Sounds like track 1 is recording ok then, just not playing back. What does track 8 sound like played back as track 1 with a flipped tape? I'd switch the tape sync off (not sure which track this affects) and recording with dbx switched off (does this switch all tracks?)
I'm still wondering if it is a fault with the track 1 dbx decoder, as happened with my 244 - for me it was luckily just a loose connector.
Also - I think there are separate output jacks for tracks 1/2, 3/4, 5/6 and 7/8. Presumably no playback signal at the 1/2 output jack either?