JD-800 Aftertouch Modification

Use the full range of aftertouch without straining your fingers.

DISCLAIMER:

This is a Mod to the JD-800 circuit. Do it at your own risk.

Photo: Gwydion Elderwyn

All JD-800 users knows we need to put a lot of pressure to use the aftertouch.

As someone sayd: one needs to stand on the keyboard to get an effect.

I took here the work of two great guys from the JD-800 Tech group. As I know, they had many messages about this work and here is the result with comments from both.

Preben Friis, synth lover an expert in electronics who explain how to do a simple mod to make the aftertouch become better, really light to play.

Gwydion Elderwyn, a guy who have great merits and a lovely work and story.

From now, I merge material from both and something more, including one or two words of mine.

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The Idea:

Preben:

In basic, just change how the circuit "feel" the input value.

A 270 kOhm resistor across R27 will make it possible to use full range of aftertouch without straining your fingers.

R27 position.
Photo: Gwydion Elderwyn

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Please read all the comments before doing anything you will repent! (LOL)

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Researching:

I poked around with a multimeter on the aftertouch circuit today. The pressure
ribbon starts out at 150 kOhm and by full pressure it is around 1 kOhm...

Shunting the ribbon will make it possible to move the threshold to make it start
earlier but will not make it more sensitive.

Preben

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CAUTION:

This is a modification you can do inside the JD if you are comfortable with a soldering iron that makes the aftertouch much more responsive.

This mod might require to unsolder a SMD resistor ... not something for people with ten thumbs.

For sure you can ask an electronics technician to perform this modification.

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About opening the JD-800:

Removing all screws may do your internal components fall.

You must remove only the screws AROUND the bottom and two screws in the center line, they are at 1/4 from side, and may be you can see a few bigger hole where thy fit to.

Sometimes the bottom have some "dust" acting like glue to.

Photo: Gwydion Elderwyn

How to remove the panel.
JD-800 Manual. (click to enlarge)
Source: Roland Co.

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The Solution:

Original post: groups.yahoo.com/group/JD-800_Tech

Preben:

Thanks to Gwydion Elderwyn I received exactly what I needed to calculate and test a better modification of the aftertouch circuit in the JD-800.

The short version: 

Change resistor R27 to 120 kOhm instead of 220 kOhm. Done.

The long version:

Since SMD components are very difficult to remove, and I did not have any SMD components at hand, I chose to solder a 270 kOhm resistor parallel with R27. 

R27 is located on the mainboard in the left corner right by the ribbon cable to the keyboard.

The picture shows my "huge" resistor piggyback on R27. It is not very pretty, but it works! 

Image by: Preben Friis

 

Actually the picture shows a 100k resistor, which was the first one I tried, but that made the aftertouch sensitivity too light. I could easily press the aftertouch from 0 to 127 with my pinkie finger. The 270k resistor is in my opinion the best value. If I use my pinkie with this value it allmost hurts to press it to max. Feel free to experiment - Reasonable values are in the range from 100k to about 1M

Find a soldering iron with a very thin tip and add a little solder to the two pads on R27 first. Then cut and bend the legs of the new resistor to fit across it and add a little solder blob to the legs of it. Hold the new resistor in position with a set of pliers and touch the iron to the legs of the resistor. This should melt both solder blobs to one and the connection is made. Repeat for the other side.

The smaller the better: 

A 1/8W resistor would be preferred over a 1/4W. 

Smaller resistors have thinner legs and that makes them easier to solder to the SMD resistor.
(Acutally I'm currently using a 1/2W resistor and it is BIG!).


The tolerance does not matter much. You can use 220k or 180k if you really want to use the full aftertouch range when playing monofonic lines.
 

An alternate idea:

Marc (daemons_ca) also suggest using a trimmer instead of a fixed resistor:

What I'd like to do instead on this mod is replace the resistor by a lower value, cut the trace next to it and connect a 500Kohm trimmer pot with modding wire (26-28Awg) and have it adjustable. 

I imagine not every aftertouch ribbon has exactly the same value, so this would make it tweekable. Nice thing is if the value of the ribbon changes in time, all you have to do it open it up and adjust it a little. The fixed resistor is there as safeguard so the value doesnt go below a safe value and draws too much current.

Also, I would not connect a pot in parallel with the current value. The response of the pot would not be linear at all, and hard to adjust. 

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Another mod using the same idea:

See that was placed a wire connecting the potentiometer (at left) to position R27 in the circuit board.

Foto: Yuri Alfons Peeters Charlotte

Remember to power off before you solder anything... 

If you connect the resistor properly without shorting anything there is no risk involved in this modification. The current in the pressure ribbon will be double from around 40 uA to 80 uA, but those currents are still very far from anything that can destroy any components.

I can tell you, that a well adjusted aftertouch is almost like getting a new synth. Many of the factory sounds is way more fun to play around with, when you can change the sound by a light pressure. 
For example, try for instance patch I-22, Classic Sweeper .. 

Thanks again Gwydion Elderwyn, I couldn't have done this without your help :-)

Have fun.

Preben Friis

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The Circuit and the mods


Draw and commented by Gwydion Elderwyn.

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As we say here in Georgia, WEEEEEE HAAAAAA!

Thanks Preben!  This mod was easy and it works great!  I used the 270k and find the aftertouch to now be similar to my K2000.

Most excellent.  Kinda makes you wonder why Roland didn’t get it right in the first place.

Jim Combs
www.touchxtone.com

 

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