Howdy. I've got a couple machines with auto thread trimmers that I'm currently ripping my hair out over, and have poured way too many hours into them. I've never felt completely competent working on thread trimmers...any time I've ever had to fix them in tackers, etc., I've mostly just tried to set everything up by the service manual and hope it works. This has been an OK approach, but I still don't have a great understanding of the fundamentals behind it (what needs to be timed where/when during operation).
Issue 1: Juki dln5410n-7. I've got another one of these that runs perfect, but it's an H designation. This one is not. This came to me with an inop trimmer...someone must have jammed it hard and broke the thread guide below the feed dog. I put a new guide in and sharpened the fixed knife. It does a beautiful job trimming threads, but always loses the needle thread if I don't start sewing with it underneath the foot. Adjusted the thread tail length excessively long, no change. Thread trimmer cam timing is spot-on via the manual. Hook timing is also book-perfect, and set identically to the other 5410. However, it's hard to use the working 5410 to cheat off of, since the H uses a little different trimmer configuration. My working theory is that the H has a spring to retain the bobbin thread, whereas the normal (non-H) one doesn't, and it's not picking up the bobbin thread easily? The H has no problem starting even with a fairly short thread tail.
Issue 2: Brother LT2-b842-905. Kind of a long shot here, since there aren't a ton around... I'm just using this one to sew labels and have the left needle removed. This was another one that I got with thread trimmer missing parts, and I pieced it back together. Trimmer cam timing was way off when I got this one. I fixed that, sharpened the knives, and adjusted the tension release. It can consistently make a clean cut, but always leaves an extremely short thread tail (~.5") and comes unthreaded on the next cycle. I've tried everything I can think of...moving the timing belt around a tooth or two in each direction to change trimmer cam timing, playing with thread controller tension and timing, the pre-tension discs before the main tension, etc. Nothing seems to have any noticeable effect. The manual sucks for this one, and offers a bit of contradictory information to itself on trimmer timing and adjustment. The cam on the lower shaft is also non-adjustable, so changing timing on this one involves popping the timing belt off and moving the whole shaft. It's not a great time.
Material and thread aren't anything too demanding...1000d cordura and T70 thread.
Thanks much in advance to anyone that spends the time to look over this and offer suggestions!