I assume it's a playback problem and since you cleaned the heads, it's not dirt.
I suggest checking the cables (reverse the leads to see if the problem changes channels). Check the tape monitor and input switch controls on the receiver/pre-amp. Try using headphones, if the deck has a 'phone jack, to confirm if it's the rear panel outputs, or all output. Exercise all switches and knobs on the deck, especially output volume and tape monitor buttons, if any-listen for any intermittent operation and clean accordingly.
If you open it up, there might be fuses for each channel, but I doubt it. Otherwise, it's a trip to the shop, unless you are handy with a soldering iron and have test equipment.