When i plug the cable in my boss rc-20 it turns on and instantly it turns off, how can i fix that?

Request directly from manufacturer

Has the unit ever had another ac adapter plugged into it?

If your Roland BOSS RC-20XL Loop Station will not record or play, there are a few troubleshooting steps you can try:

1. Check the power source: Make sure that the Loop Station is properly powered and that the batteries or power supply are providing sufficient power. If using batteries, try replacing them with fresh ones.
2. Check the connections: Ensure that the input and output connections are properly connected and that there are no loose cables. Try using different cables to see if that resolves the issue.
3. Check the settings: Make sure that the Loop Station is set to the correct mode and that the input level is set appropriately. Refer to the user manual for guidance.
4. Reset the device: Try resetting the Loop Station to its factory settings. Refer to the user manual for instructions on how to do this.
5. Service: If none of the above steps work, the Loop Station may need to be serviced. Contact the manufacturer or a certified repair center for assistance.

Hopefully one of these steps will help you get your Roland BOSS RC-20XL Loop Station working again.

http://www.roland.com/support/article/index.cfm?q=manuals&p=RC-50

I tried to look at this and get the valuable information but it requires a few steps. This is a link for the product manual, what you are looking for is on page 20 I think

Make sure the power supply for this is powered from the SAME receptacle as your amplifier is powered. Keep the cable from this to the amp fairly short and high quality cable. Keep guitar away from magnetic field sources... the wall wart adapters are notorious for generating magnetic fields.

This could POSSIBLY be done using an external source generating a MIDI sync clock fed to both.

Usually in any type of situation when data is being transferred and the connection is prematurely lost there is data lost and or file corruption.

My suggestion:

Connect the Boss Rc 50 to your computer and backup its entire contents to your computer if possible.

Next perform a Hard Reset: While simultaneously holding down the "PLAY MODE" and "LOOP SYNC" buttons, turn the power on. Everything should be restored to factory settings.

If your backup was successful, then you can transfer desired files (one at time) to determine if the are usable. If a file causes errors or malfunction, then most likely it is corrupt.

Hope this helps

Most PROBABLE is the output jack either has been broken (common due to mishandling and accidents with cables such as stepping on the plug while plugged into the pedal) or the soldering of the jack in the pedal has broken loose. Less common is a circuit board crack in the vicinity of the jack. Open pedal and inspect for the above suggestions.

CHeck the voltage out of the adapter. It should be around 9 to 11 volts. If it is not, it is possible that the OC3 has reversed polarity AND a protector that put a short on an incorrect supply. If so, it could have damaged the RC50 adapter. If the RC50 adapter is putting out correct voltage, then I suspect the inout connector on the RC50 may be damaged or not quite seated. Visually inspect the connector first.