Dear all,
I have recently became a DCC user cause I admire the technology and complexity of it. But looks like that complexity played a joke on me… I have read all threads on DCC600 issues here and while some sound similar, I still think there is something uniquely wrong with mine 
So when I received it it played fine for an hour or so. After I have listened to the couple of tapes, it started behaving strange. I would push play, it would play for 1.5 sec and stop. That would happen only on first 1 or 3 songs on the side, song or five would play normally. Having some experience with fixing usual tape decks, I have disassembled to deck and found capstan belt to be quite loos (at least it was very easy to move it with my finger). I have ordered new set of belts from eBay. When it arrived I was dazzled to see that these new belts were really diffferent from mine. Old capstan belt was like 4.5mm wide and new one was 3mm. Old tray belt was round and new one as square. What’s even more strange, both of the new belts were slightly longer than old ones! Upon reading the specs I found out that new ones were closer to the specs than the old ones. So I replaced them and now deck would play only side B on my DCC tapes! On side A it would start and stop after 1.5 sec. I have disassembled it deeper to check rollers and gear only to see them looking fine. I think I am lost now and would REALLY appreciate any advise you might have!
I could be wrong but it sounds like the electronics aren’t sensing the rotation of the side A take-up reel.
If it spills tape into the deck or into the cassette, it’s almost certainly a problem where the take-up reel doesn’t spin.
Possibly the idler doesn’t engage properly with the gears of the take-up reel, which could just be a matter of cleaning the mechanism and adding a little grease in the right place.
Or the idler needs to be replaced. They are the infamous soft-plastic gears, which get brittle and break. The DCC museum sells a replacement.
Less likely, it could be a problem with the optical sensor that senses the rotation of the take-up reel, or its wiring.
Loose belts, as you already figured, could also be a problem: They can cause the torque on the take-up reel to be too low.
===Jac
What Jac said 
Dear all, thank you very much for your help. I disassebled unit again (2nd gen is tough to access belts and tape mechanism) removed old grease and put new. I also managed to find suitable belts, which were about 5mm shorter than original, therefore tighter. I installed them - and now it works!
Thanks again! DCC rules!