No, it's not դաշն/ա/կահար , it's դաշնակ/ա/հար (դաշնակ means piano). I know դաշնամուր exists too, but I don't think I hardly ever heard it. I don't know the etymology of this suffix, but I think you can find somethingThanks; however, I've tried Wiktionary but it did not help.
Sure, the first bit must be short for դաշնամուր 'pianoforte' almost in the same way that 'piano' is short for 'pianoforte'.
But what about the rest of the word, the [ա]կահար?