Hohner Melodica Professional restoration
November 3, 2018 at 8:57 pm #10401
A while back Alan was kind enough to sell me a HMProf for parts (it’s the one described here). I decided I would try to restore it, and I finally found some time.
I went through and replaced all the gaskets, checked for leaks, etc, but it still had a huge leak (as Alan describes in the other thread). Turns out it was the reed bed: the top wooden plate delaminated from the base.
The surprising thing to me is that the top wood plate is only 1mm thick! (measured with calipers) Perhaps that is for acoustic reasons; otherwise it seems crazy how thin it is. Also, it is not glued to the reed bed at all, it’s just sitting on there, and held down on one side with the screws that hold the “guide rail” in place (i.e. the small arms that keep the keys straight). It is apparently “glued” to the reed bed only by the black glue/paint/coating that is sprayed inside the reed chambers. You can see this in the pictures: the wood is totally clean/dry/glue-free where it contacts the reed bed.
These melodicas have a reputation of needing lots of wind to play; I’m now wondering if part of the reason is that they get cracks in this 1mm wood veneer. (It also has lots of fine cracks in it, which you can see in the pictures.) The rest of the melodica was well sealed; the valve pads seem good, the reeds are in excellent condition. Anyway, time will tell, but I’m tempted to take apart my other HMProf now to see if there are any cracks to be sealed.
A couple questions:
1 – I thought I might replace the wood veneer with a new sheet of wood; I thought I might use cedar for this, for rot resistance. Does the black coating from the factory serve as a moisture barrier? Can I use raw unprotected wood or would that be a mistake? Then I would spread glue over all of the reed bed (for tight air sealing) and clamp the veneer to it to cure. I’m just not sure how careful I need to be about moisture in the wood of the reed chambers, and obviously the less moisture that gets in to the wood veneer, the better.
2 – I’m tempted to slightly widen the holes in all the keys to make re-insertion of the wire rod easier (I am dreading it). It doesn’t seem like there would be any down side to this: I believe the keys are held against the wire rod with leverage from the spring, so there wouldn’t be any clicking. Am I missing anything here?
Here are some pictures to add to the collective internet knowledge of these melodicas:
I made a metal piece to cover the overall reed chamber for leak testing; worked ok:
All the keys removed:
You can see the delamination of the veneer here:
I haven’t seen the ends of the central section pictured, so here are a couple pictures:
And in case anyone is interested, here is the first airway that passes down the body of the instrument with the cover removed; I shouldn’t have done this because it was perfectly sealed, but I was desperately trying to find leaks:
Other general notes, in case they are helpful to others:
– I read or heard somewhere that the HMProf reeds were brass, despite their color, but they are steel (or at least something ferromagnetic: they are strongly attracted by magnets. The reed plate seems like aluminum or something similarly non-magnetic.
– the spit valve thread fits 3/8″-16 fittings (same spit valve as the HMPianos); I used a 3/8-16 brass cap and it works great
– the bolts along the center line of the back seem to be 2mm x 8mm (2mm x 12mm might fit too). I had to drill out and thread tap one of these holes because it had been fouled; I went to 6-32 and it worked fine. It might even be a good idea to just do this for all the holes, since the tiny bolts are difficult to use.
– #0-3/8 and #1-3/8 wood screws are about right for replacing any corroded screwsNovember 6, 2018 at 9:37 pm #10424
Can you say a little more about drilling out and thread tapping the holes, Clep? Doing this with all of them seems like a good idea.November 6, 2018 at 10:00 pm #10426
Sure — speaking just of the metal “spine” down the middle of the melodica: it uses those above-described tiny 2mm bolts. I used a 6-32 tap and a #32 drill bit (online info says to use a #36, but I used a #32 per my local hardware store; it’s possible that I made a mistake and the #32 is for a different kind of metal, or maybe I’m right and the online info is incomplete, but it has worked fine so far). I just drilled it out and tapped it (take care to prevent metal shards from getting in the reeds). The hole in the outer metal casing needed to be widened slightly to fit thw new bolt. The bolt is of course larger than the remaining 2mm bolts, so there’s a slight aesthetic issue, but at least it’s brass. Getting the correct length of bolt is important, since it can bump against the valves when the keys are pressed.
There is be plenty of room in the “spine” for this widening, so that wasn’t an issue. It does seem like this might be a good thing to do in general, just to prevent any future issues, but the remaining bolts and bolt holes in this melodica seem fine so I didn’t bother. The screws, on the other hand… 🙂November 6, 2018 at 11:14 pm #10427
Just got a piece of 1mm cedar veneer from a brilliant guitar luthier friend; he’s a luthier, so of course it’s a gorgeous piece of wood, precisely milled within 1 or 2 thousandths of an inch. 🙂
It seems a little foolish to leave it as raw wood, given the humid air that will be flowing through the reed chambers, so my thought was to paint it with vapor barrier primer. I can’t imagine this will have much (if any) impact on the sound of the melodica, but if anyone disagrees, let me know!November 7, 2018 at 5:45 am #10428
Hello Clep, I would definitely seal the material prior to glueing it back together, especially on the side of the reed. Moisture will build up and if not sealed completely, moisture will be absorbed in the veneer and cause a mold build up. I normally seal with a urethane or marine varnish.
Melodica-MeNovember 7, 2018 at 6:00 am #10431
Thank you MM — I did paint it with an exterior “sealant” primer; hopefully that will be enough! I wonder if a piece of plastic would be smarter and easier here instead of wood that gets covered in plastic anyway… 🙂
Today I cleaned off a lot of glue from various places, basically getting the pieces ready for reassembly.
The veneer before priming:
Yet more hole drilling and refilling:
Prepping the end blocks:
I also very lightly drilled out the holes in the keys for the pivot rod so reinsertion will hopefully be easier (used a drill bit the same diameter as the rod: close to 5/64″). For the black plastic tines of the frame I just reinserted and removed the pivot rod many times until it loosened up.
Things are pretty much cleaned up and ready for reassembly, which will hopefully happen tomorrow once all the glue dries. I’ll probably use a little wax along the “bypass airway” channel, since otherwise it’s just metal-on-metal for a seal.
Assuming I get it all back together and sealed up, then the real work of tuning and gapping the reeds can start… that might have to wait a little. 🙂November 7, 2018 at 6:23 am #10432
If you covered it completely you should be ok. I usually use a marine varnish but there is no reason primer would not work. You can also use a 1/64″-3/32″ neoprene rubber and use rubber contact cement. This much easier to use. You should seal both side as the side you do not seal will tend to cup with time.
Melodica-MeNovember 7, 2018 at 6:24 am #10433
Thanks — I did seal both sides (and the edges). I also figured that since it will be glued to the reed frame there won’t be much chance of cupping or splitting.November 7, 2018 at 5:38 pm #10434
Great project, Clep.November 7, 2018 at 7:01 pm #10435
Thanks, Alan! Made possible by you. 🙂
@MM — contact cement is a really interesting idea. I know you were talking about using neoprene rubber and I’m using wood, but it’s still interesting to think about… I was planning to glue the primed wood to the reed frame with wood glue, but now I’m wondering which would be smarter… it seems like contact cement would be a bit more conformal and maybe have a more reliable air seal, and if there was ever a need to remove the cover veneer in the future, it might be possible with contact cement, but with the wood glue it’s going to be more or less permanently fused to the wood of the reed frame.
I’m still leaning towards wood glue, but… interesting…November 7, 2018 at 7:56 pm #10436
Clep, I am not sure if wood glue would be a good idea as it would be permanent. The original application is with a paper backer (Like a gasket) with adhesive. This is what separated from the wood base. As you can see the material cupped (warped) with time and lifted from the edge. also if you get a little bit of the glue falls onto the reed after you clamp it you will more than likely need to replace the reed. An even layer of contact cement on both the veneer and the base will make a better seal. let both dry about 20 minutes before you stick them together and no glue will fall onto the reed.
Melodica-MeNovember 7, 2018 at 8:08 pm #10437
Ok, thanks — I’ll use contact cement. I was planning to glue it upside down to prevent any drips of glue, but yeah, not an issue with the cement, so that’s nice. I’ll have to be careful about alignment as I know you only get one chance to put the two pieces together!November 8, 2018 at 6:19 am #10440
Quick update — got the veneer glued in place:
I spent a long time tracing the pattern of reed chambers and creating a stencil and transferring that pattern to the underside of the veneer so I could limit the glue to the correct spots, and then realized it was a waste of time because the glue was hard to precisely control, so I just spread glue over the entire surface.
Tomorrow I hope to reassemble and seal it.
Does anyone have any tips on replacing the tiny white plastic spacers that stand on the end of the key guides that break off? Talking about these things (blurry pic):
…when I work on the Piano 36’s, I just use daubs of hot glue that I shape appropriately. Works well, but they can have a little bit of friction with the keys, and sometimes they fall off, etc.
I was thinking of winding fine copper wire around the posts until the right size, and then touching them with a bit of solder to hold them together.
Epoxy seems like it would be too difficult to control (i.e. it would run down the post pretty easily.)
Any other brainstorms?
Thanks!November 9, 2018 at 12:03 am #10442
Tomorrow I hope to reassemble and seal it.
…arrgh, but there are always more holes to drill out and re-fill and re-drill… 🙂November 9, 2018 at 12:48 am #10443
You’ll be in our thoughts and prayers, Clep.
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