MATRIXSYNTH: aftertouch repair

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Wednesday, May 06, 2009

aftertouch repair

The topic of aftertouch repair came up on the Analog Heaven email list. The following are a few links that came in. If you know of more feel free to comment or send me email directly.

Roland JX-8P / Juno-2 aftertouch repair
Roland JX-8P aftertouch repair
AKAI MX100 aftertouch repair

These came from Robert on the list with the following:
"What's the keyboard pressure circuit like?
On my JX-8P, I think it has a resistive pressure sensor, an op amp, and a few resistors to set the gain.

What's the most likely thing to fail?
I'd say the pressure sensor itself. On the JX-8P, you change out one resistor, to give the op amp circuit more gain. That's all you had to do to make the aftertouch usable again.

What if you don't have schematics?
Open the synth up, unscrew the keyboard bed. Take it apart enough to find the pressure sensor. It's most likely a long tape like thing going the length of the key bed. Each key will have a part that extends down to press on the pressure sensor. I can't remember if the JX sensor has two wires or three coming off of it. I think it's two. It was separate from the key switch wiring, different connector.

Use a volt/ohm meter to trace where those wires go. Most likely, they will lead you to some resistors and an op amp. Look at some op amp circuits, inverting and non-inverting. It's most likely an inverting op amp circuit. The two resistors set the gain. You need more gain. Once I found the right op amp, I experimented with the resistors. Seems like I cut one end of the resistor and soldered in a second one in series with it.

I have the schematics somewhere. I used to be a Roland warranty service tech (cough, cough, 25 years ago).

Wierd, new stuff on the net, a new way to fix it. It is the pressure sensor.

Here's another take on the pressure problem: the brass strip in the sensor has tarnished.
Your DSS-1 sensor may be like the Roland sensor.

Maybe I'm lazy. I just tweaked the resistor and it worked on stage for the next 10 years. I haven't played it for a while. I should. I also have a MKS-70. The JX-8P seems to have more punch, but the key contacts are flakey.

Here's another fix site:
Cool, here's an Akai one:

Here's a schematic for a Korg DW-8000. Might help...

Anyway, looks like there is more than one way to fix the problem. Heh, learn new stuff everyday. :)"

via Dave on the AH list.

"Roland JX-8P / Juno-2 aftertouch repair

If you own an early Roland synth that has aftertouch you have probably experienced that the aftertouch function is not working as is should. You have to press very hard, and the response is not even over the whole keyboard length.

This is caused by oxidation of the aftertouch sensor. There were reports of this problem already when the JX-8P hit the streets so most probably the sensor was already oxidized at delivery...

However, by doing a fairly easy, but time consuming job, the aftertouch can be restored to better than new condition!

This diagram shows a cut through the sensor. When pressure is applied the two brass conductors will be electrically connected through the conductive rubber and the metal foil.
Problem is that the brass oxidize and the surface stops conducting. There is also a build-up of oxide on the conductive rubber strips.

PS: You might have heard that it is possible to get the aftertouch working by a modification to the sensor electronics. This only applies to the very first JX's and it is mostly unlikely that this is necessary on your instrument. Only way to get the aftertouch fully working is by following the procedure below.

How to repair:
* Remove keyboard assembly from synth. Placing them in correct order on the table makes assembly easier...
* Remove all key's: First remove the transparent plastic strip that that prevents the key's from being pulled towards the front. This strip is not needed. Then loosen all springs. The keys can then be removed by pulling towards the front and then lifted up.
* Carefully remove the upper rubber housing with felt-strip. (This is secured with glue at both ends.)
* Remove the metal foil and all bits of conductive rubber. Be very careful not to bend the foil.
* The two brass conductors surface must now be cleaned! Usually it's enough to clean it with q-tips wetted with metal polish. If severe oxidation has developed then use water-proof sanding paper and water to clean it. If you work carefully it's possible to re-use the double sided tape.
* Clean the brass-strips with q-tips wetted with acetone until no dis-coloration of the q-tips is visible.
* Clean both sides of the conductive rubber strips with q-tips wetted with alcohol until no dis-coloration of the q-tips is visible.
* Clean the metal foil with q-tips wetted with acetone until no dis-coloration of the q-tips is visible. (Careful not to bend it.)
* Then assemble in reverse order. Sometimes it is necessary to replace the double sided tapes. Also, secure the top housing with a very small amount of super glue at both ends.

After completing this procedure your JX-8P aftertouch should be working better than new!

Additionally, there are gain-setting resistors there, which can be changed to..." It cuts off at this point."

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