Do you have any tape that is already recorded that you can try to play back? If not, you can test playback to see if it's working by recording something on tape without using your recorder. Stretch out a few feet of tape on a table, oxide side up (black down, brown up). Get a magnet, like a fairly strong ceramic magnet from a hardware store, and "record" pops on the tape every couple of inches by placing the magnet on the tape then taking it away. When you run this tape through your machine in "repro" you should hear the pops go by, and they should flick the meters, all channels. If this works, you have a record problem, something common to all channels. The bias oscillator comes to mind. If you get no playback of your pops test tape on any channels, look for something common to all play channel amps, like the power supply, a bunch of dead caps (many caps will all die at about the same age), or the like.
Jim