How do you fix a broken key?


  I was pretty upset the first time I discovered one of the keys on my Kurzweil RG100SE electronic piano lying there, dead. I was happy when I discovered that it wasn't hard to fix. But I was upset when it happened again a few years later and I had forgotten how to get inside the case to fix it.

  If you've ever done these kinds of repairs, you know that opening the case without damaging it is often the hardest part. If you look on the back of the piano there's a lot of screws. If you look on the bottom, there's many, many more. If you just start randomly removing screws, you'll waste a lot of time, and you'll always have some extras after you've put it back together again. So here's how to get inside one of these pianos- and also some info on how the keys work.

open!
  OK, this is where you want to be. They actually thought about this, and the top is made to flip up. They even have a cord on the near end that holds the top in place for you. There are a couple of tabs on the rear of the top that support it in its open position. Please note the three very cheesy looking brackets tacked on to the top- they are the key to opening this thing up.

bottom
  This is a look at the bottom of the piano, before it was opened. The red circles indicate the three screws that go into the three cheesy-looking brackets in the top photo. Please note that it's standing on its "nose" in this picture- front down- and also know that the red circles are not actually painted on your Kurzweil. Finding these screws is up to you, I'm just here to let you know that they exist, OK? (there's exactly a billion screws on the bottom- go ahead, count 'em!)

rear1
  Not only do you have to remove those three bottom screws, you have to remove a row of screws from the back of the unit. Here's three removed from the upper-key end of the piano. Oh, I almost forgot- there's a screw through the rear of the top into the end piece. That has to come out. There's another screw through the bottom of the piano into the end member, which I also removed. I don't think I had to take that one out. Can you see them on the left, one above the other?

rear2
  Here's a view of the low-key side of the rear, showing three more screws not being there. Bad photo- there's a screw off to the right into the end member that has to come out, and I believe there's another in the middle of the back, for a total of 7 in the row. It'll be obvious once you see it for real.

  With all those screws out, (and the bottom three, of course), it should be possible to raise the front of the top- the edge that comes down over the backs of the keys- and tilt the top up into the position shown in the first picture. I didn't do this, exactly, because I took one end off completely when I was trying to figure out how it opened, before I discovered (well, re-discovered, actually) the three cheesy looking brackets deal. That middle bracket will drive you nuts if you don't know it's there...

rear2
  Here at last is the reason why a key will drop dead. There are special springs that fit into notches in the back ends of the keys. The springs pull down, which pulls the front of the key up. The springs are sturdy enough that I doubt they'll ever break- but they do sometimes (how?) slip out of the notches. (I blame the cat)

rear2
  And here, finally, is that special little spring that makes it all worth while. If you have a dead key, and you succeed in getting the thing open, you'll be looking around inside for one of these. They could fly anywhere, but I found this one up front, hiding under the keys. I would have needed to take an end off anyway to get to it, so that all worked out for the best. Your mileage may vary.

Putting the spring back on is a snap if you use a hemostat or a pair of needle-nose pliers. It's much better with the hemostat. The spring won't be as likely to spring loose while you're fiddling with it if it's clamped in a hemostat. Where in the world did I get these things?

The Plot Thickens- 08/09/2008


  Another key failure, which I lived with for quite a while, but last night I decided to tear into the thing again and fix it. Oh, poor pityful me!

  This time I discovered that that the plastic that "anchors" the spring has cracked. It may have been cracked for some time, because this is the same key that failed before. This time it was very obvious that something was wrong with the part- it was twisted all out of whack by the force of the other springs nearby pulling on it.

broken!

  The circle is where the main crack is. That's where the spring is missing. The four springs to the left of the missing spring are twisting the black plastic bracket out of position. Compare them to the normal spring position on the right. Oh- everything is up-side-down in this photo. I removed the keyboard assembly from the case to get to this point. The upper side of the keys are flat on the workbench.

broken!

  Here's a view from the left side. The arrow points to the main crack, which occurred between the two "fingers" that held the missing spring in place. Sadly, that crack has extended this way along the part, allowing the other springs to twist it out of shape.

the evil crack

  Here it is- the nasty crack as seen from the top (really, bottom) side, after the springs and keys have been removed in that area. In this view you can clearly see the fingers which form slots that hold the springs. The crack starts on the left, between two fingers near the standoff and runs to the right through several "braces".

This kind of plastic is tough to repair- it's some kind of "filled nylon" I believe. Anyone have a suggestion?




The Brace- 08/11/2008


  I tested some epoxy on the plastic. I could get a decent bond in shear, so rather than attempt to glue the old piece back together I decided to attach a brace to take the load. I pondered the break until I had a good idea of the area I had to work with- then went looking for something to make a brace.


weatherstripping
  This aluminum extrusion has come in handy on many repairs. It's an off-cut from some weatherstripping.


the
  Here's a view of the overall area. I need to clamp the break in the right position until the epoxy sets up. I can apply pressure from the right direction by spanning the part, but I don't want to cause more damage, so I have to brace my clamp to someplace solid. A domino helps me get under those bits that stick out.


dry fit
  The brace is filed into shape, and now it's test fitted in place.


roughed up
  I suspect that the glue bond will be better if I take some of the shine off the plastic. Here I've roughed up the surface a bit with sandpaper.


spoo
  I've coated the area with epoxy- it's just the ordinary two-part stuff that you mix up as you need it.


ooze
  Here I've worked my brace into the bed of epoxy until it has oozed out all around. Some air bubbles came out and then my brace snuggled in nice and tight.


stay!
  I clamped the break into the right position before I applied the epoxy. There is very little pressure being applied, actually. The idea isn't to glue the crack together, it's to attach a solid brace. In my experience, when you attempt to fill a crack like this with glue you get a poor bond. Time will tell if this brace idea works!


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No sound! 01/16/10


  This keyboard has sure kept me busy! In answer to some of the comments here, no, it isn't really a weighted touch. It's better than most synths, in as much as the keys have little weights glued into them (so maybe it really is a "weighted" touch?). They don't feel quite as insubstantial as a regular synth, but it doesn't replicate the feel of real piano action. I've played some keyboards lately that really do well at that- but then, I didn't like the sound of them as much as this thing.

  Not that it's perfect, by any means. I've begun to notice some distortion, some weird overtones, in the sound, but they don't detract from (and maybe they actually add to) the realism of it. Let me put it this way- I still get a lot of pleasure out of just sitting down and playing this keyboard after 20 years. I don't know of any other electronic keyboard I could say that about. Which is why I keep patching this thing up, rather than buy a new one.   So. The latest problem was a long time coming. Years ago, before the last repair, my B below middle C key got weak. It would still sound, only it was not as loud as every other key. When I opened it up to fix that nasty crack I had a look at the mechanism of that key, but couldn't see anything wrong with it, so I left it be. Sometimes I do more damage just messing around with stuff than it's worth. Well, recently a strange thing happened. I completely lost the Eb above middle C key, while at the same time that B key fixed itself!

  This had to be related, but how? The keys operate like this-

key action

  This is a side view of a key. Over there to the right, there are two "triggers" that stick down. These press down on two rubber "domes" when you press down the key. The inside of the domes have a conductive material in the top. This gets pressed onto conductive areas on a circuitboard and completes an electronic circuit, which the Kurzweil then registers as a key press. It's basically a switch, and is like the switches in a cell phone or TV changer. With one difference...

  There are two switches per key. This is how the Kurzweil determines how hard you strike the key. See, it doesn't literally measure the force you apply. What matters is how fast you hit it. The lightning-quick computer in the keyboard actually can tell the length of time between when the first switch makes contact and when the second one touches. It then calculates the speed of (and therefore, force on) the key and adjusts the volume of the note accordingly. Clever.

  Now, when the B key got weak, I sort of thought that one of these domes had cracked and wasn't making contact at the right time- therefore leading the keyboard "brain" to think that a heavy hit was a light hit. But the domes were both in good shape, and I didn't want to dis-assemble that part of the keyboard at the time. When the Eb key stopped working completely, I knew I had to get in there. A key that's a little quiet is "character". A key that doesn't work is "broken".


contact board
  This is a view of the underside of the key assembly. (I leave it to you to review the previous fixes to determine how to get to this point) You see the brown circuit boards? On the left the screws holding one on are still in place- on the right, the screws have been removed. Also, there are those red connectors here and there that need to be carefully disconnected. (You may notice that I've also removed some keys in this area- that wasn't really necessary).


under keys
  Here I've removed all the screws from one section of contact circuit board and flipped it over. On the lower half of the picture you can see the triggers which stick out under the keys. On the top half you can see the rubber domes that those triggers press onto. (my drawing wasn't exactly right, but the idea is the same). The domes in that area look fine from this side- now we gotta go in there!


contacts
  Now, this a view of the contact board with the rubber domes removed. You can see there on the right a bit of the rubber piece- it's actually about an octave or more worth of switches in one chunk. Little nubby bits poke through holes in the circuit board to hold it on. Be careful when removing it- I pushed these through the holes from the far side, rather than pulling them through, to keep them from tearing off.

  You see a black dot inside the domes of the right? That's the conductive stuff that gets pressed down onto the black oval-shapes on the circuit board. It actually contacts two ovals, closing a circuit. If these get dirty, there can be problems... wait a minute! What's that there?


speck
  Oddly enough, right next to the contacts of the non-functional Eb key, there was this little tiny fleck of something (circled in red). If I didn't know better, I'd say it was a piece of peanut skin, but of course, I would never shell peanuts near my Kurzweil! Well, maybe... The yellow boxes indicate the areas where the conductive spots on the rubber domes contact the circuit board. It isn't hard to imagine that if that little bit of dirt was in the wrong spot it would prevent that dome from making contact- and the key wouldn't work.

  How might it get in there? Well, the domes have holes in the sides of them to let air in and out when they are pressed down. This "pumping" action might have been the reason for the simultaneous "self-healing" of one key and failure of another- maybe that same little fleck of dirt got blown out of the B key and sucked up into the Eb key! Anyway, I carefully brushed all these contacts with a camel hair artists brush, then vacuumed out the inside of the keyboard after I had reassembled the rubber dome piece and contact board. Fixed! It's probably a good idea not to let your Kurzweil get too dirty.


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