JVC made two different designs 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 both 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 only disable the DD system 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 lost. 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 equipment.
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 screws, technically no tape transport alignment is needed for doing so, the chassis plate has tapered nipples that mesh with holes on the bottom of the video drum assembly giving a 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.
Before we start lets go through a brief description of the DD drum parts and how they work:
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).
4- DD gears and electronics
1- 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 gears (gear reduction assembly), when moved it changes the helical angle of the video pickup heads:
2- 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 gears (gear reduction assembly). 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):
3- 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:
4- The DD gears and electronics, it uses a set of gears reduction system and a worm gear on each side, a pulse wheel/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 a series of other gears until the final gear whose shaft is screwed into the bottom of the chassis plate. One of the two final gears if driven its shaft pushes the drum away from the chassis plate, the other final gear if driven it pushes 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 reading the RF signal from the heads until the maximum signal has been detected. A pulse wheel and sensor is used to precisely track the movement in both direction and compare the readings to the position of the head and lock on the strongest signal.
Now that we know how the DD system works, how do we lock it to the neutral position? The answer is pretty simple, Keep the drum assembly undisturbed, but how do we achieve that?
- First, we have to break the transmission of movement from the DD motor to the gears then 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 keep the electronics of the DD system alive to avoid VCR shutdowns and trick it into thinking that everything is working fine, We must keep the motor connected to the system 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.
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):
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:
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:
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:
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Last edited by dellsam34; 27th Nov 2020 at 18:40.
Picture links fixed.
Last edited by dellsam34; 7th Jun 2019 at 02:57.
Hell of a post, I don't understand why it hasn't gotten more attention. Also, thank you for posting this here, The dFAQ version keeps ending in page errors for me.
This is utterly beyond my current capabilities or comprehension (I don't even have a DD VCR), but assuming it works it seems to make HR-S9*00U decks worth having again after all, no? They continue to be highly rated in the dFAQ VCRs guide, but Lordsmurf has more recently emphasized that he no longer recommends them due to this "un-repairable" issue. For those of us not so adept at this type of work, I wonder if it's something Tom Grant or someone else would offer as a professional service?
On the other hand, does this essentially negate the advantages of these decks in the first place (such that it's a valuable fix for those who already have one, but not for those of us looking to buy)?
Thanks for your interest, not so many people repair VCR's nowadays that's why it was buried in the ashes.
My useless decks are just waiting on better 3D printing tech. I may know of a method now, and somebody who could do it, but I currently have no way to convince him to do it. He's overbooked on projects, something I understand. So for now, still a waiting game.
DD system was a great feature back then when VCR's were the main entertainment piece in the living room, So having to pause, rewind and fast forward movies or home video with crystal clear picture with no noise bars is priceless. Not so much now, most people use a VCR to convert tapes to digital, even the ones who do play a tape once in a while are not interested in high quality pause, rewind or fast forward.
So there is no reason now to repair the DD system when you can just disable it, and no 3D printer can produce high quality high strength plastic gears at that size at least not the few thousands of dollars ones even the Polyjet ones.
davideck wrote this years ago:
I should also mention that the Dynamic Drum System that many JVC VCRs have ...allow them to playback almost any tape. This system dynamically tilts the scanner to optimize the tracking in a way that other VCRs can't.
Great post! Made it sticky!
For some of you who don't know what does the Dynamic Drum System do, It basically makes the video frame clean even in fast forward search speeds, frame by frame speeds, pause, reverse frame by frame speeds, reverse search speeds and reverse playback, Such features are no longer important nor being used in the digital world of today, see attached video:
I have one of these VCRs with the high-end drum, which recently just stopped playing tapes. After finding this thread I took the drum out to see that the problem was a cracked gear, so DDS was probably stuck. After removing the 2 small black gears and retracting the other gears to deactivate DDS, it can play tapes again, but something is wrong with the picture. With TBC/NR on, there is no picture in play mode, however in forward/rewind mode the picture seems fine (just as any VCR with no DDS). When I turn TBC/NR off, the picture comes on intermittently in play mode, but it seems like something is not aligned properly, because there is a constant noise in the middle. Do you have any idea what might have gone wrong?
Yes, it is miss aligned. The head is not in it's resting position, You need to follow the procedure step by step.
I tried this ultimate fix, but sadly got less than ultimate results.
I have a JVC HR-S7600U that I picked up for what I thought was a good price. With a solid cleaning, it played my test tape well. A few months went by and when I finally tried to use it for a project I got the dreaded "shuts down in 3 seconds and flashes Auto."
I followed this tutorial and indeed found one split gear, and another on it's way. I removed the recommended gears, backed off the other 2, reassembled but the same problem remained.
[Attachment 57690 - Click to enlarge]
[Attachment 57689 - Click to enlarge]
I occurred to me that I had never heard the whir-whir that others talked about, so I dug a little deeper. It turns out the small DC brush motor was stalled. I was able to remove it and the circuit board, and cleaned and lubed it. With 5V on the bench it spun fine.
[Attachment 57691 - Click to enlarge]
After reassembly into the gear train, I tested it again and got a distinctive clicking or knocking. After I removed power and restarted it, sometimes it would spin, other times it would stall.
Removing one gear at a time to determine the source of the clicking, I eventually determined that it was coming from the horizontal shaft with the worm gears. A very (very) close inspection revealed that the center worm gear was cracked, and that one of the teeth was cracked and shifted. This would account for the rhythmic clicking, and that the motor would jam on this tooth when stopped and not be able to restart.
[Attachment 57692 - Click to enlarge]
[Attachment 57693 - Click to enlarge]
[Attachment 57694 - Click to enlarge]
Knowing that the VCR would never operate without the motor (and optical sensor), I decided to pull the shaft and worm gears out and let the motor free run with no load of the gear train. Sadly this had absolutely no effect on the operation. While I do now hear the whir of the motor, the VCR still times out at 3 seconds and shuts down.
I do see that at the final stage of the gear train is another optical sensor, and the last gear has a feature that obscures the sensor for half it's rotation, and then gives a clear view of the sensor for the other half. I have tried the gear in both positions and it doesn't make any difference.
[Attachment 57695 - Click to enlarge]
So now I have several questions:
1) With no load is the motor spinning too fast and affecting the timing sensor?
2) Does the 2nd optical sensor have any affect?
3) Has anyone found a way to defeat this circuit in it's entirety so that the vcr thinks everything is operating nominally?
After so many disassemlies and reassemblies hoping for the silver bullet, I'm at my wits end!
Thanks for the test feedback, really appreciated, your problem seem to be worse than any problem I've seen before, I have one machine on the bench and I will inspect the worm gear shaft for cracks after seeing this, You brought up an interesting set of questions.
1- I don't know if the frequency of the pulses matters or not unfortunately, That is something the JVC engineers would know for sure but we can always find out by experimenting, I believe the motor sensor is just a self check that the motor is alive, and second sensor is what actually counts the degree of movement in either way.
2- I would assume so, A sensor means a signal feedback, no signal equals a problem in the processor language.
3- I have not tried, I thought about it though but would require some electronic and coding skills and understand the processor circuit diagram, Basically a small circuit has to be built to generate the necessary tones to mimic the sensors' feedback and fed with the same voltage available to the DD PCB, Then all the gears and motor are removed and the new PCB will sit in place of the DD PCB.
All what you can do is keep experimenting, Try to find a way to make the second sensor register a signal, I'm pretty sure that's the reason for the shutdown.
I don't remember if I noted this already, but I think the two variants may be older and newer rather than high and low end. JVC changed the mechanism a fair bit in the 1998 models (like HR-S7500, HR-J658 and so on, generally the second number indicates the lineup it's from) compared to the previous gen, and slightly more for the next year (HR-S7600, HR-DD868 etc). They then seem to have kept it mostly the same for a few years, but then changed to a different one for the last few ones (don't think there are any DD decks with that one).
If you look at the SMs for e.g the HR-DD868EK, HR-S7600AM and 8600EU, they show the same mech parts in both (other than the tape stabilizer thingy). They even used the same main pcb and ICs, just with SVHS and TBC boards added in the SVHS versions, so one would think it would be possible to use parts from the cheaper model to repair the more expensive SVHS ones.
Also, the SM mentions some stuff about what faults will cause it to shut down. Also, one thing you could maybe also test is whether it acts differently if you put it in mechanism service mode, you have to bridge two wires somewhere, it's mentioned which in the manual. It let's you play, ffwd etc without a tape inserted.
Maybe, But I've seen the belt type DD system in low budget VHS machines and the worm gear type in high end S-VHS machines, though it is a handful of VCR's only, not enough to make it a general rule.
I haven't looked up error codes but I would assume if they are listed it will just say DD system, I don't think it will tell you exactly what's wrong with the DD system.
Thanks to the member Hodgey over at Digitalfaq we have a new technical document about the DD system by JVC.
I may have a fix for you. I had the same issue- the main drive gear is cracked. I'll reuse your pic here because I don't want to take my DD mech apart again and break something else.
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 60532 - Click to enlarge]
Note that in retrospect, I don't think this step is required now that I have figured out where to position the broken tooth.
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 60541 - Click to enlarge]
[Attachment 60542 - Click to enlarge]
[Attachment 60545 - Click to enlarge]
When the VCR powers up, it initializes the DD by spinning the motor clockwise (which means the worm gear is 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 worm gear shaft 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 (see drawing).
[Attachment 60552 - Click to enlarge]
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 60543 - Click to enlarge]
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 60544 - Click to enlarge]
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 but will initialize. I tried simple things like running the FF and RW pins to ground, but that didn't fool that wily VCR LOL
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.
Last edited by drzapp; 2nd Sep 2021 at 15:43.
Thanks for the info, I never though about the motor shaft gear being cracked when I initially started this thread, So good to know that you've found an additional work around for that.
As to bypassing the whole thing, The ideal way is to make a small smart circuit that can comunicate with the VCR's system control chip and mimic the pulses, This requires someone to have a deck with a DD system and an oscilloscope to pull all the frequencies out of the pins so a circuit can be designed accordingly, I have the deck and the oscilloscope but I'm not knowlegeable enough to design such a circuit from scratches, We have members in this forum with such skills but not sure if they are willing to dive in.
Also it is a good idea to use a perfectly working DD system for normal playback to improve tracking LP tapes, miss aligned recordings and shrunk or stretched tapes by externally controling the DD motor via a potentionmeter and a small circuit.