This is the answer I got from this man that has helped in the past with Tach questions when I asked him about bench testing..He has a had some tachs on Ebay. Yes I do, I would be glad to test your tach for you, no cost to test, as it only takes a few minutes on the bench, just return shipping, please include a return Shipping Label & package your Tach(s) well. After I test it, if it's working correctly, then I'll return it, if not, then I'll determine what it needs. I am not set up to work on the "Blue line" Sun Super Tachs, or the ST 602, 635 series as these are a Factory sealed unit. Mick McCann 217 Rundall Pl. Binghamton. NY 13905 Mick tachtinkerer@xxxxxxx No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.503 / Virus Database: 269.17.4/1187 - Release Date: 12/16/2007 11:36 AM