Fix for dynamic drum with broken motor shaft worm gear?
 

As a follow-up to this thread, I had the same issue but it was not fixed by removing the gears because the main drive worm gear is cracked, causing the motor to jam and stall.
I used a very fine razor (Feather surgical blade #11) to trim down the broken half of the tooth of the gear so there would be less moving plastic to bind up the motor. Be careful not to damage the gear teeth on either side! Attachment 14030

Note that in retrospect, I don't think this step is required.

Then I greased and put everything back together, leaving out all unused gears. It is important to get everything aligned correctly- the two sensors that give gear movement feedback have an initial starting position. The one that senses the motor shaft is completely blocked, while the one that senses gear motion is halfway blocked (see attached pic). I've circled the gear, and marked how the half circle blocks half of the sensors.
Attachment 14031Attachment 14032 Attachment 14033
When the VCR powers up, it initializes the DD by spinning the motor clockwise (which means the worm gear is also turning clockwise... it gets confusing) just enough to move the gear sensor 1/8th of a turn so it blocks both sensors, then it moves back to the original position. The secret is it only turns the cross shaft worm gear 3/4 of a turn to do this. So I reasoned that if I positioned the broken tooth in this 1/4 section that is not touched in the initialization sequence, it would not jam the motor. And that is exactly what happens. I've tested it extensively, and empirically determined the optimal position for my cracked gear. When looking down the worm gear axis, it should be in the lower left quadrant.Attachment 14040
To make this easier, I made an extension cable for the DD mechanism so I could have it outside the VCR body while a tape was being played. Attachment 14035
If you don't want to go through this, there is an easier way- just plug it into the board and flip the drum out of the way Attachment 14036
That way, you can easily adjust the main drive gear and test with power cycles. After turning the motor shaft to adjust, be sure to reset the sensor gear to occlude half of the sensor, as pictured above.
Some caveats- for this to work reliably, you must not use any features that activate the DD! So no FF or RW while playing. Pausing and stepping by single frames does no use the DD, so that is ok. While I had the DD hooked up via cable, I tried using the features that would activate it, and it mostly worked, but was very loud, and eventually jammed again.
I think if we can find the right CPU pins to attach to, we could bypass the FF and RW signal somehow so the DD doesn't move during playback but will initialize. I tried simple things like running the FF and RW pins to ground, but that didn't fool that wily VCR :laugh:
I'm still working on mapping the functions of the DD circuit in the hopes that I can make an arduino that emulates it, so we can pitch the whole assembly. If anyone has any skills in this area, help would be greatly appreciated.