TLS Guy I think I have identified the cause of the problem it appears that there is a crack or small hole puncture in the hook up cable,that is why only the right channel was producing sound.
That may well be your problem, but you need to switch the working right channel to the left channel of the tape 2 input, and make sure you have sound. No sound equals amp problem. Sound equals tape unit problem, or the suspicious problem in the lead.
If the sound comes out of the left channel when you plug the right into it, cut out the suspicious area of the lead you have identified.
Now solder a female RCA socket on the end from the tape recorder and a male RCA plug on the end going to the amplifier. Check your work with a meter, and hopefully you will be back in business. If not you have problem with that tape recorder.