Have a number of chimes that are out of tune...would like to have 2 octaves checked and replaced or fixed, as needed... also am missing one chime... how do we go about fixing our needs?

What model? Diodes are either burned up (but you would see the ends still present) or missing because they aren't applicable to that model. It's more likely the rubber pads under the keys need to be replaced

Unfortunately, the manual on
http://www.korg.com/us/products/tuners/ga_40/

indicates that 12 string is not supported.

The owners manual can be downloaded at
http://i.korg.com/uploads/Support/GA40_EFG1_633661472559800000.pdf

The YouTube tutorial is at
https://www.youtube.com/watch?v=Of4CRajvq0E

with a 12 string, you can often tune the octave until the tuner sees the matching note. It won't see the correct string number, but an A note is an A note, whether or not it's an octave higher.

Hope that this helps.

Unless there is damage you should only have to clean them. Do NOT compare pitch to piano since on a piano the high notes are stretched up to 22 cents sharp and at the low end they are about 20 centts flat. If any of the tubes are cracked, it is fatal. Also do not be confused by worn or deteriorated hammers that may appear to be out of tune.

Suzuki does sell individual replacements, I bought a few last year. If there are a few unusable chimes now, you can try to tune them, nothing to lose, I was able to tune 8 of ours and 2 others were too far gone and had to be replaced. They tune like a tuning fork, which is what they are. If the chime is flat, you either file the tips, in effect making the legs shorter, this must be done evenly or sustain will be affected, or, the inside of the legs near the tips can be filed, again evenly, to reduce mass. If the chime is sharp, filing is done down in the "******" area.

Forgot to mention... the clapper has almost nothing to do with the tone.

"sounds flat" ... What are you comparing this to? Pianos at that range are sharped due to stretch tuning so this would appear flat compared to the piano. Use a Korg or other brand tuner to verify. If it is flat, my educated guess is there is a crack in it. Dip it in a vegetable dye water mixture and usually cracks will show up. This is almost like Magnafluxing they use in the aircraft industry. These are likely made of aluminum and repair if cracked would be hard... one could try to have it welded and then filed down to tune it. I have done this with sleigh bells on a pipe organ, but those were brass.

Inspect for damage, especially hairline cracks. Also inspect for contamination or dirt that will change the resonant frequency. AND VERIFY the tuner you are using is accurate. Do not compare to a piano which MAY be out of tune... in fact, pianos are STRETCH tuned, that is the higher notes are SHARPED and the LOW notes are FLATTED deliberately. Comparing your chimes to a piano, the E5 would appear flat. Use a good electronic tuner to verify the pitch, not a piano,

You will have a near impossible task to find a replacement. Try at a music store that deals in student instruments. This is not high tech and the technology has not changed much so you might luck out.. DO NOT leave any part of your set with them... they tend to get lost.

If you can't get the part then your choice is to MAKE a suitable part. It is often not hard to come up with a substitute. If the metal of the chime has cracked, then have it welded and then tune it with a file...

Frankly, I would analyze what the clappers ar made of and MAKE them yourself. If they are plastic, find suitable material and make your own. If metal, you can have a metalshop make some for you.

Finding replacements either piecemeal or individual chimes is NOT likely to happen.

The chimes have jumper plugs to select the preset songs.

You CANNOT load your own like cell phones allow.