Use JVC HR-S9800 parts in a HR-S9900?
 

I found a JVC 9900u that works except for no s-video output. In looking around for replacement parts, I find a couple sources for 9800u and 9911u parts and service manuals for both, but nothing for a 9900u. Can I use 9800u parts (specifically, the terminal board assembly) to fix the 9800u?

No, don't assume 9800 and 9900 are complete matches.

I was already asked about that 9900 on eBay. I'd stay away from it, especially at the $200 asking price. You know nothing about how it tracks, performs, head condition, etc. It physically looks dirty and beat up. The seller's knowledge is extremely suspect, especially with statements like "component output works fine". It's composite, not component. Therefore you can't trust anything he writes.

And remember, 9800/9900 decks are all starting to develop dynamic drum gear issues, not unlike Panasonic caps issues. Except there's no replacement parts for the gear, and 3D printing is still not durable enough. My own 9800 is now downed for this reason, though I can manually jumpstart the gear if I really need to use it. Others can even be jumpstarted anymore. Like an AG1980, assume it's broken or near-failed unless specifically known otherwise.