What is the power cable - Music

Try using a completely different power cable to rule out the problem.

Without more information of your speakers and amps we can't really help you. The Mackie board puts out line-level signals and that should drive any connected amp or powered speaker to the full capability. Make sure you are using BALANCED interconnect lines either XLR or TRS cables. Use of TS unbalanced cables costs you 6 DB and also noise pickup. Make sure ALL your gear, amps, speakers etc are powered from a SINGLE receptacle or power conditioning module. Normally you will use the AUX outputs of the mixer by way of TRS cables or balanced lines in a snake to power amplifiers onstage which then connect to the stage monitors. If you have powered monitors the returns of the aux to the stage go to the powered monitors. A heavy duty extension cord brings power back from thee stage power to the mixer.

if its not right next to where the AC power plugs in, or part of the power plug itself ( lil notch in cable recepticle that will allow it to slide out ) then it would have to be inside the mixer on the power pcb. ( the board the power cable connector is soldered to)

Isolate one channel at a time to see if a particular channel is the problem. Make sure ALL interconnected equipment is powered from the SAME receptacle or power conditioner. A remote amplifier MUST be powered from the same source as the mixer, even if it requires a separate run to power it. Make sure all cables are BALANCED, either XLR or TRS cables. Sometimes a cable can have a broken wire or shield and cause this.

This likely uses the standard power cords found on tower computer. You can ALSO often find these cords at Thrift shops.

You will need either powered speakers or an amplifier for the speakers. If you have powered speakers with built in amplifiers, use balanced audio connection cables from the MAIN outs of the mixer to the speakers, Use XLR or 1/4 inch TRS cables.
If you do NOT have powered speakers you will need amplifier as well and speaker cables in addition to the audio connection cables above.

The monitors will have to be POWERED monitors with built in amps.

You can simply use a wye splitter cable to drive the two monitors. Run the cable from the mixer to one monitor and put a wye cable and an extention to the second powered monitor speaker.

If you mean power adapter, it is a 12 volt 2 amp one and I don't know which polarity.
Here is an Ebay seller that has them for the E86:

http://cgi.ebay.com/AC-power-adapter-4-Roland-E-86-E86-Keyboard-Synthesizer-/110432738764?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item19b64dddcc

If it is the audio cable you are after, just get a standard INSTRUMENT cable (not speaker cable) with 1/4 inch plugs.

You would NOT use a stereo cable to plug in your guitar.

The stereo cable would carry the battery voltage back to the amp and MAYBE cause the fuse to blow.

You use a MONO instrument cable for your guitar.

The ring connection is use to complete the circuit to turn on the battery in most guitars.

Replace the fuse ONCE and get the proper instrument cable (NOT consumer audio cable) for your guitar.

If fuse blows again, take amp to shop for service.

Sounds like a ground loop. MAKE SURE ALL of your equipment is fed from the SAME receptacle. Make sure the ground pin on the power cable is good (try another cord or swap). Swap the audio cable to verify the cable has a good ground.