Forgive me, but the description doesn't give us much to go-on. The idea that there is no sound with the mode switch in "stereo" -but some poor sound in "mono" makes little sense. The only thing I can imagine is if one output channel is dead and all speakers were connected to the 'good" channel and then that channel's preamp failed and the other preamp was also defective.
The best suggestions I can offer at this point is to check the connections to the pickup -both at the cartridge and the input plug on the amp. Then also check the plug for the volume control. Also check the speaker connections.
Beyond these checks the problem is probably an amp in need of repair. The unit is 36 years old.
If you feel competent to work on it, look up the schematics here:
http://arcarc.xmission.com/PDF_Jukebox/ ... chematics/Otherwise it is time to look for a technician or skilled hobbyist to service it.
Rob/NYC