Although it's covered in the user manual, there are some clever heads on this forum who may help me out.
I have a UCX, all analogue channels used, 8 ADAT channels going in via TOSLINK cable, and it's all synced fine using the UCX clock as the master. I picked up a cheap-ass A/D convertor (£2 from a charity shop) which has 2 phono inputs and a coaxial out and a TOSLINK out.  When connected using a coax cable, the SPDIF LED flashes - clock/sync error. If I set the cheap-ass A/D convertor to master, it works. But I'd rather trust the RME clock, sure you understand!
Am I wasting my time here, just to gain 2 more inputs? Or does anyone have a suggestion? The A/D convertor has no controls apart from 44.1 and 48kHz (which I use as my sampling rate anyway).
I was considering running a spdif out from the UCX to the convertor, but I'm sure it would freak out/ send that signal back to the UCX or something). It's cheap junk which will probably become a doorstop with a nice red light on...