I have a rumble 100 bass amp which used to work fine until now! I've had it about 2 months and have only played it about a dozen times. Today, while practicing with a new band, 90% of volume got lost. Now there's hardly any noise and a crackelly speaker. The volume was not even 3/4 the way up while I was playing. I've tried 3 different bass guitars and 3 different cables.

How new is the amp?

A single faulty solder can do that...I'd try returning in to the seller.
Have you tried a different amp with the same equipment?

Sounds like a wiring issue.

Hope I've helped
PvT

Try plugging another sound source into the unit. The buzz MAY be pickup of magnetic fields in your guitar pickup that clobber your notes from the strings. Bad guitar cables are another possibility. By putting clean music from say a CD player into the unit you can find where the source is. If the CD player generates the same noises, then likely you have a failure of filter caps in the amp or other component failures. Also make sure your guitar can't "hear" the amp as feedback can do strange things.

A common reason for 'distortion' with all bass amps can be either that the screws holding the speaker unit to the cabinet have worked loose or that there has been a build up of dust particles in the corrugations around the edge of the speaker cone, which rattle especially on the lower notes.
The solution is simple - remove the front grille and check the screws are tight. Use a vacuum cleaner to remove dust from the rim of the speaker cone.

Try this link:http://www.fender.com/support/amplifier_schematics.php If your amp isn't listed contact them and they will send you one.

It should be adequate... If necessary put shields around the drumset and tell the drummer to take it easy and don't showboat.

First check speaker and speaker wire.

There is an inconsistency in your text. You indicate that cables were swapped and problem still existed...
You said:... "i have tried using it on a different amp and still fine".

If you tried it on a different amp and it was still fine then the guitar is probably OK.

THEN your amplifier is in question as the root of your problem.

What is the amp you have? The guitar is the CB-42, but what amp do you have?

MANY intermittent problems with amplifiers are often with the input jack itself. These jacks are easily broken by tripping over a cable plugged into them.

Suspect the input jacks. Wiggle them and see if you get anything. They get yanked sideways because the musicians don't loop their cord thru a handle and they get yanked sideways.

Also you can try spraying sparingly CRC226 into the switches and pots. You can get it at Home Depot in the electrical department.

I just fixed my bass amp with a similar problem last week: the problem is related to a loose connection on the PCB board (likely).
A bass amp is a rough place for circuitry: the low rumble and power shake a bass amp more than a guitar amp.
In my case, the power transistors needed to be re-soldered and then the grounding bolts and screws needed to be removed and cleaned because the connection was bad. Remember that when two different metals needed for an electric connection are toughing, there is the likelyhood that corrosion WILL happen.
My bass amp did basically the same thing yours did: sounded good at low volumes, but at higher volumes it would act up.
In short, there is nothing you can do (unless you are a repair person): you need to take it to a local music shop and have a tech fix the intermittant contact problem: someone who knows what they are doing should only take an hour at most.

Hope that helps!
-mike

Replace C21 (10uF/50V) and C38 (2.2uF/50V) and it will work great.