There are a couple ways to approach this depending on what you are going to end up doing.
In you turn on smart navigation for the page, ASP.NET will automatically scroll the page and focus onto the control that was last active. However, based on what you said about the textboxes this might not work perfectly. Assuming that your postback is being caused by a textbox change even, you have to leave the textbox for that clientside event to fire, which then fires the postback. So your return focus might not be exactly where you want it.
In the second scenario you'll have to employ some fancy code-behind footwork. When you process the postback event of the text change you'll have to find the repeater item where that textbox exists. You should be able to ask for textbox.Parent (maybe more .Parents) and eventually work your way back to the repeater ITEM, where you can get the item index, and then the repeater itself, where you can get the next item (n+1). Then you can drill down into the item and find the textbox. Grab the textbox's ".ClientID", then you can use that in some javascript
: document.getElementById("<value from clientid>").focus();
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Peter