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06-08-2024, 09:55 AM
aramkolt aramkolt is offline
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I don't claim to know all reports of different types of Dynamic Drum failures, but I've now had TWO instances with a relatively small sample size of JVC Dynamic Drum decks 7600/9600 series units that can't be bypassed with the typical removing of gears as it is a different gear failure.

What happens is that the transverse shaft right after the motor that moves it cracks, and you'll first get a clicking sound and eventually the gear will spread far enough at the crack that the worm gear causes the motor to get stuck, preventing the VCR from seeing the feedback to "sense" that the DD is in the correct place. When this happens, you'll get the classic shutdown usually right after inserting a tape and the VCR will be unresponsive until you unplug it again.

The old fix addressed a crack later in the chain (by removing the very small tooth gears that are almost always cracked after 20+ years) that locks the mechanism. It works because the drive chain to the DD position sensor remains intact.

I've attached a video showing the warning clicking sign (and that unit also will freeze on occasion) and the labeled DD PCB.

I think the only way to reliably fix this is to have an electrical solution that mimics that the VCR is wanting to see from the dynamic drum sensor in forward in one condition and reverse in the other.

The motor itself still needs to be there because the machine is expecting to see pulses that tell it that the motor is spinning (that sensor is non-directional though - it'll just pulse whether the motor is in forward or reverse and the plastic that makes and breaks that is directly connected to the motor shaft, so nothing that'll break there, especially since it would be under no load once the transverse bar is removed.

Clicking but moving DD.MOV

Transverse center gear crack.jpg

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So... any ideas on how to electrically fake that the sensor is reading open when power is applied in one direction to the motor and closed in the other? The other requirement is that the reading stays the same until the voltages to the motor are reversed. Power is cut to the motor after it moves, so the circuit would need to "remember" if it's on or off after motor power is stopped to the motor and would probably need to tap power from one of the other always active lines I would think.

Really seems like a transistor plus diode in the right configuration would do, but my electronic circuit design knowledge is rather limited unfortunately.

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06-08-2024, 10:01 AM
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lordsmurf lordsmurf is offline
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Not helpful, but just an "I told you so" to some, as I've long said some DD decks cannot be fixed with current bypass.

I didn't investigate those further, so kudos to you for making more progress than I did here.

FYI, to anybody thinking of doing a bypass, sometimes merely attempting to bypass gets fiddly, and it's easy to break stuff, even when you're oh-so-careful like I am.

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