Hey, recently my Marshall JMD 50watt combo's master volume stopped working properly. as soon as you turn it on the amp just goes as if its cranked, even if the volume is off. I noticed that the knob is loose and it works properly if I push the knob in but it doesn't stay. Can anyone help me fix this?
I assume that like most Marshalls it has a splined shaft? If so, you can try placing a bit of paper over the shaft and then push the knob back on. If that doesn't work, replace the knob. If the spline on the shaft itself is worn round then you'll have to replace the pot.
What and where is the splined shaft?
The spline is the bit on the left that the knob pushes onto.
With the knob off, it still moves and has to be pushed in, now what should I do?
take it to an electronics/amp repair guy and have them put a new pot in the amp. You can do it yourself if you are good soldering on PCB's
What they said. You need a new pot.