After monkeying around with a broken JVC Dynamic Drum system, I'm posting my observations so far. Anyone else please add on.
So, my first job has been to bypass the dynamic drum which I've now done making the system operate like the newer (non dynamic drum) models.
There are 2 sets of gears on each side. they all move together in the same direction (clockwise or counterclockwise) as a team though one set will move at a slightly different speed. One gear is a screw that pushes against a metal post actually raising and lowering the entire side of the drum. The other gear pushes into a plastic ring that has another screw. When perfectly aligned, the gear will raise and lower a metal guide (in relation to what would be the bottom of the chassis). When not aligned, it spins in place slipping in the ring and doing nothing.
To get my system working again, I put slight pressure on the metal posts with the metal screw gears until the 2nd gear got traction and raised the metal guide on both sides all the way towards the top (closing the gap between guide and heads). The final position allowed me to remove both of the metal screw gears (one is black, one is white). and leave just the post gears that fit in the plastic rings. The ring and drum were still in correct alignment allowing me to move the ring slightly down on both sides (but not too much or it catches and you can't move it back up without inserting the metal screw gear again). With both sides of the guide pushed all the way up (closed), I assembled the dynamic drum without the metal screw gears. Now the tape plays perfectly in SP and EP (I checked some EP tapes recorded on this unit when the dynamic drum was new) and the dynamic drum gear train does nothing (spins but doesn't move the drum). I can FF and Rew just fine without lockup or shutdown and return to play with perfect alignment. Slow motion and 1.5x are clear, but after that noise enters the picture as expected because the drum cannot stay aligned without the missing gears.
I'll probably play around with it to try to get the dynamic drum working again, but this should help out those of you who have a broken gear to get the system playable again.
As far as head alignment, I don't have access to the equipment needed to check head alignment, but I can say the EP tapes I recorded a decade ago were playing rock solid on tracking without any need for any of the TBC features so it appears I found the "neutral" position for the dynamic drum.
If anyone has a factory assembled dynamic drum, I would love some close ups of the gear positions and gaps to maybe get to a spot where we can align these ourselves.
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your workaround looking like gold, pictures or video would be really helpfull.
I would definitely want to try to bypass the DD system.
I serviced my VCR and noticed the two tiny black gears are split, I added some grease to them and I wished I didn't, After putting everything back together I noticed a loud grinding noise coming from the drum, I tried to rewind the tape and it quits sometimes. I bought a HR-DD8400U so I can get some spare parts hoping they share the same DD system and they are in good shape.
Does anyone know how to align the gears on this system, The service manual doesn't mention the dynamic drum at all. Note that I haven't pulled the drum out, Just the gear mechanism on the bottom of the head.