TheDosss said:
Hey guys lately I have been noticing my amp makes a loud crackling noise when I let chords ring out. I have no idea what it is or how to fix it so if anyone has any idea please let me know. I have uploaded a couple recordings of the noise.
https://soundcloud.com/jason-fados/mesaampbuzz1
https://soundcloud.com/jason-fados/mesaampbuzz2
Yes I know what it is.
First I want you to disconnect the power before opening the amp or working on anything.
You are going to look at the component side of the circuit board.
On some amps, the components are located so you cannot access them. You will have to remove the circuit board and turn it upside down.
On others, most, you do not have to turn the board upside down...
Looking at the components, hopefully with a large lighted magnifying glass,
you need to look carefully at the resistors.
You will notice resistors laying directly on top of circuit tracks.
The circuit tracks are colored green, and the resistors lay directly on top.
There is no space between the resistor and the circuit tracks.
One of the resistors is arcing down to a circuit track.
This problem is often mistaken for a bad solder connection.
But re-soldering will not make the problem stop.
Now looking carefully, at the point where the resistor lays on, and contacts the board.
The resistor you are looking for is directly on top of a circuit track or more than one track.
You will spot, hopefully, the point where you can see black carbon, or burning residue,
right at the base of the resistor, where it contacts the board and the track..
You will have to look carefully, closely, one at a time to spot it.
This is where a big lighted magnifier will be very useful...
it can be a job to find it.
Now, I mostly see this happen in the vicinity of V1, V2 preamp tubes.
This usually occurs where the plate resistor, connected to pin 1 or 6 of the preamp tube...
is laying on top of a cathode circuit track (pin 3,8)
OR it occurs where the plate resistor is laying on top of the filament track (pins 4-5, 9)
OR it occurs where the plate resistor is laying on top of the grid track (pin 7 or 2)
OR it occurs where a grid resistor (pin7,2) or a cathode resistor (3,8) is laying on top of a high voltage track...
and that high voltage track is connected to pin 1 or 6 of a preamp tube.
In other words,
anywhere the high potential voltage and the lower voltage contact each other on the board...
is where it can arc.
once it arcs, it forms a carbon track across the fiberglass.
Once this carbon track forms on the fiberglass, an extra resistor is created, and the arcing or burning continues.
This continual arcing, and resultant carbon forming an extra resistor on the board, across the tracks,
is
causing the crackling noise you are hearing.
All you need is to find the SMALL spot and remove the carbon.
Basically, the resistor has a good insulated coating...
And the board also has green insulating shellac over the copper...
But occasionally, this insulation breaks down. It mostly happens when
the amp is used in high humidity conditions, near the ocean, in a rainy climate,
OR, of course, if somebody spilled a drink in the amplifier. Even if the drink has long dried up.
(hopefully a good English ale, lads.)
As I said, it may or may not be easy to spot it.
If the burning has been going on for some time, there will be more black carbon residue...
If it just started, there may be a lot less to spot.
The alternative method, being basic, is to lift the resistors off the board, one lead...
and inspect under each resistor. Keep looking, you will probably find it.
BUT, the board is very delicate, the tracks break easily, so
it is better to have an experienced tech do it for you.
The circuit board is easily damaged by an inexperienced person.
You need a very good soldering station, and tools and experience to do it right.
This is NOT the amp to "learn" on.
Then when laying the resistors back down, leave a small space between the bottom of the resistor and the circuit tracks.
instead of laying the resistor on top of the tracks like the original.
When you find the resistor that is burning, to the track below it, remove the resistor,
clean ALL the black carbon SPOT off the board.
OK this is important. This is a very small spot! NOT the entire board!
If you find the spot where it's burning, the black carbon acts like a resistor.
ALL the black must be removed, or the carbon will form an "extra" resistor on the circuit board.
This means cutting, scrapping, drilling, grinding until ALL the carbon is removed, IN THIS SMALL SPOT AREA.
Sometimes, you will have to cut out part of a circuit track and bypass it around the burn mark...with wire.
Then, reinstall your fresh replacement resistor.
Sometimes, more rarely, circuit tracks will also arc to each other.
Instead of burning under a resistor.
Where the tracks are located very closely together, and you have a high voltage track,
right next to a cathode track, a grid track or a filament track, arcing can occur, BUT RARELY.
This arcing will burn a carbon thread in the fiberglass. This forms an extra resistor in the circuit.
So inspect carefully, again with your magnifier lamp, for any SMALL black spots or burn marks in the fiberglass.
It can be very helpful, to remove the board. But difficult...
Shine a flashlight through the fiberglass, and look at the other side of the board, with your magnifier.
Sometimes a flashlight will help you find a SMALL burn spot in the fiberglass.
To help you find a tiny point where 2 arcing circuit tracks have burned a SMALL carbon spot in the board.
Sometimes it is extremely difficult to find the point of arcing and burning.
Narrow down the problem to ONE preamp Stage.
Remove the components from the circuit, use a mega-ohm meter to check
between plate and cathode, plate and filament, plate and grid.
The carbon spot between the circuit tracks will show up on the ohm meter.
Even when it is too small to see....
Once again, when you find the spot, ALL the carbon In that SMALL spot must be removed by drilling, grinding, scrapping.
remove the carbon completely. clean the board thoroughly. Leave no trace of conductive material, flux or dirt.
Bypass any bad area with wire...AFTER removing the carbon.
CARBON DOES NOT "CLEAN" off the board with spray cleaner SOLVENT! It has to be physically removed, completely.
OK, that being said, once you find the SMALL spot, remove the carbon...
the amp will work like new again!!!
If I can do it, you can too. Rather, have a good tech do this for you.
OK I can do it for you in my shop,
if all else fails.
I'm sure you want to look for yourself first.
YOU CAN HIRE ANY TECH YOU LIKE.
But please, stick with experienced professionals.
A HACK can make a real mess out of a Mesa Boogie.
Drop me a PM for any questions.
AND be careful.
High voltage, which is stored in capacitors, even when the power is OFF!
Yes it's dangerous.
You need to discharge any capacitors that store voltage FIRST- Before the amp is safe to work on.
soundguruman