I purchased a 1970 Raleigh 10spd at a LBS in April. They changed the cassette to a 6 making it a 12spd.

On the last ride the chain started jamming between the derailleur and the large chain ring when shifting from the small chain-ring to large chain-ring. This morning I attempted (without good results) to adjust thing. This is a non-clicking old fashion shifting system. It had been working o.k.
Any thoughts on the problem?