I broke the master volume switch off. How I do I

First, plug it in. Next connect the speakers. Center all eq controls both master and channel. Turn off any reverb or effects. Turn the master volume 1/2 way up. Plug a sound source into channel 1 wiyj the channel volume all the way off. Turn on the amp. Slowly raise the channel volume until you heat output. If you get nothing, it's broken.

Hi, I can help you with your question. On the face of the effects, towards the right side, there are three settings that can be adjusted: DRIVE, TONE and VOLUME. The VOLUME knob is your master volume. The expression pedal can adjust volume, but it's output volume is controlled by the master VOLUME knob we just identified. I hope this has helped you, and if it has, could you please click on the "helpful" button ? Thank you!

All i can suggest is to use electrical contact cleaner on it from the inside. But you,ve already done that. And i would imagine that the master volumes will be the same to, Harry

It's about hardwares.
Wiring connection sockets is loose.
Inside the korg PA50,Wiring connection to the keyboard is separate to left and right.
Open/disassemble your korg.
Look for connection cable to the left keyboard.
One end is on the keyboard,the other end is at the mainboard.
Unplug both of it, then mounted again.
For master volume,better buy new one.

Do you turn on standby every time and wait for the tubes to heat up first? If you don't, it can severely damage the tubes.
But a common cause of this could be just old tubes. Like any other piece of technology, tubes get old. If you like to play your amp LOUD then they wear out a lot quicker. You have to replace tubes every now and then. Because I'm not there to check out your amp, I can't give you an exact answer but it sounds like that's the problem. This isn't always a bad thing. You can make the sound of your amp exponentially better with nicer, new tubes. Your amp has 12AX7 tubes. Very common tubes to use actually. I'm not sure what kind of music you play, I'm assuming mostly rock and blues. But here is a list of tubes and reviews on them.
http://thetubestore.com/12ax7review.html
This is how to replace a tube:
http://www.ehow.com/video_4438285_change-pre_amp-tubes-guitar-amps.html
Upkeep on a tube amp:
http://www.ehow.com/video_4438295_replace-tubes-guitar-amp.html
A note he doesn't make in the video is to dust out your amp. I use the compressed dust spray for the inside of computers.
So overall, stop turning up your amp to 11, make sure it's well ventilated and dust it out from time to time. Hope this helped.

your RCA from your mixers output has to go into the input of your amp. just switch that and you should be fine

There are many models of Marshal amps icluding 15Watt types.

First thing to try is to see if headphones have normal volume. If they do, then the headphone jack switch that connects the speaker is LIKELY bad.

If the headphones do not have any volume, LIKELY causes are:

1. Fried output power amp

2. Volume pot is broken or has bad solder joints to the circuit board.

Contrary to what's posted above, the Marshall MG series does not pass the speaker power output through the headphone jack, and inserting the headphones into the jack does not disable the speaker. The headphone amp is a separate circuit and it's input is taken before the master volume. So Jan's post is exactly right. Turn off the master volume and turn up all the other gain/volumes as high as they will go. The other issue is that the headphone jack's output does not drive low efficiency headphones well, so you need some headphones similar to Sony's MDR-V700s to hear it well. Earbuds just don't cut it.

Sounds like your master volume is turned up too much. The channel volume will make the squealing go away(caused by high gain and humbuckers). The best thing to do is dial in your favorite presets and change the channel volume to the correct level with the master volume where you usually keep it. I use my Spider Valve at master volume at about noon and the channel volume between nine o'clock and noon. Let me know if this does not solve the problem......

There are 2 channels, Which means there are 2 sets of knobs ... One for clean and one for crunch. Start out with a low master volume and set up the crunch, Then the clen and switch between the 2 and make adjustments until they match volume wise ... Then turn the master up.