Gain pots are shot

Firstly, the two kinds of currently available pots are A and B, these are logarithmic (A) and linear (B) in their function.

Older pots were marked differently though, it used to be that linear was marked A, and logarithmic marked C.

A gain control should use a logarithmic control ( A, in the new coding system, or C in the older system ) so there's nothing wrong here.

As for the value to use, I would suggest replacing with 100K, the same value as the pot marked with the C, as this is an older marking code this pot was probably in the circuit originally and the 50K pot marked with the A probably found it's way in there in the more recent past.

I hope this helps

You need to replace the potentiometer with the broken shaft. You can open the unit and try to get a part number or go to Behringer parts to get an exact replacement. The pot must be matched for value, taper, and mounting and shaft dimensions.

Disconnect all speaker connections and rca cables. Leave power, ground and remote wires intact. Try turning on again. If you still have a protection light your amp is faulty. Hopefully you have warranty. Double check all connections. Send back ri factory for best repairs.
A thumbs up would be greatly appreciated if this answer is helpful to you.

punk and metal tones (late 80s): treb-7 mid-6-7 bass 6-8, presence 6-10, gain/distortion all the way up. classic rock just turn down the gain/distortion to 6-7. bridge pickup

metal mid 90s & up: treb 6-7, mids 3-5, bass 7-9, gain all the way up, presence (which is a mid and treble boost) 4-6. bridge pickup

blues SRV: treb 6, mid 6, bass 7.5, presence 8, gain 5, reverb (if you got it). use your neck pick up

of course this is subjective and depends on the guitar, cable quality, etc. it looks like a small speaker with a low watt output. that little speaker in there is going to be hard to articulate any tones. more than likely it will have a harsh bit to it cause the value of the pots and the logarythm you hear. reduce treble as needed to smooth out the harsh, keep the mids up.

Hi there. Try re-soldering the control pot to circuit board connections. If not, use ohm meter to verify pot functionality. If bad, replace.
Thanks
Paul

If you have the old pots look on the edge of the cans (metal backs) for the code. Take them to a good electronics supplier and they can match them or measure the resistance from each outside pin with all pins free of interconnection. You will also need to know the audio tapers. The supplier should be able to tell from the codes.

This is usually caused by oxidation in the potentiometer (fancy name for your volume control, also called a pot). The solution is to buy a small spray can of tuner cleaner, then unplug the amp and partially extract the electronics chassis from the case -- usually by removing screws in the top (but NOT usually the top handle screws), then gently sliding the chassis forward until you can easily see and reach behind your gain knob. You should see a squashed metal cylinder attached thru the faceplate to that knob -- that's the pot. It will probably have 3 wires attached -- don't worry: these never have voltages high enough to feel even when the amp is on. Either near where these wires are attached or elsewhere on that cylinder there should be one or more small holes. Using the small plastic tube that came with that spray can of tuner cleaner, spray a small amount of cleaner into the pot, then turn the gain knob from maximum to minimum and back several times. I usually do this by plugging the amp into a receptacle and moving the knob this way BEFORE using the spray, listening for the noises, then I spray it, manipulate the knob as described and hear the noise disappear. Unless you're confident about doing that, DON'T! Instead, slide the chassis back into the case and plug the amp in, then see if the noise is gone. If so, put in all the screws and you're done. If there's a noticeable improvement, but still some noise, move the knob bank and forth a few more times to see if that fixes it; if not, unplug the amp, slide the chassis out again and repeat the spraying procedure one time, then reassemble the amp and plug it in. If your problem is still not gone, you'll have to replace the pot. If you know how to solder WELL, you can order another volume pot thru a Marshall dealer or online, OR you can remove the pot and take it to a local electronics store where they can probably find an equivalent potentiometer at a lower price than the genuine Marshall part -- be sure to ask for one with "an audio taper" (a logarithmic change in resistance that corresponds to how audio amplifiers control sound). Good luck, and please post your results on this website.

relatively simple 2 leg pot. fix.
Under $50 to repair.

Thanks for using FixYa - a FixYa rating is appreciated for answering your FREE question.

amp gain controll need to be changed

Not really, other than sending it to MTX or an authorized repair shop.