on earth would anybody disable a MAGNATONE vibrato?? (BTW, thanks to the schematic in the Tube Amp Book, I was able to restore it.)
     
  3. jhundt

    jhundt Doctor of Teleocity

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    had a very similar experience with a Fender something-Reverb amp. The RCA plugs and jacks were creating a problem, but I was too dumb to figure it out for a while. When I finally cleaned them with some emery paper, everything worked fine.
     
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  4. mgreene

    mgreene Tele-Holic

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    I love tremolo - so that's up there in weirdness :lol:
     
  5. keithb7

    keithb7 Friend of Leo's

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    My self-built clone amp. 5F2A. After 7 years of playing it on 10 it started acting up. Power tube was arcing.

    I got this. No sweat. I built the amp, right?

    I found cracked resistors. From heat I think. Replaced them. Also added zener diodes to drop voltages.

    Amp would not function properly. Checked and re-checked my work multiple times. Lifted board at least 4 times. Got frustrated. Started throwing parts at it. New cathode caps. Coupling caps. More resitors. Rewired most of it. Lol. Still frustrated and fed up I put it down for a few days. I put my energy into a different amp. Rested my eyes.

    Came back to it and started out this time, where I should have started. With a muti-meter. Found voltage problems. Led me to the schematic. Again should gave looked at it long ago.

    Located one wrong resistor lead location. 10 seconds with a soldering iron. Fixed. Hours of time with smoke coming out of my ears, wasted over this one.

    I can only blame the builder.
    He’s thick headed sometimes. Over confident at times and was gob-smacked back to reality by this example.

    One of the simplest amps ever built. Idiot.
     
    Last edited: Dec 7, 2017
  6. clintj

    clintj Friend of Leo's

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    Here's my 2.

    Number 1: a 5F2-A Princeton clone with intermittent static on loud bass notes. Chopsticked the whole board, cleaned the sockets, replaced the tubes, you name it. Then I was reaching in with the stick to poke something, and bumped the resistors in the artificial center tap. Static. One bad joint there was causing static in the sound.

    2. Ex-bandmate buys a new reverb tank for his Mesa, and starts complaining about how they shipped him a bad tank, this brand is junk, yada yada. I quickly verify the wiring and test the cables, give it a thump to check for reverb crash, seems to be generally working. Unscrew the tank and flip it - still stuffed chock full of foam shipping blocks.
     
  7. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    I recapped a 76 Marshall Super Bass after the cap job it had a weird vibration like flutter but only at higher volumes. The cap cans ground though one of the cap mounting clamp screws. The screw was broken but it and the nut were held in place by corrosion. I must have broken the screw pulling the old cap or putting in the new one.When the chassis vibrated the ground would make and break. I found it chopsticking the amp. A new screw and the amp was good to go
     
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  8. Mr. Lumbergh

    Mr. Lumbergh Poster Extraordinaire

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    I did a cap job on an old Lafayette-branded Valco from around '65. The trem would work on an intensity and rate setting of ~6, and nowhere else. Dirty pot(s), right?
    Cleaned them, same problem. Ordered another pair and replaced. Same problem. Replaced the power stage electrolytics and resistors; they were old enough that they needed it, and it's a bias-wiggle amp. Nope. Replaced the 3 caps (IIRC) in the trem circuit, and back it came. They're not electrolytic, but had still drifted so badly since the mid-60's that the circuit only worked on that one very specific setting.
     
    Last edited: Dec 8, 2017
  9. ChopSauce

    ChopSauce Tele-Meister

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    I'm better not to say it. That was on a tube amp I built. You're better not know how bad it was...

    :confused:

    ... I'll never admit that I ever did this :D
    (so you'll never know ;-)

    Am I the ony one with such a kind of experience?

    Thanks for the info!
     
  10. MrCairo46

    MrCairo46 Friend of Leo's

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    Back in 1981, my Fender Bassman 135.
    Amp stopped working after playing and amp on for a few hours. Light still on, tubes glowing, no buzz, no hiss, and fuse fine. Opened it up and saw several wires comnecting different components had never been soldered. I learned it was sorta not unusual with Fender at the time an was called a "Friday 5" or a "Quiting Time" amp.
    Each time the amp heated up real good the heat moved the wires just slightly enough to break contact. When amp cooled the wires touched again
     
  11. dkevin

    dkevin Tele-Meister

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    OOOh! OOOh! I had a Champ experience like this...The amp would play but crackle and cut out when warmed up. I endured it until I finally got up my courage and opened the chassis to have a look-see...The OT plate wire to the output tube was neatly wrapped around the clean, never-soldered to, tube socket spade and was making intermittent contact after the amp warmed up..As far as I could tell it had been that way since 1977 or so....
     
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  12. Tele-phone man

    Tele-phone man Tele-Afflicted

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    About 27 years ago I was working at a radio repair shop, and I was the audio guy. I had some kind of stereo with a discreet output section. It was oscillating. Finally found the trouble to a SHORTED wire wound resistor (should have been something like 33 ohms, measured about 1/2 that; forgive me for being a little imprecise, it was a long time ago). I learned a lot at that job, but I don't think that kind of job really exists anymore.
     
  13. fidopunk

    fidopunk Tele-Meister Silver Supporter

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    I was given a Rickenbacker B-22 out of a dumpster. Had been recently recapped, retubed, and the rectifier had been refreshed. The guy that rescued it couldn’t get it to function. The output transformer had three leads; ground, 8 ohm, and 16 ohm. Someone had wired it with 8 ohm to ground and 16 to the tip of the output jack. Straightened that out, and it’s been my main amp for years.
     
  14. clintj

    clintj Friend of Leo's

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    You might be surprised. There's a electronic supply store in Salt Lake that does some in house repair work like that, and has a network of area techs to handle more specialized work (guitar amps, hi-fi gear, etc). They've seen a resurgence of repair work the last few years with the poor economy making it more attractive to repair vs replace good stuff.

    I plan on seeing about getting added to their network next year.
     
  15. Gibsonsmu

    Gibsonsmu Tele-Holic

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    I built a 6g3 - kept cutting out over and over again randomly. Mostly when I moved it in and out of the car. Would work when I put it in but not when I took it out. Builder had forgotten to solder one of the connections from the speaker jack to output transformer. Of all the ones to stupid out on?
     
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  16. Platefire

    Platefire Friend of Leo's

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    Was working on one of my DIY amps. Installed my filter cap bleeder wire with Alligator clips and a 10k 3 watt resistor to ground to be safe. Only problem is when I finished making changes, I forgot to remove bleeder jumper to ground when I fired it back up. Fire, smoke and blown fuse. The fuse saved me.

    Another time working on a SE EL84 amp I converted from a Revere reel to reel. I installed a new SS Rectifier doubler, a Champ style prramp/tone stack and left the power amp set up as is. Fired it up and there was fire and smoke so I quickly turned off thinking I must have missed wired something. It burned up my first main power resistor. After scratching my head for a while not finding any wiring mistakes, I finally discovered I had installed the 12ax7 in the EL84 socket and the EL84 in the 12ax7 socket:>[ Platefire
     
  17. jhundt

    jhundt Doctor of Teleocity

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    Fender PR type amp.
    when playing the guitar lightly it was fine. When picking or strumming vigorously, a very unpleasant distortion came on, and died out as the note attack decayed.

    I tried and tested everything. Replaced power tubes, pre-amp tubes, speaker, etc. I did NOT suspect anything in the reverb circuit, because reverb was turned off, and I even disconnected the tank to eliminate that as a cause.

    Finally, I tried replacing the reverb driver tube. BINGO! problem solved. I am still not clear on how the distortion was created at this point, but it works, and I'm happy.
     
  18. Pullshocks

    Pullshocks TDPRI Member

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    I had not played my D-clone for quite a while. Turned it on, plugged in a guitar. Faint hum but no guitar.

    I was cringing at the thought of having to repair it, because it is soldered with....lead-free solder, which is a bi**ch to solder, and virtually impossible to do rework. (If any of you know the secret, let me know).

    Fortunately it turned out to be something simpler. I had borrowed a preamp tube and forgot to put it back. Wish they were all that simple.
     
  19. alnicopu

    alnicopu Friend of Leo's

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    bought a 76 princeton reverb for 300 bucks. Guy said the reverb was flakey. Worked intermittently. Turned it on with the reverb set at about 5, sure enough, nothing. Put it to 0 and the thing started popping like small arms fire at Dick Dale playing next to waves crashing on the beach. It was so loud I jumped. Like Rambo movie effects loud and it should have been completely off. Then would quit for a minute and do it again. Ended up being a cap lead broken just under the plastic overspray right at the cap where there was a warp in the board. It was so low to the board that it took me a while to get it move enough to find it. I must of broke the connection completely at the first chop sticking. Trimmed the plastic back a bit a resoldered to the broken part.
     
  20. charlie chitlin

    charlie chitlin Doctor of Teleocity Silver Supporter

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    A piece of very thick, stranded wire...maybe 6 or 8 gauge...about 2" long with some insulation stripped off both ends...jammed in where a fuse should have been.
    Also...
    The screw on cover on the 1/4" plug for the speaker wire came loose, shorted the terminals and cost me an OT.
     
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