You have three options that I can think of.
1. Open up the casing and check the soldering.
2. Use a head cleaner (http://www.amazon.co.uk/AUDIO-CASSETTE-CLEANER-CLEANING-FLUID/dp/B005DPORQA/ref=sr_1_1?ie=UTF8&qid=1320186895&sr=8-1)
3. Obtain a new one (http://www.amazon.co.uk/s/?ie=UTF8&keywords=sony+cassette+recorder+939&tag=googhydr-21&index=aps&hvadid=9026590265&ref=pd_sl_1oz4rnwug8_b)
Martin R, November 2011
