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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    I will go again. I got a blackface V-Champ last year that for some reason just would not bias. I did the measurements, calculated the wattage, tried different operating points but the thing kept running away (red-plating)!

    Well, I AM MR AMP EXPERT, so I KNOW that the only problem could possibly be the 6V6 plate resistor! DERP!

    After being flummoxed to the point of thinking it might be the OPT - I momentarily resigned from my expert status and actually looked at the rest of the circuit.

    At a vast distance of about 3 inches away from where I was working, I noticed - after a couple of hours of beavering away - that both bias grounds from the 12AX7s had somehow popped out of their solder joints on the chassis. Never seen this on a Fender amp.

    There is no indication of shock damage that might have caused this or visible overheating on the circuit - WEIRD! Wonder how long it may have been like that.....

    Took about 1 minute to repair the actual fault and about 15min to put the 6V6 bias back to where it was when I started :cry:
     
  2. clintj

    clintj Friend of Leo's

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    You know that little under board jumper on a 5E3 that completes the power tube grid leak path? Yeah, the UPS gorilla dropped one hard enough to pop that loose on one of my repairs. Mighty hard to inspect something under the board, and led to nearly runaway bias. That one right there is why I ask for ALL the pin voltages when checking power tube bias numbers.
     
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  3. printer2

    printer2 Poster Extraordinaire

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    I built a small p-p amp, decided to ass something to the preamp section. No room left but some copper left at the edge of the board. Slice through it a bit, drill some holes solder, spark her up. Sounds fine but now I have a low level ticking sound at a 2 second rate. Well this will be a pain to figure out. Flip the amp over, looking at the board and listening... ..tick, tick, tick. And where I added some parts a tiny spark, then another one, then another, to the time of the ticking. Almost like painting an arrow to the problem. The voltage would build up on the filter cap to the point where it broke down the resistance of the air, short the cap for a length of time, it started charging up again till the breakdown voltage, again and again. Flash, flash, flash, a little spark. Widened the gap on the strip of copper and all was good.
     
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  4. Gunny

    Gunny Tele-Holic

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    A customer said the output volume was (always) low on his previously owned 50 Watt Marshall tube amp. Scope showed no obvious problems in the output waveform. Checking the bias, I found that someone previously did NOT install the bias resistor on the grid of one power tube. There was no component there. Easy fix to add the resistor and everything returned to stock performance.
     
  5. kennl

    kennl Tele-Afflicted

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    I did some guitar repair work at a mom&pop store years ago. They took in a Traynor YGA-1 amp on trade and it sounded awful. I heard a bunch of people try it out and walk away with negative comments about the amp. The owner asked if he was wasting his time trying to sell it for $75. I told him it wouldn't sell at any price sounding like that, and that I would give it a quick dioxit treatment (the full extent of my amp repair skills). While spraying the speaker output jack, I checked the wiring to discover that the speakers were wired out of phase. I corrected the wiring, fired up the amp and removed the price tag. The next customer in the store bought the amp for $300.
     
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  6. schmee

    schmee Poster Extraordinaire Silver Supporter

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    I had a BF Deluxe that would intermittently take a significant notch down in volume.... with some crackle. Checked the usual things to no avail. Finally realized that with the volume low for testing etc it worked fine, but at gig volume there as a dead spot in the Volume pot! If I had it adjusted right at the edge of that dead spot, it would make and break at times!

    Bu the most difficult was a Fender amp with Reverb, that would howl sometimes and cut out. It took forever to realize the reverb cable RCA jacks had to be cleaned up well. I discovered it by happening to move the cables while working in the back of the assembled amp. Of course on the bench, with no reverb active, the amp worked fine!
     
  7. cousinpaul

    cousinpaul Friend of Leo's

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    There was a silver panel Super Reverb that tested fine on the bench but would inexplicably go silent at random when installed in the cab. Turns out the top of the cab was shielded with metal screening that would make a mechanical ground to the chassis when screwed together. Over time the screening had become frayed and a strand was dangling onto the board and shorting out the amp. Simple fix but it took many hours on the bench to eliminate everything else it could have been. The owner had already been to a couple of techs and was very pleased that the problem had been eliminated. One of life's small victories...
     
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  8. muscmp

    muscmp Tele-Afflicted

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    mine was my first 5e3 build that seemingly only made a crackling noise in either hi or lo ch.1 input upon plugging in the cable. but, it seemed to only happen on thursdays!! well, yesterday, friday, i pulled the chassis and tightened up any and all mechanical connections to the amp. then, when i looked at the ch. 1 input jack i noticed one lug was bent more than the others. so, i bent it back up and it still happened but not as much. then i noticed that the aluminum tape on the back of the back panel had a crease that stuck out. so i smoothed the crease down further, made sure the jack didn't touch it and solved the problem. i could have saved all of this time IF i would have pushed down the tape further and made sure the jack didn't touch it. here i was getting ready to gut the wiring assuming that it was a ground problem. sheeesssshhh!

    play music!
     
  9. JD0x0

    JD0x0 Poster Extraordinaire

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    This is actually incredibly common in Fenders, to randomly have a ground pop off the chassis. We found a Princeton in the trash that just needed tubes and a ground resoldered.
     
  10. 24 track

    24 track Doctor of Teleocity Silver Supporter

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    i have a G&K ss 112 with abuilt in 12"celestion ,Nice SS amp for clean always dependable ,one night last yaer I decided to to practice ( I know its hard to beleive)
    so I start in to play echoes from pink floyd and every time I hit F#minor the speaker goes into death throws and if it werent bolted down would have flapped about on the floor like a fish out of water, only on that chord

    -2.jpg

    so after much deliberation and test retest I'm going to replace the speaker on next payday so I figure it wont hurt to deoxyt the pots and clean the contacts in the switches I take it apart and find this

    -1.jpg

    the molex connector from the board to the speakers is frayed and both wires shorting to ground ,SS amps dont use a chassis ground but this was vibrating enough on that frequency (F# minor) to cause all sorts of maynhem a simple fix but the symptoms pointed me in a totally different direction .
     
  11. Dacious

    Dacious Poster Extraordinaire

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    It's funny how a thin layer of corrosion can work but then build resistance on hot parts to the point a low current will stop flowing - I had a Ampeg VT40 that did that - turned out to be goop on the FX loop connectors. Q-tip and switch cleaner, bingo.
     
  12. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Here's another wierd one. A Pro Reverb kept crackling and getting weak output etc. It drove me crazy. The death cap had been clipped but was still there in place hidden nearby the power tubes and switches. After a long time trying to figure it out, I found out the end of the cap was occassionally contacting the chassis as the thing vibrated from sound.
    [​IMG]
     
  13. Bob Womack

    Bob Womack Tele-Afflicted

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    I had a '73 Traynor YRM-1 Reverb Master for twenty years but needed to make space for a growing family so I sold it. Ten years later I still missed it so I tracked down another built the same more and year. When it arrived I was thrilled because it sounded better than the first... but there was this ghostly fizz and noise in the background behind everything but only when there were notes sounding. I applied an orange wood stick and tapped the components and tubes and couldn't figure it out. Finally I used DeOxit on the power tube sockets and the tube connectors themselves. Gone. Didn't see that coming.

    Probably the most dramatic was the absolutely worst static crackle I'd ever heard on a Gibson GA-55RVT, loud and crackly and only on certain notes. All the usual stuff turned up nothing until I removed the chassis and noticed that one of the top 10" speakers had a washer on one of the screws that was hidden behind one of the tubes. I checked and the screw was nice and tight... but... I cranked up, produced the crackle, and stuck a finger on the washer. Gone. 1/8th of a turn on the screw ended that torment. The surprise was that it didn't sound mechanical at all - it really sounded electronic.

    Bob
     
  14. mgreene

    mgreene Tele-Holic

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    Really? This was completely new to me - never heard anyone mention it before.

    Where is this mystical trash can, you mention? :D
     
  15. flyswatter

    flyswatter Friend of Leo's

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    I fixed an early-60s Brownface Super that had had its OT replaced and welded -- WELDED -- onto the chassis. It was a vintage replacement but only 30 watts and the leads were a mess. So I replaced it with the proper Hammond replacement. Since a disk grinder is not normally a part of an amp guy's toolkit, I had to get a friend to grind the weld off of the chassis.
     
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