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 19:40.
Picture links fixed.
Last edited by dellsam34; 7th Jun 2019 at 03:57.
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 16:43.
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.
Wow... what a great description of the DD system and potential fixes!
With a view to digitizing some old tapes of school plays etc, I dug out my HR-S9500, gave the drum etc a clean, and checked that it was working. Played an hour of LOTR faultlessly. Then ordered an AV cable for my capture device and after a couple of weeks set it up. More or less straight away it started the 3-second shutdown issue. This thread has at least delayed my search for another machine. Removal of the drum assembly from the main chassis, and removal of the DD gears assembly confirmed a cracked small gear as in dellsam34's first picture. (the cracks to the frame where the shafts run don't seem to be an issue).
I removed the small gears as indicated (might be worth mentioning that the larger gear shafts unscrew). Putting it back together didn't change the error, and I found it necessary to align the sensor occlusions as beautifully photographed by drzapp. Now it powere up fine and plays, but with serious noise bars, so the head alignment is not correct.
The fix describes 'backing off the large gears so that they don't interfere with their neutral resting position, but I removed them entirely as shown in drzapp's picture. Was this a mistake? I assumed that with them out of the way, that the drum would snap to the neutral position as explained by dellsam34.
I didn't separate the drum from the intermediate plate or chassis plate. Toying with possibilities for repair of the small gear!
By using the manual tracking function (channel +/-) buttons I can pretty much get rid of the noise bars, but subjectively the picture is still not as sharp as before the failure.....?? What does manual tracking move, as presumably doesn't use the inoperative DD gears?
Last edited by Foxone; 20th Jan 2022 at 15:08.
On one of my machines putting the drum assembly in the neutral position worked perfectly and I posted a sample capture above, The only explanation I can come up with is that there is a possibility the drum assembly is adjusted as whole at the factory, meaning that the entrance and exit guides are not aligned (using an alignment tape) when the drum is at its resting position but rather offset a little bit.
If this is true (which I don't believe it is) then there is two options, find a tape recorded with the machine when the DD system was working fine (or an alignment tape or at least a well known tape) and re-adjust the guides. Or put the small screw gears back on and try to turn them manually until you get to that factory offset position using the tapes I mentioned above (assuming the motor linkage is disabled so it won't move the gears again). The second option won't be easy as the access to the screw gears is hard when the drum is pit back together. Pay attention not to hit the drum while spinning if doing so.
Thanks for the fast response! I think part of the problem is just deterioration of the SVHS tapes...they have been stored for however long it is since Sky+ came along, and are showing evidence of print-through. They only have TV movies on them, so are of no value. Tweaking the guides has improved the tracking further, a technique I didn't even know about. Your suggestion plus YouTube has filled that gap in my knowledge. Realistically I must stop being fussy, as the VHS tapes of school are a bit rubbish quality....probably filmed on VHS-C and then reproduced in bulk.
Once the transfers are done the VCR will be available free to anyone in UK that needs spares!
I also have a Grundig Video2000 machine that I must dig out and test....I've seen the same model in a science museum!!
Judging from the faulty model I got, the "JVC turns off automatically after a few seconds" problem isn't inherent to models with the Dynamic Drum (DD) system. JVC applied for patents related to the DD system in March 1995 and planned to have it on the market in early summer 1995, as described in the same article. The first VCR with the DD system was apparently introduced in Europe in November 1995.
My JVC HR-S4700EG has a sticker with the (assembly line?) code 07A01759 inside. The manual included with it has a September 1991 "Printed in Japan" notice and I found some late 1991/early 1992 reviews for the model, so its design was around long before the DD system was introduced. Just some research to back this up as I'm basically clueless when it comes to VCRs.
I don't know how precisely others measured the time it took for their models to turn off. My HR-S4700EG takes some 5.5 seconds, it seems. After turning it on, the display shows "PR 1" and you can hear a buzzing sound for 3 seconds. Then the display shows "EJECT" and you can hear the same buzzing sound for 1 second. After that, the VCR shuts down automatically and the display shows "SET CLOCK" (which I suppose is normal for a VCR that had no power and has its clock not set). That's with no tape inserted, mind you. Trying to insert a tape yields mostly the same result, possibly with the exception of the shorter 1-second buzzing sound. It's hard to hear if it's there with the tape popping back out. Either way, the tape doesn't go in all the way and the VCR spits it back out, shutting down afterwards.
I can't tell if this is the same problem everyone else is having. Similar issues and possible fixes for them have been described over the years: 2003, 2004, 2006, 2011, 2012, 2013, 2018 and 2019. The bold ones may be the most detailed threads. The possible solution by Blackout won't apply to my VCR as its drum doesn't have those black gears underneath it. So far, I'm finding hardly any videos of an opened VCR showing what it does for others (This one probably though it's a bit different from my problem. Check the links of the other threads for some more videos and photos.) Therefore I'll provide some videos of my faulty HR-S4700EG with this post. Oh yeah, I can do without your comments on the quality-brand TV and the test tape.
Anyway, if I had to take a guess, probably some mechanical issue with one or any number of gears or springs. The dirt sure doesn't help though, overall, the device is in great condition. Probably hard to prevent this problem from occurring after all these years. I've read time and time again not to rewind and fast-forward your tapes with the JVC VCR you use to digitize your tapes but to use a second, cheaper-brand model to do that, reserving the good JVC VCR only for playback/recording. I may take this one to a repairman sometime and keep you posted once I do. It'll take a while, though.
This post may not provide a solution to anyone, but maybe it helps having this information in one place, given how many devices are affected by this.
Turning off or freezing if there is something wrong when loading or turning on is something a lot of VCRs do. I's generally to prevent risking further damage, either the system control cpu shuts down if seeing something abnormal (as with the dynamic drum thing), or it could be via PTC thermistor or something. Many will also have internal fuses to protect circuits.
The HR-S4700EG is quite a bit older than the dynamic drum models, and the mechanism differs quite a lot so I don't think any videos of the newer ones will be all that helpful to fix issues with it. It was used in models from around 1989 to 1993 I think. Maybe you can find some info on similar issues with other decks with the same mech (this may or may not have something.) I have the HR-S5800EG from 1991/1992 with the same mech that shares a fair bit with it but I've not had that issue, (It did randomly start having some servo issue instead). Expect to need to replace some SMD electrolytic capacitors on the audio module, under the video head drum and possibly in the power supply to get it working well if you do get the mechanical problems fixed.
I hate necro-threading, but hopefully this will help others as well...
My DD locked up and when I unbound the gear train and reassembled, the tracking was completely off. After several tries, I ended up breaking the 5 pin flat ribbon during reassembly. You can repair these. Determine the side that the metal is exposed and carefully scrape away the insulation. Once you have the 5 pins exposed you can peel back the rest of the insulation and snip leaving the 5 bare wires poking out. Use some liquid (not gel) super glue on the tip of the clear plastic reinforcement and reattach the clear plastic reinforcement to the insulation of the cable. Spread some liquid on the rest of the clear plastic reinforcement and push the metal wires down effectively gluing them to the clear plastic. Wipe off excess glue. Now you have a working reinforced cable again.
As for the DD. What I had to do is back off the screws on the lead ring on both sides completely. These are the screws that have a plastic gear cover on top that the 2 gears you remove to disable DD push into. So now the lead ring is in the "neutral position" but ready to be moved. The threaded gears I screw in until I feel them hit the plate. I then back them out 1 full revolution. After reassembly I got my DD close enough I have noiseless playback up to 5x and my EP recorded tapes look good. Your mileage may vary. Good luck.
I didn't have any cracked gears that I could see but all the worm gears were dry as a bone so I lubricated the gear train. Hopefully will last me again until a gear breaks (*eyeroll*). I'm not as confident as others about printing the parts. The teeth and worm threads are tiny. Well beyond the precision of consumer printers. Not to mention if you don't get the thread or teeth perfect, you'll sheer the other parts. If you're in the market for an SVHS...avoid the DD ones at all cost.
Last edited by ybarra; 18th Jun 2022 at 12:33.
I've been looking for someone to help me circumvent the electronics on the DD system to avoid the automatic shutdown if a normal non DD system head assembly can be fitted in and the failed DD one is removed. Without tricking the system control into thinking the DD system is there, there is no way to just replace the head assembly and call it a day.
Also for a perfectly working DD system, if it can be controlled externally it could benefit from playing back tapes with irregular speeds, shrunk, stretched tapes ...etc. by tilting the head and observe the picture. We all know the DD system is not active during playback by design.