I have a roland kc 150 65w amp that begins humming the moment i turn it on. its a normal low frequency hum. nothing is plugged into it, and when i ajust the volume knobs it does not effect the sound. How can i fix this

If it hums when running on battery power, either your cable from your instrument needs repair, or it's the wrong type (unshielded or TRS) or your instrument has a problem, or you're in an area that has too much RF or electromagnetic interference.

If it's humming when something is plugged into the input, but not when nothing is plugged in, it's most likely the cable. If it hums when nothing is plugged in, the amp is defective and you need to return/exchange it.

I used to be a hi-fi enthusiast many years ago.
A low frequency hum can be caused by inadvertently creating a "hum loop". This is when audio separates are earthed or grounded where they are plugged into the power outlets and then connected together using screened audio leads where the screening is connected to both plugs.
The situation can become worse when one of the separates is a type that isn't grounded.

Ineffective or insufficient power supply smoothing and/or shielding leaving an AC ripple on the DC also creates a hum or in the case of poor design or bad shielding can radiate AC magnetism into the high gain input stages of amplification.

Whatever the cause of the hum it can be aggravated by poor matching of separates with regard to input/output sensitivities and impedances. If the pre-amp stage gains have to be high to compensate for a low input signal the signal-to-noise ratio will always be poor through every stage. Small signals requiring large amounts of amplification requires low impedance circuitry and appropriate sensitivity.

Food for thought...

Try different cables. That would be the most simple and cheapest fix.

You are using a Gretsch with this? This is a tiny amp useful for ONLY low levels as a practice amp. If you turn up the gain the unit will distort as the tiny speaker just can't handle high levels. An appropriate level would be if *********** could tolerate sitting alongside it. For the hum situation... does it hum with NOTHING plugged in? If so, probably the wrong adapter is being used. If it doesn't, then your guitar is probably picking up magnetic interference... major source of that type interference is "wall warts" power adapters. The only solution is to keep distance from the source to your guitar. Poor quality guitar cables can let in hum as well.

any music store should have them or homedepot lowes?

something could possibly be giving it feetback or it could be Faulty i would check with the store you bought it from

If this ties to a keyboard, make sure the keyboard and this are BOTH plugged into the same receptacle. You should send a higher level from the keyboard (turn volume up to around 70% or so) and use less volume at the input gain of this amp. Use more at master volume if the amp has one if needed. If the hiss is there with NO input cables plugged in and input gains at 50%, that is objectionable, the amp probably needs servicing. Unused channel gains should be clear down.

Does the hum exist with no cable plugged in? If so, take in for service. If there is no hum with cable out, then try new cable and/or musical instrument be it guitar or keyboard as the problem is likely in those.

Likely a filter capacitor in power supply is failing if it has started doing this. SOMETIMES devices have a very small amount of hum normally. Also sometimes unbalanced instrument cables can pick up hum.