When we had a 6620 (this has been many years ago), we sent in our dig. tach....(sorry, I can not recall the name of the electronics place...they quoted us one price on the phone, and then when they got it, "all kinds" of other stuff was wrong..(?))and it ended up costing within about $20 of a ~new one.
You might come out better ($) by checking w/some combine salvage yards. Abilene Machine is a good one to start with. I'm sure they will 'Google right up' for ya'. They are usually pretty good about taking stuff back, too, if it does not work for you.