Hello from the interwebs! I know this is an old post, but I know the fix to this and want to post this as I'm certain others will be looking for answers in the future. If you are having the problem Tokyo_DIY shows in the video attached his last post above, it's a bad "end sensor", bad "start sensor", or both. What that video shows is the tape transport motor doesn't know when to stop. It will keep ejecting the tape, then pull it back in. You'll hear the sound of the tape transport motor spinning. So frustrating! This is due to the "end sensor" which is referenced in the service manual as "Q3002". You can find "service manuals" for these JVC VCRs for free posted all over online. I replaced both the "end sensor" and "start senor" though just for good measure (Q3002 and Q3002) and I wasn't sure which was faulty haha (but I think it's the end sensor - Q3002). Once replace, I was back in business!
Here are the part numbers for the 9800, they are likely similiar or identical to many other JVC VCRs made between 1997-2003ish:
Q3002: PTZ-NV16 PHOTO TRANSISTOR or PTZ-NV16A PHOTO TRANSISTOR
Q3003: PTZ-NV16 PHOTO TRANSISTOR or PTZ-NV16A PHOTO TRANSISTOR
Now what if you cannot find these replacement parts anywhere??? Or at least for cheap??? I have you covered. Buy a "for parts" JVC deck on ebay
like the HR-S7600U or one of the bit more modern JVC MiniDV / SVHS (for parts) and you are likely to find donor parts for a purchase price of $150 or less. Especially the S7600 which I've seen go for $80. My donors came from a JVC/MiniDV-SVHS deck I bought for $75 (It was a dump, but had these working parts), and I was off to the races. (plus you will have a lot of other spare parts on these you can use to fix future issues!)
PS Regarding the parts numbers above, I am specifically referring to JVC 9600, 9800 and 9900s, but MANY other JVC models will do this with bad start or end sensors.