Roland RS-640 What is causing this shifting

It definitely looks like 'encoder' something. Strip would be my first thought and unless you have a lotta overspray clogging up your strip, ya might look into replacing the eye.

 

Definitely the encoder strip is dirty or needs replaced.

 

Have our machine since 2005. Replaced the eye once, about 6 years ago. Replaced the strip once, but have cleaned it several times and reversed it to make it work.

 

Take 2 sponge swabs, not cotton but sponge ones, and dip them into 70% iso Alcohol. DO NOT use 99%, only 70%. Squeeze the excess alky out of them so they are just damp.
Pinch the strip and slide the length of it. Move the carriage one way or the other so you can get the part of the strip behind it too.
Allow it to dry about 3-4 minutes and try a print.

 

I don’t run a Roland, but that looks like a media feed issue to me. Are you sure the material isn’t walking? Maybe wipe down your pinch rollers and mark the platen with some tape at the media edge...

 

The encoder sensors rarely go bad in my experience. There should be no physical contact with them and the power supplied to them is generally steady. There's an LED on one side and a light sensor on the other side. If you don't have your encoder strip centered between the two arms, it can rub on the sensor and ruin both the sensor and the strip.
I have seen them get dirty, which will block the light passing between the two legs. Usually a blast from some canned air will fix that. Don't use a compressor for this as the pressure is probably too high and the air is contaminated unless you've got filters and traps on your air supply.

The closeup on the letters shows that it's stepping at each pass which is classic encoder problems.

Blast the sensor with the canned air and clean the encoder strip. I like C5's technique. I usually use an alcohol wipe from the pharmacy (or 2 if it's really dirty) but there are usually sharp edges right around there and you have to be careful of cutting yourself. Using swabs would keep the fingers safe.

 

You can also double up a pair of those white cotton gloves, soak your fingers in alcohol, then clean as mentioned....same process, just easier without having to jam a long foam brush inside the machine.

 

I’ve seen the same looking artifact from having to tug fabric to avoid head strikes. We run grand format, and 50-75 foot prints are fairly common. We have to take great care in keeping rollers clean and tensioning the load perfect or we have walking issues.

Never fun to find out 400 inches in!

I keep my machine pretty clean so the encoder doesn’t have a chance to get funky. The carriage rails are exposed so I have to clean and lube them daily. Any time an oily rag gets near the encoder strip, it gets wiped clean too!

 

Clean Encoder strip. If banner material is shifting too, use all of the pinch rollers