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Crate FXT 120 Crackling Noise!

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  • Crate FXT 120 Crackling Noise!

    In SoCal, here's the schematic:crate_fxt-65_fxt-120_07s194_sch.pdf

    I got this with the input jack torn off the board, the effects board that has the Channel/Gain Select & DSP jacks loose inside the chassis, and the chassis somewhat bent on the back where these jacks come through. It had taken a mighty hit there. It was not treated well. Straighten the chassis some, pulled the main circuit board to solder in a new input jack, got new nuts and washers for the effects board jacks and put it all back together hoping for the best. Low volume on clean channel, the volume pot went from off to on and did not increase in volume from that point. Overdrive channel seemed to work well, gobs of volume and distortion of the good kind. Possibly a bad pot but tried cleaning it. Used DeoxIT D5. Similar to this in construction:
    Click image for larger version

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    No real good place to spritz, and I think I took lube out as the operation felt easier to turn than the pots I did not attempt to clean. Used the smallest of spritzes when I applied it. Then turned the shaft back and forth about twenty times. Also reflowed the solder on this volume pot in the clean channel. I had the mounds of solder left from the input jack that was pulled out of the circuit board. So I just put the legs of the new input jack fully through those mounds. Then took my ungar 25 watt soldering iron and touched the solid mounds until I saw them liquefy and pulled the iron away. Did the same with the legs of the volume pot sticking through the circuit board. Let me know if my technique is wanting. Now I had a working volume pot in the clean channel! Put it back together and dimed it and played. This is one loud amp! I put the distortion on Gain 2 with a good amount and wailed for half hour or so. I noticed my playing become unintelligible, and realized that I had a crackling noise that was causing this. It happened with clean, gain 1 and gain 2. I got my dead blow sledge and gently tapped the chassis on the back when the crackling happened and it would seize to crackle. I worked every pot twenty times, hoping it might get rid of the noise. No deal. This amp is a great amp, but unuseable as is. Where to start to find this?

  • #2
    wooden chopstick, and start poking individual parts. SOmething likely has a cracked solder joint. Primary suspects are large parts like filter caps and 10 watt resistors, but really anything can have a cracked solder joint.

    Especially if the chassis was bent from a big hit.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Yeah will pull the chassis again Monday and do as you say. I was able to do the last pull of the chassis and repair without completely removing the circuit board as I did the first time. I was able to weasel it out and flip it over as you have mentioned to me in past posts. Then flip it back over, and put a rag to keep the board from shorting against the chassis and powered it up. Thought I had it, so put it all back together and played it only to find the crackling. May have to pull the chassis a few more times. But I'll find the cause eventually. With your assistance, thanks for the reply.

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      • #4
        It is best not to reuse solder. If you don't have a solder sucker, pick one up, they're cheap enough.
        Remove the old solder with the sucker, re-solder with fresh.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


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        • #5
          Thanks for that I'll pick up one of the pen style solder suckers. I have one from Radio Shack, a soldering iron with a hole and has a rubber bulb to suck up the solder. But the bulb is toast. Worked ok before the bulb cracked. Have no idea where I would get a replacement bulb.

          When you reflow solder do you remove all the old from a component and then resolder? Is that what you would call reflowing solder? I did ask if my technique was wanting and it is apparently. But that's how one learns.

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          • #6
            Got back to this amp this weekend and found a crack in the solder on one of the big ceramic resistors so just reflowed the solder added a bit more. This cured the crackling awful distortion that was happening! Yay! But I still have the problem with the clean channel volume pot.

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