JVC Dynamic Drum Ultimate Fix

[hkorpim]

2 things.
I have just finished writing the new technical document for DD emulator and it's installation, that I share below. Any comments welcome. The document also contains instructions for guide ring alignment, if lost.

I also now have a new PCB version of the DD emulator, that has an extra row for 6 pin 2.0mm header to be connected directly to main board connector CN3501. (This PCB also has reservation for some extra resistors in case of optional features like sensors and PWM signal, but not used today).
It works, if DD gearbox has been removed, but has some issues:

1) It's a bit difficult to locate to CN3501.

[Attachment 85642 - Click to enlarge]

2) The space under drum is very limited even if DD gearbox is removed, because the drum leans left and almost touches DD emulator.

3) Plastic part in the picture below slides in direction of red arrow and touches DD emulator slightly.

[Attachment 85643 - Click to enlarge]

This picture was borrowed from Colin/video 99 as a screenshot:
https://youtu.be/LiP8CQ-j3oc?si=bb5f_46CO37yhnAb

[lordsmurf]

It's a grammatical mess, but I understand the non-install aspects. It could use an editor. I'll volunteer, help you with it later.

In terms of the step-by-step install guide, it won't make much sense until we/I actually have a DD emu in hand, and go through the process. You surely forgot something, or somebody will run into an issue you never saw or thought of. Just be prepared for it.

Overall, you've done quite well.

[hkorpim]

...in Finland we call grammatical mess "rally english"... If you have ever seen/heard a famous Finnish rally driver in an interview...

[lordsmurf]

...in Finland we call grammatical mess "rally english"... If you have ever seen/heard a famous Finnish rally driver in an interview...

I like it.

[dellsam34]

I offerend the seller $40 on this head which is linked in an earlier post and got it yesterday, Unfortunatly it does not have a FE head, so the ribbon cable is narrower by 2 tracks, But I can confirm that the physical dimensions are identical for a straight forwad mechanical swap. I will attempt to test it on the weekend, I got all the parts I need to fix the VCR, the fuse, the ribbon cables, It will be a determining weekend.

[hkorpim]

dellsam34, I look forward to hear the results.
FYI: It has now been confirmed, that DD emulator also works with the 1st GEN Dynamic Drum system. It has been succesfully tested in The Netherlands this week. 1st GEN has been called 'low end' version before on this thread, but all 1st GEN models I am aware of, are produced between 1995-1997 and all later models have the 2nd GEN DD. So my conclusion is, that it's about the deck/drive generations, that make also 2 different DD systems.

Both generations of DD system have same sensor signals, but the order of terminals in connector/main board socket is different. It just means, that there must be GEN specific ribbon cables. In both cases the cable connects with 6 pin/2.0mm pitch header.

[dellsam34]

Well folks, I got some good news and some bad news. Let me just go through the process and you'll see:

I replaced the blown fuse, Installed the DDE board and cable and that took care of the 3 minutes shuttdown caused by a seized DD gearbox, By the way I found a better way to route the DDE cable as it hit the P guide numerous times (second picture was taken then), You can just unscrew the two rear screws of the mechanism and gently lift the corner of it to get the tip of the cable thru:





I then bolted the brand new head on and transfered the magnetic rotor and stator from the old head to the new one noting the position of the heads vs rotor ring to not offset the switching point if that even matters. I connected the head's ribbon cable and slid it to one side, the side that aligns with it naturally when hanging free, Unfortunatly I did not get any usefull picture as you can see from this video, I paused a little bit in the middle of it:

https://forum.videohelp.com/attachment.php?attachmentid=85858&stc=1&d=1740899011

I then put the old head back after removing the DD gearbox and all the remaining gears, bottomed out the spring loaded screws and without any tape path modification the picture came out crystal clear as seen in this footage (the green strip on top of the frame is in the tape, I watched it before):

https://forum.videohelp.com/attachment.php?attachmentid=85859&stc=1&d=1740899250

The conclusion:
-The head must be identical in terms of ribbon cable width and/or the correct pinout, It could be that I did not align the ribbon cable tracks correctly but it is not easy to fiddle with it due to being too tight and short.
- No need to align the DD gears (at least in my case), just remove everything, tighten the screw snag without stripping them and you should be good to go.
- Ideally a whole deck (mechanism and heads) can be swapped if the DDE is in place.

[hkorpim]

I'm glad to see, dellsam34, that you got your VCR back to good working order!

I would like to see someone, who has badly misaligned dynamic drum, to test this simple rule of guide ring alignment: Turn the guide ring alignment screws visible from the side of drum using the small gears fully clockwise until stop. (To do this, you must have the DD gearbox removed.) Then unscrew both back 3/4 of turns, mount the drum into deck and test playback. The VCR shuts off, but if DD gearbox' worm gears are still OK and it runs, make an expansion lead from socket CN3501 and run DD gearbox remotely from head drum (or get yourself DD emulator...) to get the required signals for SYSTEM CONTROL PROCESSOR. I have learned by testing, that the kinematics and adjustment of gears is symmetric.
Don't do this, if the dynamic drum has good alignment and VCR has good picture!

See also my pdf-guide pasted to this thread on 20th February 2025.

I originally created the following video for myself to better understand how the gears work, then added some text and uploaded it to youtube:

https://youtu.be/Y5A8nUbm9ag

[dellsam34]

It shouldn't be any mis alignement, once everything related to the DD gearbox removed, both the spinning drum and lead ring would rest in their neutral position, As I mentioned I removed all gears, none left on the drum assembly, But if the gearbox is to be left on the head then that would matter.

It's good that you are experimenting with the DD system to see if we can repurpose it for future fine adjustements for problematic tapes. I would like to see two buttons instead of a potentiometer, one labeled TL and one TR for tilt left and tilt right and a full duty cycle is delivered to the motor but only when a button is pressed momentarily.

[hkorpim]

I little bit disagree the issue of alignment, because the screws, which actually adjust the lead ring remain inside the head drum after the gears have been removed and depending on their position there is either good alignment or misalignment. If the gears have cracked/splitted the alignment may be gone. With good gears not. Mine got completely misaligned and it took several attempts, before I discovered how to get them back to alignment. Now my VCR is in perfect alignment and all old tapes recorded with several VCRs during 40 years play well.If there is lead ring misalignment, the VCR is more sensitive for some tapes not playing well.

I believe, that your idea to adjust the lead ring with DD motor works, but before testing that, I suggest to mark the adjustment screw collars in order to easier get them back to current setting, if alignment is lost. For such test the gear screws (drum tilt) should be left off (drum stays in neutral pos.). This makes it also safer, because then, if hitting the end of travel, the gear teeth don't get smashed: the gear axle simply starts to slip inside the adjustmen screw collar.

I used potentiometer, because I wanted adjustable speed and smooth start, but also easy direction change without a switch. The problem with DD motor is, that it is so weak - it has jerky start if worm gears are damaged in any way. Running with on/off button would be easier solution, but more difficult to regulate accurately. JVC should have used stepper motor on this.

[dellsam34]

On the gears issue I don't know exactly what screws are left inside, I took everything out of my head drum, or so I believe. Anyway, yes the motor is jerkey but the gear ratio is so big that the kick start won't have an effect, Stepper motors don't need higher gear ratios, they are accurate on their own. I still think that buttons should work fine for the future mod, but the gears need to be in perfect condition, no cracks. We will find a way to produce them with the right resin, it's just a matter of time.

[hkorpim]

I am referring to this screw on the picture, which I marked
[Attachment 85891 - Click to enlarge]on my PART 3 video. When rotating it with the small gear, guide ring (=lead ring) moves up or down.

[dellsam34]

Interesting, I spun those gears endlessly but they don't seem to move anything, so I removed them and I could see a loose piece of plastic trapped in there that I could not extract, But before I put the head back on; I noticed the lead ring moves in both directions freely and seems to rest on its neutral position, then I tested it and got the video I posted above without any mis-alignment symptoms, lets assume I was lucky this time but I will find out for sure when I take the DD system apart again in the near future.

[lordsmurf]

Well folks, I got some good news and some bad news

Where did that unit come from? The internal looks horrible.

I don't know exactly what screws are left inside,

LOL!
<nods head in understanding>

Tip:
- Open it, take pictures.
- Do small amount of stuff, take more pictures.
- Repeat, over and over.

^ Lesson from the school of hard knocks.

Be thankful it's all digital now. I remember wasting Polaroids at $1 per shot.

I want one, but I'll let dellsam34 be the guinea pig

I made a good decision.

[dellsam34]

No, I did not lose screws, I was responding to hkorpim that I didn't see any screws (threaded rods) left inside.

The bad news are about swapping a non DD head, So if you want to fix the DD system you still need the DD emulator, I made the good decision.

[hkorpim]

Dellsam34, it's just the collar of the guide ring ajustment screw, that shows up in my photo from the side of drum. The axle of the small gear goes inside the collar with friction fit. If the gear has cracket, the gear cannot rotate the axle and the adjustment does not work. If the collar of the adjustment screw has cracket, the gear axle just rotates loose inside the collar and can also not rotate the adjustment screw. You have been lucky, if the screws have cracked in well aligned position and you have good picture.

[dellsam34]

I will have to investigate further, For now I don't see anything in the way of the lead ring, it moves both ways and settles in the center on 4 nipples. It has been a while since I took this apart when I started this thread, so I don't remember exactly how the said screws work.

[dellsam34]

From the DD technical guide you can see that in the neutral position all 8 pivots are in full contacts and the screws are not touching any part of the drum or the lead ring, Which where I had my drum at:

[hkorpim]

That's a good picture, that clearly illustrates the principle. I got got the best lead ring alignment in my VCR after unscrewing the lead ring screws 3/4 rev from fully in position.

[dellsam34]

Are you the original owner of the VCR? I'm thinking that someone messed with the tape transport before you and now the head has to sit at a slight angle to get a good picture, just a guess.

[hkorpim]

I purchased my VCR as new - it's only me, who has opened it. The DD worked normally until about 2 years ago, when it first started to shut down itself. The drum sits well aligned in neutral position exactly as you have described on fixed stands. It was just the lead ring, that caused misalignment to tape path until I adjusted it. Now I have it with all gears and DD gearbox removed and it works well. I only have academic interest to get the gearbox right... No real need for a VCR, because todays HDTV and 4K Netflix & Youtube are so much better on big screen. But it's nice to own some working VCRs, because I have 200+ tapes with some history of Finnish TV. Even if I could get DD gearbox back to working, I would not use it anymore, because the benefit is so marginal: 5x/7x/9x not really relevant without stripes anymore. All the other functions are close to perfect even without DD functionality, but also 5x/7x/9x very stable and only thin stripes.

[Aristarhos]

If you can pop the cover and take some pictures that would help tracking down some candidate heads to replace the DD heads now that we have a way to disable the electronics, I believe this head LP20617-006A JVC DRUM SUB ASSY with 2SP, 2LP, 2HiFi and FE heads should replace high end models with the DD system.

Here is the inside of a European JVC HR-S7600.

I have 2 fully operational units HR-S7600, 3 HR-S9500 units, of which 2 operational and one with split gears.

[hkorpim]

The mechanism on Aristarhos' pictures look identical to my HR-S9700EU.

To me it looks like JCV made heavy standardization in VCR mechanism for last production years. Regardless of model, all used the same basic mechanism with or without Dynamic Drum system.
If all models share same mechanical parts - tape quides, audio/control head, cassette mechanism, common head drum mounting interface etc., it means, that the head drum must have identical tape path in DD models and non-DD models. European HR-S7600 does not have DD, but otherwise mechanism is same as on HR-S9700EU.
See picture of European HR-S9700 EU below. (Note, that DD gearbox and all gears removed):

[Attachment 86077 - Click to enlarge]

[lordsmurf]

To me it looks like JCV made heavy standardization

Beware, looks can be deceiving. Sometimes you have to read very carefully, as the part numbers only differ by a digit or letter.

Early on, in my refurb'ing for others days, I made this mistake. I bricked at least one VCR this way. After inserting the wrong part, it was a door stop.

Be careful, diligent, and detail-oriented!

[dellsam34]

Few things to consider about swapping an entire mechanism:
1- Drum motor version, There are 5 pin and 4 pin ribbon cables, I believe mono speed servos such as NTSC have 4 pin ribbon cables, European and multi standard 2 speed servos have 5 pin ribbon cables.
2- Heads rotary transformers ribbon cables are based on head count, ranging from basic 4 head (2 SP/LP, 2 HiFi) to 7 head (2 SP, 2 LP, 2 HiFi, 1 FEH).
3- DD systems need electronic bypass.
4- The AC head seems to be standard 7 pin across all models with the same modern mechanism.

Anything else is not important as long as the mechanism has the same mounting points.

I will be attemting more swaps in the future as time permits and I will post results.

[hkorpim]

Good points, Lordsmurf & Dellsam34, electrical features can differ considerably thus better be very careful before connecting cables and if different make them match if possible. Luckily most service manuals are available from electrotanya.
PCBs and electrical solutions can differ alot, but mechnical platform with tape related dimensions seem well standardized.

[TOM11]

Hi guys,
I managed to buy a JVC HR-S9700EU with a working dynamic drum.
The question is...
Is it possible to save the original dynamic drum so that it never breaks?
For example, by lubricating the gear wheels, worms...?
Is it possible, for example, to strengthen the supporting plastic for the metal axes?

[Alex9876]

Hi guys,
I managed to buy a JVC HR-S9700EU with a working dynamic drum.
The question is...
Is it possible to save the original dynamic drum so that it never breaks?
For example, by lubricating the gear wheels, worms...?
Is it possible, for example, to strengthen the supporting plastic for the metal axes?

Hi! It's not about the grease. You need to understand that the gears break down due to the metal-nylon connection! The metal does not change its geometric dimensions. Nylon decreases in size over time. As a result, the nylon has a crack. And under the influence of mechanical forces, the crack increases rapidly. How will this phenomenon be avoided? It seems that for this you need to strengthen the gears in advance, for example, by applying a bandage made of a strong thread soaked with glue. You can see my method of restoring the mechanism DD. Perhaps one of my methods will suit you.

P.S - But in general, I would recommend getting rid of the VCR with this mechanism DD as soon as possible and purchasing the same thing but without the mechanism DD. After all, the accelerated browsing function is not so necessary nowadays.

[hkorpim]

Hello Tom11, As Alex9876 states, nylon shrinks over time and the gears crack due to internal stresses.
Otherwise HR-S9700EU is an excellent VCR - I have one. No reason to get rid of it, because we have working final solution to keep it working after DD fails permanently. It has been discussed on this forum - the solution is presented on my video: https://youtu.be/VaJ18J-bpzM .
You can continue to use you VCR as long as DD works on it, but after first symptoms, do the mod shown on video. Today there are no other solutions known to prevent VCR from shutting off, if it's system control IC cannot sense the DD sensor signals. Of course the best solution would be to replace the nylon gears with new metal gears or improved nylon gears, but nobody has yet developed them for a reasonable prise.

[dellsam34]

I got an entire mechanism coming in the mail, it was pulled from a Philips VR960B Super VHS VCR a clone of a JVC model, I'm pretty sure it has a basic dual azimut head with no FE head, but I think I made a mistake last time when I installed a similar head on my DD VCR, I believe I did not match the phase of the upper drum assembly and the rotor magnet as described in the service manual, I will give it another try.