Have a low noise like hum when i turn on thump 15A It doesn t change by moving level but when I plug in anything it s feeding a noisy tale on every signal attack.

buy a canister cold spray and spray on some active components if the noise begins You have a thermical failure in a component on the board can be faltering resistorcircuits or resoldering some active components.

Your best bet is to contact Mackie Tech Support. Here is a link to their Suppot page. Mackie Tech Support
it's likely it needs to go to a repair shop. Here is a link to their service center finder. Mackie Service Center Locater

As Tom Jones sang 'It's not unusual....' The amp has probably gone and you need to get it fixed. These are not the most reliable and I would suggest getting something better from another manufacturer if you can afford it, perhaps a local shop would do you a trade in if they can fix it?

Jose Miguel Carcamo Cruces do you have pictures of these j112 fet ? Thank you!

"Noise" is not a very good deescription... is it hum? is it static like? is it a buzz? Hum pickup by the pickups of guitars are common and some pickups are worse. If it is hum move the guitar away from magnetic sources like amps, motors, flourescent lights, dimmers... etc. Sme guitars have grounding problems and touching them varies hum. Also, do NOT let the guitar "hear" the amp as it can feedback at frequencies above your hearing even. Do some "sniffing" by moving the guitar and the orientation around the area seeing if you can find hot spots of noise to avoid.

Check that: You are using BALANCED interconnection lines (TRS or XLR) everywhere. Low quality cables can cause or let in noise and hiss.

Check that ALL, and I mean ALL interconnected equipment is powered from the SAME receptacle or power conditioner.

Check that you are not using a noisy source or recording system like the internal audio of a PC for instance.

Short the inputs to ground and see if the output noise goes away. Some effects are noisy because the inut signal after effects gets added back into the output.

You haven't described your setup and without that information there is not much more I can add. Ferreting out noise is one of my specialties. In general your input level should NOT be too small as this leads to less signal to noise ratio due to digitizing noise. Look at the Lexicon specs to find what the levels should be. Without the description of your setup I can add no more...

You are using a Gretsch with this? This is a tiny amp useful for ONLY low levels as a practice amp. If you turn up the gain the unit will distort as the tiny speaker just can't handle high levels. An appropriate level would be if *********** could tolerate sitting alongside it. For the hum situation... does it hum with NOTHING plugged in? If so, probably the wrong adapter is being used. If it doesn't, then your guitar is probably picking up magnetic interference... major source of that type interference is "wall warts" power adapters. The only solution is to keep distance from the source to your guitar. Poor quality guitar cables can let in hum as well.

Make sure ALL interconnected equipment is powered from the SAME receptacle. Use extension cord if necessaary. Use ONLY balanced interconnecting audio lines as well. You have a ground loop... make sure all power connections are three wire.

Crackling is usally one or two things my guess would be bad grounding of the amp or bad connection with the jacks or plug it self try to make it crackle on purpose by jiggling the jacks see if it has any effect on the crackling try to narrow your possibilitys

Hums are usually related to power, but not usually the actual power adapter. It's most commonly a "ground loop" issue, with the infamous 60Hz hum. The solution for this sometimes is to try plugging the keyboard (or amp) into a different power outlet, perhaps on a separate circuit.

Also try an audio cable of higher quality, which may have shielding or braiding in it to reduce external noise and cancel out internal noise. Also, shorter cables tend to produce less noise than longer ones