Behringer's Ultrabass BXL3000 Gain knob broken

I am afraid to give you bad news but behringer stuff is something that is very difficult to get parts for. The question you should be thinking is more, "Can you get that board?", "Can the repairmen at the service center get it?" OR, "Can you or the repairman get the parts to repair the board?" I have had this experience many times trying to repair behringer equipment and I can't even find a cross reference to a single transistor on a behringer circuit board, as if they don't exist. It seems that the manufacturer has most of their components custom made to their specs so they would be known as "genuine behringer parts" that they will not sell to a consumer. They don't want you to repair it, they want you to buy another one. It's the American marketing process. Think about it, Westinghouse wouldn't sell many light bulbs if they lasted forever. The best option you have on this is to make a 1-15 cabinet out of it, by drilling a hole for a 1/4 inch socket and wire the speaker to it and mount it, then you can put an amp head on top of it. Sorry. Didn't want you to go through a lot of time, travel, and aggravation to find this out the way that I did.

Very simple. Remove all black screws at top and sides of cabinet. Gently slide the amp unit out (towards rear). Done! :)
Don't be alarmed if you have to "tug" on it a bit. This is normal. Just don't try to "FORCE" it out.

I would check it with a different instrument cable AND bass guitar. If you end up with the same results, it's a thermal issue with the amp and will require factory authorized repair. The amp heats up and the intermittent component starts failing....more heat, more failure. It's not an easy fix and your Behringer repair shop will have their work cut out for them with this repair.

Suspected Faulty PC board Dead Capacitor Or Mod Pot I would suggest returning it for an exchange the problem is your speaker will work on a regular stereo because a stereo powers the sound internally causing the magnets in the speaker to vibrate Headphone jacks are designed to pick up off a seperate circuit board with a different connection to the main board hence the no sound from the speaker.

Foot pedals are usually NOT interchangeable... The correct footswitch would come with the amp so be sure it says Behringer on on the footswitch. Apparently some sellers have foisted off VOX footswitches which may not work with the Berhinger.

How do you know you are at 40 WATTS? This takes electronic test equipment to measure.
Tests to run. Observe the speaker cone with power off. Now turn it on... did it displace and stay there? If so you have a failure of the amplifier itself. Next, If you can place finger on the cone, move it gently in and out feeling for any roughness. If you feel roughness, the voice coil is fried and you be buying new speaker. A lot of the Ultra bass Behringer's use LM3886 power amp chips... sometimes in parallel, and a failure of one could cause this.

Since you just bought this, exercise your warranty (assuming you bought this new). The amp probably will have a slight buzz with nothing connected... the question is how loud it is. If you can hear it loud at five feet, that is too much. If you bought this used, prepare to have it serviced at a shop.

Most of these use two stick type amplifiers driving the speaker in what is called "bridged mode". If one of the sticks fails, the unit will do what you are observing. Unless you are adept at electronic repair this is NOT DIY.

Here is a BX1200 that uses a pair of LM3886's in parallel rather than bridged mode as it is a smaller unit. You will find simularities though. Your unit will either use the 3886's of possibly either TDA7293's or TDA7294's.

http://elektrotanya.com/behringer_bx1200_sch.zip/download.html

The file is compressed and has two sub files with different revision levels. Click on the brown dot in the upper left to download.

You will PROBABLY find the 4 screws on the sides that screw into the chassis and then get frustrated that it will not come out... MANY of the Behringer things HIDE the other screws as the ones holding the handle... The trick is to get at them... The plastic end covers over the handle have ridges that snap under a metal plate that is over the handle. Carefully pry out the plastic near the handle to release these ridges... pry SIDEWAYS a bit... The ridges snap into small cutouts in the metal... what a pain these are. Use a small 1/4 inch screwdriver with blade vertical right where the handle comes out to bend the plastic outward... Dont reach in very far as the blade will catch the metal... just far enough to catch the plastic cover edge... it is scary. The two handle screws usually go into nut-plates that are a part of the amp chassis.

The DI consists of two sections of an op amp and is truly driven differentally. There usually is a couple of 120 ohm series resistors and a couple of hf bypass caps near the connectors

Common problem is to be using the wrong one of the two input jacks... one has higher gain. Also make sure both gain controls are turned up some.