mesa boogie dual rectifier odd feedback

[ingenious621]

i have had my dual rec for 6 years now and about 5 months ago is started getting feedback whenever i would stop playing. it never fluctuates in frequency and is always the same sound. i thought it was the tubes changed all the tubes and it still does it. i even got a new guitar and still does it with that. so i brought it in to get fixed and the repair shop said it was bad solder points. well needless to say it still does it so i called mesa to see if they had any ideas and the tech there said that its definitely not the amp it has to be something im introducing to the amp. which is ridiculous cause ive had the same gear ever since ive gotten this amp and had no problems up until 5 months ago. got all new cords, still does it. so i brought it over to my uncles house cause he is real good with electronics so we had it apart and he recorded the feedback on his little zoom recorder and played it back through headphones moving it around the amp to try to find the area that is sensitive to that frequency. there is one area by all of the input wires which is right by the 2nd pre amp tube plug in that was really sensitive and microphonic but the schematics that he found online did not match my head. any ideas? this is very frustrating and to have the idiot at mesa basically tell me i had to live with it is ridiculous... thanks

matt

[Mars Amp Repair]

well,
Does the amp make the noise with no input? Even so, on some of the Mesa's the input jack is the grounding point for the preamp. If the ground is bad, you can get all kinds of werid sounds & symptoms.

you might try either loosening & retightening the input jack or remove the jack & file or fine sand the metal behind where the jack mounts to provide a better ground.

Also the shorting pins on the input jack as well as the point that contacts the guitar plug and the inside of the collar could have some oxidation on them that needs to be burnished or cleaned.

glen

[ingenious621]

when i took the amp in they replaced the input jack, the ground, and some crossover... and re-soldered some soldering points. i have no clue about electronics but my uncle said its very noisy by the transistors where the input wires are soldered in. and where one of the tube plug-in inputs are. very strange. it doesnt happen when volume is low but i know its a problem with the amp because it just started doing it 5 months ago and if you tap the circuit board or just bang it lightly you can hear the same frequency that it feedsback at very softly. so something is going on with the piece of crap ha

[mykey]

matt[/QUOTE]

first of all you need to understand that mesa amps are very badly designed, that being said the second thing is to look at the resistors on the circuit board. these resistors are laying flat against the printed circuit tracks.
you need to use a soldering iron to very gently lift all the resistors up and away from the circuit tracks- so that the resistors are no longer laying against the circuit board. the problem is that one of these resistors is arching high voltage to the circuit track that is underneath it. use needle nose pliers after heating each connection to raise the resistors off the board as high as possible. only an experienced person should do this and watch out, high voltage that can shock the crap out of you.
if you find the resistor thats been arching, replace it with a new one, it will have a burn spot on the underside, and leave plenty of space between the resistor and the board (leave the leads longer than original).
another problem is arching between the circuit tracks themselves, on the fiberglass board. turn out the lights and turn on the amp, you will see the high voltage arching, little lightning bolts flashing in the dark.
its probably an arching resistor as above but I have seen many, many problems in these amps.
and yes you're right it IS the amp, not you're equipment. mesa has never admitted or corrected these problems. you are lucky if one of these amps lasts five years. in humid conditions, expect many problems. in very dry desert, it might keep working.--mykey
PS my name is not junior, I have repaired guitar amps for 34 years. I have also managed a mesa boogie authorized repair center.