A supervisor in Garmin's product support group has been posting on BT. Below is a compilation of his posts regarding the nav board issue from the SB. Anyone with unanswered questions probably should contact Trek directly.
Hello Everyone,
this was an unfortunate problem that was found with some NAV boards that were built by our vendor. There is a serial number break for the defective boards, the problem is we know what the date codes are on the defective boards and when the boards arrived. However there was also another date code of board during this time frame as well so it's impossible to identify for sure which units have the defective board. So what we have to do is bring all the units that fall into this serial number range, take apart the unit, if it has the known bad date code we're replacing the NAV board and re-aligning. If does not have the bad date code the unit is returned to service, mod 1 is marked and the unit returned. Unfortunately we're limited on how many units we can do each day as they do have to go back through our production facility. I apologize for the inconvenience this may cause, however the problem with these boards is not consistent as it had to do with the way the board itself with the multilayer's was assembled. let me know if there is any questions.
Here's a list of the units that possibly are affected.
GTN 650 (Black face) S/N's 1Z8001498 through 1Z8001801
GTN 750 (Black Face) S/N's 1ZA001270 through 1ZA001876, 1ZA000644, 1ZA000709, and 1ZA001230
GTN 750 (Gray Face) S/N's 1ZA200303 through 1ZA200319
here are no consistent problems that will be seen. In some cases you'll have intermittent problems in some it will be more prevalent with the NAV not working. The problem is with the board manufacturing process and not any components on it. The problems are very rare and even with the bad date code board you may never see any issues as the problems have been very rare. However it was a decision to remove them.
Best Regards,
_________________
Trek A. Lawler
Supervisor, Garmin Aviation Field Service Engineering
Aviation Product Support