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?

Did they also change the chain? If you are moving from a five cog cassette to a 6 cog cassette you may need a different chain. However, that is not likely what is causing your problem. If it is jamming between the derailleur and large chain ring it sounds like an over-shift. I'd take it back to the bike shop and ask them to make the adjustment while you watch. This way, you'll either learn how to make the adjustment yourself or learn that you need to frequent a different LBS.

Most modern chains are narrower than the original equipment chains, which exacerbates the adjustment issues NOS88 mentions. Keep your derailleur adjusted and avoid 8 speed or higher chains if at all possible. SRAM makes a decent 7-speed chain that works fine on my bikes.

Thanks for the replies. I guess some of my concern is it has been fine for 3 months. I'm going to the LBS today and watch what they do.

Had this on the Tandem with a triple and it was combination of two things. Originally it was 8 speed but I changed to 9. Narrower chain on the 9 speed and I did not change the front derailleur. I also changed the granny to a 24 from the 28 fitted. It was the chainring that was the problem. I fitted one out of the spares box and it was steel and quite thin. The chain was not being thrown far enough with the 8 spd derailleur and the thin chain ring allowed to chain to slip between granny and middle. I got a new set of matched chainrings to cure the gap between the rings but still had a problem going from middle to granny. New proper 9 spd derailleur and all cured.

Been a long-long time but kind of remember raising the front der. up a bit along with tweaking the inner guide of the front der. with a channel lock. Put a little twist at the front of the plate towards the chain ring. Of course this was on the Huret or Simplex derailleurs back then.

https://www.parktool.com/blog/repair-...ur-adjustments

Yeah, I would check two things right away, first of all is the derailleur square to the chainring, I've seen where the derailleur got whacked at some point and it was fine, but changing the freewheel or cassette to one with a different dimension, could cause it from being slightly off to far enough to give issues. Another thing is, the height of the derailleur, from the outside plate of the derailleur to the largest chain ring at the top should be no more then 2-3mm and this gap should be for most of the length, if it's more then this the chain might be jamming in between.

Took it to LBS and found the cable on the down tube had moved to the outside of some hose clamps I had used to hold a water-holder to the frame. Moved the cable back and spent 20 minutes tweaking the chain-ring derailleur...moving it and bending it. It works o.k., or at least as good as it did.

When I bought this I had them change the drops to a straight bar and move the shifters from the drop tube to the straight bar and they also put on the 6-cassette. Basically the LBS guy working with me yesterday was a different wrencher than the one who had modified the bike originally and was not surprised that I would have some problems with the modifications being fussy.

Riding this morning. I'll see if it works. Thanks for the replies.