I have a 4 year old George Steck digital grand model #GDS390GT-V. Been wonderful except now the middle range keys have a muted sound unlike the rest of the key board. The dealer is out of business and this brand, I believe, has been purchased by Samik. They don’t respond to my service/repair inquiries. Any suggestions on what to do next? Is this condition repairable? Are factory parts required? Thanks in advance for any help!