I have been gathering 840 projects and decided to look deeper into one of them. This one failed the diagnostic because none of the tact switches after "display" will work. I was concerned about the lack of operable zip drive and bought a few for stock. But when I took this unit apart I found a deal-breaker. The channel 3 cue pot had cracked the circuit board about 1-2 inches in several directions. It looks as though something heavy fell on the #3 cue pot and knob and caused the board to crack. I don't think this board is salvageable. I may use this unit for spares. Take a look at the pictures and "let the buyer beware!"

https://imgur.com/a/s2pRr4f