I have a Meridian 506 which has developed the same problem after six years. I plan on sending it to Meridian for repair after the holidays.
Someone else who auctioned a Meridian 508 on Audiogon had the same problem. He sent it back to Meridian before putting it up for auction and, in response to my e-mail, said it cost him less than $100, including postage, to have it repaired.
If you decide to repair it yourself, let me know how it goes.
Good luck.