Introduction: How to Rebuild the Sticky Pads for a Hohner Pianet
A place in Los Angeles called Ken Rich Sound Services is selling reproduction sticky pads designed to fit Pianet C, Pianet L, Pianet M, Pianet N, and (I'm assuming) Combo Pianet.
I installed a set and they seem to work well; they stick to the reeds nicely and there are no harsh damping or attack sounds. If you have the money and you'd rather not spend many hours rebuilding the stock sticky pads, I'd recommend going with the Ken Rich version. Here's a link to the product page:
This Instructable will cover how to rebuild the original "foam rubber sandwich" sticky pads used in the Hohner Pianet C, Pianet L, Pianet M, Pianet N, and Combo Pianet. The sticky pads used in the Pianet T and Clavinet/Pianet Duo are different and will not be covered here.
Most of the original sandwich-style sticky pads have ceased to function due to the decomposition of the foam rubber layer. Further complicating matters, the exact composition of the bottom layer of the original sticky pads is still a mystery, from the type of leather used, to the silicone oil which impregnates the leather and causes the pads to stick to the reeds. Only the Hohner company knows exactly how they were made.
By doing some research and comparing the properties of the original sticky pads to other materials, I have successfully rebuilt a full set of sticky pads for a Hohner Combo Pianet, and the keyboard now plays evenly and in tune across all 5 octaves, and sounds loud, bright and clear, without problematic damping or attack noise. The effectiveness of the adhesive is high enough so that the bottom 5 reeds will come within about 1/32" to 1/16" of re-striking the pad if an aggressive attack is used.
I found that each set of 12 pads took about 1.5 to 2 hours of active time to rebuild. There are an additional 16 hours of waiting that can be done for many sets of pads in parallel. I will explain later.
(Below are some technical notes for those so inclined, but they are not needed to start making pads.)
Crucial to this project was the original UK patent filed for the keyboard action, which can be found here (much thanks to the administrator of pianet.8.forumer.com):
On page 3, line 63, a "pressure-sensitive adhesive" in the form of a "non-drying resin or oil" is mentioned, and specifically the chemical "poly-dimethyl-siloxane" is named. This chemical is also known as "dimethylpolysiloxane", "PDMS", and "dimethicone", and it's commercially available in a non-curing form as "dielectric grease". Dielectric grease is not pure PDMS, it is somewhere between 70% and 85% PDMS, with other ingredients mixed in. However, it works for this application. Here is the material safety data sheet for the product that I used:
The first few experimental pads that I made used pieces of 1/16" thick cowhide leather cut from saddle strings. These did not age well, because the grease would soak further into the pad over time, causing the concentration of grease on the surface to become too low to pluck the reed. Also, the leather would shed fibers onto the reed, making the problem worse. Comparing the leather pad to an original pad under a microscope, I found that the collagen fibers in the original pad were much finer and more densely packed than those of the cowhide pad, leading me to try using lambskin, which has a smaller, tighter fiber network and turned out to work much better.
Step 1: Materials and Tools
Here's what you'll need:
1.) 3M Super Weatherstrip Adhesive: This is glue. Strong stuff. Available from Grainger. EDIT: also spotted at OSH.
2.) Permatex Dielectric Grease, 0.33 oz. tube. Available from most auto-parts stores.
There may be a better option available for the dielectric grease.
I got an email from a nice person in Germany, who said that a repair shop in Berlin had repaired his Pianet pads using several layers of foam tape and a layer of kid leather soaked in "pure silicone oil, 100,000-200,000 centiStokes if I remember correctly". He suggested a product like this one:
Possible advantages include:
a.) The oil will probably soak into the lambskin more quickly and eliminate a lot of the waiting time.
b.) There is less chance that extra grease/oil will become deposited on the reed, causing it to go flat. This happened on about 5 of my notes after about 6 months of playing, and was easily repaired with a Q-tip.
I will run a test on the silicone oil and post when I have some results.
I tested the product above and it does work a little better, and soaks in faster. Use the silicone oil for best results.
3.) X-Acto knife, X-Acto blades.
4.) Single-sided razor blades.
6.) Electrical tape.
7.) Narrow gap rubber tape, 3/8" wide by 3/8" thick, open cell. I used the Ace brand. I can't guarantee that any old brand will work.
8.) One piece of finished lamb skin, approximately 4 by 3 inches, any color. Try to get a piece that has an even texture, without a lot of fibers sticking up. I got mine from a leather supply place.
9.) Rough-grit sand paper
10.) 3/16" diameter rod. Could be plastic or metal. At least 12" long.
12.) 1 piece of canvas cloth, at least 6" by 6"
13.) One full set (61) original Hohner Pianet sticky pads, any condition as long as the black rubber collar parts are intact.
14.) Isopropyl alcohol
16.) Paper towels
17.) Glass of water
Step 2: Prepare the 3/16" Rod
Wrap one layer of electrical tape around about 12" of the 3/16" rod in a long spiral, so that there is little-to-no overlapping. This is to make the rod about as thick as the Pianet key shafts, which are about 5mm in diameter.
Step 3: Clean Up the Black Rubber Collars.
Using a pencil, mark the original locations of the sticky pads on the tops of the aluminum key shafts.
Remove all of the old fragments of sticky pads from inside the keyboard, saving the upper, black rubber parts. Make sure the reeds don't have any rust or residue on them. There are rust removal kits available online. Clean the tops of the reeds with alcohol and Q-tips.
Cut, scrape, and sand away all of the existing foam, glue, and residue from the original black rubber collars. Be careful not to damage them. Stack 12 of the collars on the 3/16" rod as shown, leaving a gap between each about the thickness of a razor blade. The flat outer surfaces of the rubber should be sanded rough, and lined up flat with each other. Clean them with some isopropyl alcohol.
Step 4: Apply Weatherstrip Adhesive
Cut a piece of rubber tape about 4 inches long.
Place the tape with the non-sticky (cloth) side up, next to the 12 rubber collars which are lined up on the 3/16" rod.
Lay a thin bead (about 1/8" wide) of weatherstrip adhesive along both the rubber tape and the flat surface of the collars. You can get the bead of weatherstrip adhesive to break free from the tube by pausing for a second, then quickly raising the tube straight up in the air.
Using the handle of a Q-tip, spread the adhesive evenly over both surfaces. Try not to get any adhesive on the side walls of the rubber tape.
Pretty quickly the adhesive will start to get tacky. At this point press the 2 parts together, being careful to keep the long edges aligned. Squeeze them together. Some excess adhesive will probably squirt out of the sides, this is OK.
Step 5: Now for the Lambskin
Cut a piece of lambskin exactly 3/8" wide, by about 4" long.
Carefully remove the liner from the adhesive side of the rubber tape. Make sure you're not peeling off the entire piece of adhesive-coated plastic.
Stick the smooth side of the lambskin to the rubber tape, being careful once again to align the long edges perfectly. To be very clear, the fibrous suede-like side of the lambskin will be exposed on the outside.
Leave the assembly on the 3/16" rod, and place it in a well-ventilated area to dry.
It will take at least 30 minutes for the weatherstrip adhesive to harden. Relax and have a glass of water.
Step 6: Burnish the Lambskin
Slide the block of 12 pads off the rod. Place the canvas cloth on a flat surface and scrub the lambskin on the canvas. This will remove the thinner, looser fibers so that they won't shed off onto the reeds while in use. You will probably only need to rub for about 20-30 seconds.
Step 7: Slice Through the Foam
Using a razor blade, slice vertically through the dried weatherstrip adhesive and the foam rubber, being careful to keep the blade both vertical and square to the length of the block.
You will probably need to "saw" through the weatherstrip adhesive and the top part of the foam, but after you get about halfway through the foam, you should be able to press straight down on the blade (still keeping it vertical and square) and cut through the rest of the foam.
Step 8: Slice Thru the Lambskin
Using the X-Acto knife, cut through the bottom layer of lambskin. Once again, be careful to keep the blade square to the block of pads. You may decide to clean up the edges of the lambskin if they're not quite straight.
Step 9: Optional: Cut Hourglass Profile
Some commercially-available replacement sticky pads require the key shafts to be bent up in order to compensate for the thickness of the pads. If you have already installed a set of these, your key shafts will need to be re-adjusted anyway, so you may decide to skip step 9.
If you are starting from scratch with an untouched Pianet, you may find that these home-made pads are a bit too thick and stiff and require some of the key shafts, particularly on the black keys, to be bent up about 1/16" to 1/8". Alternatively, you can make the pads less stiff by following step 9.
Take each pad and place it lambskin-side down. Compress the foam rubber by pressing straight down on the black rubber collar until the pad bottoms out. Using a sharp X-Acto blade, cut away the foam rubber that bulges out of the 2 longer sides of the pad. The foam rubber will now have an hourglass profile as shown. You may also decide to clean up your trimming work with some small scissors.
Step 10: Apply the Grease
EDIT 2/13/2012: Use silicone oil instead of dielectric grease if possible. See step 1. I didn't have time to take new photos.
Take each pad and apply a healthy dose of dielectric grease to the lambskin, using a rubbing and swirling motion while squeezing the tube. Try not to get any grease on the foam rubber. Use enough grease so that it builds up to about the thickness of the lambskin.
Step 11: Let the Grease Soak In
Put the greased-up pads on a paper towel and let them soak for at least 8 hours.
After 8 hours, you will want to "touch up" the grease job anywhere that the lambskin surface is starting to become visible.
Let the pads soak for at least another 8 hours after touching them up. It's very important that the lambskin is completely saturated in the dielectric grease.
***EDIT***: after 3 weeks I had to reapply grease to about 15 of the pads in the top range of notes, all pads from the same batch, because they were no longer saturated at the surface. You may want to soak the pads even longer, several days maybe?
Step 12: Wipe Off the Excess Grease
After the pads are done soaking, remove the excess grease by wiping them with a paper towel. You will want to wipe down the surface until it just barely starts to lose its gloss. I took a photo with no flash, holding the pad so that the lamp was reflecting on it, in order to show what it should look like.
Step 13: Install the Pads
Slide the new pads onto the key shafts, lining them up with the marked locations of the old pads. If you don't have a record of where the original pads were placed, this photo shows them pretty clearly if you zoom in:
If you are starting with straight key shafts, check the black keys in the middle to upper octaves to make sure the key shafts are less than about 1/32" from bottoming out on the felt stop (blue in this photo) when the key is unpressed (*). You may decide to adjust the key shafts or swap in some of the more-hourglassed, less-stiff pads (if you followed step 9).
If you are starting with bent-up key shafts, you will need to go one key at a time, remove the key, bend the key shaft back toward straight but not all the way, and then install it and check to see if the pad is making the key shaft float over the felt stop. You may want to go one octave at a time since the key springs are tough to line up on the white keys without removing the adjacent black key.
This is also a good time to grab a can of compressed air and blast out all the dust, metal filings, cigarette ashes, and whatnot from the inside of the keyboard.
After a few days, you may find that a few of the pads are losing their stick. You can add a thin layer of dielectric grease to them and they should be fine again. I couldn't figure out why this happened, my only guess is that the soaking process was less effective for some of the pads.
(*) - Note that the return springs on the black keys are only about 2/3 as strong as the springs on the white keys, in order to compensate for the longer lever arm of the white keys. This means that the black keys will have more of a tendency for the pad to be too thick or too stiff.
Step 14: Now You Should Be Up and Running
Here's a demo video showing the Combo Pianet in action.
The key action is still a little loose from side-to-side, and I haven't fixed it yet. The clicking sound you hear is the acoustic sound of the white keys striking each other.
I will update this page if and when I discover anything about how the pads react to aging/temperature/humidity etc, but since they are made of mostly car parts I'm hoping they will be pretty stable and long-lasting.
Thanks for coming!