Can't get rid of background noise on my marantz pmd660

You might consider sending it in to Audio technica for repair. They're really customer oriented and alot of times they'll repair and or replace for just cost of shipping. Try contacting them directly.

There is not enough info here to narrow this down to far. What is the sound source? The internal mic or direct via cable? Also, does the background noise stay the same regardless of the playback mode? Supplying these details will enable me to locate the problem circuit and provide potential fixes.

Thanks,
Dan

This implies that there is not enough current going to the device when running on battery (only back-light but no data) whereas running fine on AC means that the device board is most likely in a good operational condition.
I think maybe once a leaked worn battery which left forgotten inside battery compartment damaged the battery terminals or at least the wires connecting these terminals to the main board, i would recommend you to open and check on that.

i hav just bought a at2020 mic and i tried recording bought it's not workin for my dell inspiron i forgot the model anyway it's running on windows 7 so please help me out am screwed with this ****

Is it a condenser mic? Either your mic is bad or you need to add a compresser to your signal path...

Unfortunately you will have to contact them here:http://www.audio-technica.com/cms/site/74a0c4b691ac33ae/index.html as they don't seem to have that document listed on their site.

Call Shure Applications at 847-600-8440. They can help you troubleshoot. Have the wireless system with you when you call.

In your PC, right click

you can test the cable from connection to connection for continuity from end to end. There should be no resistance( a complete short) If the cable is good, there is a bad or broken connection inside the mic. Good Luck