JVC made two styles of DD systems as far as I know. One is used in lower end VCR's and uses a tiny belt and pulley to drive the pulse wheel against a sensor and a worm gear shaft to drive the gears, See picture:
The other system uses worm gear motor to drive the gears and the sensor wheel:
This fix applies to both systems as there is a lot of similarities between the two.
Note: This MOD will keep the DD system disabled it will not eliminate it, therefore it is reversible if at a later date a replacement of the broken gears with good ones to be carried out, however gear alignment maybe required and there is no procedure in the service manual for such alignment. If the gears are cracked and slipped few teeth, chances are the alignment has already been disturbed. In any case proceed at your own risk.
The procedure assumes that you already know how to disassemble/assemble VCR parts, it will not go into details on how to perform the basic tasks around electronic components.
This procedure does not require removing the mechanism from the VCR if you wish to detach the head assembly from the mechanism mounted by 3 or 4 screws depends on the model, technically no tape transport alignment is needed for doing so, the chassis plate has tapered nipples that give one unique position when the drum assembly is fastened back to the chassis.
After a lot of tests and online search I found out that DD feature engages only in the following modes: Fast forward search, fast reverse search, reverse playback, Pause, forward frame by frame, reverse frame by frame. It is not activated during normal playback/recording of the tape in any speed of any color format (SP, LP, EP .... PAL, NTSC ...). During playback/recording the video drum is resting in its neutral position in a manner that it will be explained later on in the procedure.
Procedure:
Before we start let’s go through a brief description of the DD drum parts and how they work:
JVC Dynamic Drum System.jpg
The DD drum assembly consists of 4 main components:
1-The video drum assembly (rotor, stator, rotary transformer, do not take this assembly a part, transport alignment will be required if you do so).
2-Intermediate plate
3-Chassis plate
4- DD gears and electronics
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-The video drum is clamped to the chassis plate by two spring loaded screws, 2 guide pins and 4 resting points, as long as it is not disturbed it is in its neutral position just like a normal VCR without the DD system. It can only be moved by one side set of 3 gears, when moved it changes the helical angle of the video pickup heads:
DD system.jpg
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-The intermediate plate a.k.a lead ring or tape guide ring is sandwiched between the video drum and the chassis plate and it has two springs sitting between the chassis plate and itself and two guide pins, if it is not disturbed it is clamped to the video drum by 4 resting points and that is its neutral position, it can only be moved by the opposite side set of 3 gears. When moved it changes the tape angle (this is a complicated geometrical matter, just to give you an idea, tilting the head alone will screw up the tape to drum and P guides contact, the tape itself has to be tilted in a way to keep that contact with the drum perfect, Pretty clever design from JVC engineers):
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-The chassis plate a.k.a drum base is a mounting plate for the whole assembly, it makes replacing the head easy without re-aligning the tape transport:
IMG_3538.jpg
-The DD gears and electronics, it uses 3 gear reduction system and a worm gear on each side, pulse wheel and sensor and a circuit board to communicate with the main processor. The motor spins the worm shaft, the shaft spins 2 large gears on each side, each gear spins an idler and the idler spins the final gear whose shaft is screwed into the bottom of the drum assembly. One of the two final gears that are screwed into the drum assembly if driven it moves the drum away from the chassis plate, the other final gear if driven it moves the intermediate plate away from the drum itself and vice versa (I know it's confusing).
The combination of movement of both the video drum and tape guide ring (lead ring, intermediate plate) in either direction makes the DD heads follow the tracks correctly by changing the position and observing the RF signal from the heads until the best reading has been achieved. A pulse wheel and sensor is used to precisely track the movement in both direction and adjust accordingly.
Now that we know how the DD system works, how do we lock it to the neutral position? The answer is pretty simply, Keep the drum assembly undisturbed, but how do we achieve that?
- First, we have to break the transmission of movement from the DD tiny motor to the gears to the drum assembly by physically removing a minimum of one gear from each side. One gear from one side to clear the video drum and one gear from the opposite side to clear the intermediate plate.
- Second, we have to defeat the electronics of the DD system and trick it into thinking that everything is working fine, To be able to do that the motor has to have power by keeping it connected all the time, and it has to spin and the sensor has to read pulses, any one of the above conditions is not met the VCR will shut-down.
To get started make sure you disconnect the VCR from the mains, remove the necessary hardware to get to the chassis, Remove the chassis (or video drum assembly if mechanism stays in) put on a soft pad and put the VCR away from the work area if not needed.
After removing the mechanism from the VCR (Head assembly from the mechanism if mechanism stays in) carefully flip it upside down on the soft pad to access the back side of the head assembly, Remove the DD electronic system screws (4 or 3 depends on the type), Carefully lift up the gear assembly and put it aside for now.
If you look at the bottom side of the drum you will see two spring loaded screws, mark their position and go ahead and tighten them down clockwise for 1/2 to 3/4 of turn, what that does is increase the clamping force on the video drum while in its neutral position for better adherence, this tension is very important if you decide to put the DD system back in service, that’s why you have to mark the screws' position:
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On the lower end models DD system, you can remove any gear or all of them since the pulse wheel is driven by a belt (make sure the belt is not loose):
IMG_3541.jpg
On the higher end models, you can only remove the large white gear and a small black gear on the opposite side if the drums is still attached to the mechanism:
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If the drum assembly to be removed from the chassis then you can remove the tiny black gears and leave the big gears on, the big black gear has to stay on because it drives the pulse wheel. Back off the other two drum and lead ring black gears so they don't interfere with their neutral resting position:
IMG_3540.jpg
After done put everything back together and test operation, during modes where DD should engage you will hear motor spinning but nothing happens, Noise bars in the picture will be noticed during those modes which is normal and it does not affect playback or recording.
I applied this MOD on a higher end model and I played a tape that I tested few years ago when the DD system was fully operational, here is a sample of the same tape with the MOD applied:
Here is the lost preview video:
