When I plug in a hum will start and when I touch the jack an the guitar it stops humming . It's a yamaha acoustic electric.

Assuming there isn't an instrument plugged into it when it's humming, it will be time to have it recapped,

try plugging into a different socket if that doesnt solve problem you may need to replace cord. sounds like a loose neutral in socket though.

Simple answer is yes.

You can try repair it. They are pretty basic. I would look for poor solder connections, and reflowing all solder joints in it would be prudent.

The hum is likely from a gain stage with a bad solder joint on the input isolation capacitor. Also check other wires and check for bad solder connections on the pickup connections as well. It can even pick up hum through a bad shieid/ground connection in the jack on the guitar

I ASSUME that you plugged your guitar into a DI box that then used BALANCED XLR cable to the Mackie board??? The guitar generates line level and the Mackie board inputs are mostly mic level... you will get distortion if you overdrive the Mackie board. You could try going into line level jacks, HOWEVER if you go any distance balanced interconnects must be used. Even the line level jacks require using TRS balanced cables. Try is using a DI box, and preferably one with a gain switch. Also fiddle with the ground lift switch on it for least noise or hum. I use the LiveWire brand DI boxes and they seem to work fine. Then it is important to set the gain with the trim on the MAckie so you don't clip. Also be aware that an acoustic guitar can "hear" speakers and you can get feedback or regeneration that will sound like you are in a cave.

Make you a hook out of a coat hanger. Get inside and hook the wiring to the jack, take the nut off the jack and pull it up through the F hole. You can plug in to it a patch cord so you can pull it back through once fixed. Check out Guitarelectronics.com for a wiring diagram.

Probably the ground wire is broken on this either at the jack or very near the end of th pickup stick. This acts like a condenser microphone and is fairly high impedance so any break in the ground will pickup hum. Using a high impedance guitar cable is necessary.

You have a grounding problem ! You need to BOND (electrically connect) all the metal parts including the cases and shafts of the volume controls and switches. OFTEN the cavity with the controls are painted with a conductive paint such as Aquadag or NicklePrint which is then bonded to the barrel part of the jack you plug into. Note, while this helps the buzzing due to touching the metaal parts, it will not eliminate hum if you get your guitar pickups into a magnetic field from transformers, etc.

LIKELY the internal battery is exhausted. On some of these you have to take the strings off to get to the battery inside. What a pain that is.

The next time it stops, unplug the cable from the guitar and touch the tip of the connector with your finger. If the amp hums, then the amp is still working and the problem is in the guitar. If there is no hum from doing this, then the amp has am intermittent.

Try this before it fails to see what to expect in the way of the hum from this test.

Common problems with the amp are OFTEN with a broken input connector or bad switch on the preamp output jack if it has one.

There are MANY other things and without troubleshooting it is hard to steer you to a fix.