XL-3500/ 3510/3520 FAQs & Troubleshooting XL-3500/ 3510/3520 Not your product? Sort by Updated Date Popularity 2 Results 1. 12/12/2018 Incorrect thread tension 2. 20/11/2012 The thread tension is incorrect. Sewing (Utility Stitches) FAQs & Troubleshooting