On Lew's unit there was a very long scratch under the solder mask. In this pic you see the very corner of a crack in the solder filet (center) at the ~8 o-clock position. If you follow the trajectory you can make out the scratch.
http://fidelisanalog.com/wp-content/uploads/sites/3/2018/12/LM-TT101-Crack1.png
Once the mask was scraped away you can see that the scratch actually cut clean through the trace, and that the little corner of solder which cracked was the only piece making electrical contact.
http://fidelisanalog.com/wp-content/uploads/sites/3/2018/12/LM-TT101-Crack2.png
The boards on these units are very hygroscopic and haven't stood the test of time, nor has the solder. With the 101 I now vacuum clear every single joint and re-solder fresh. The riveted vias take a bit of skill as they trap gas and bubble so you have to get the temperatures right and have patience to get a joint that hasn't blown out, and also doesn't have excess solder.
For testing I flex the board approx 30deg in all directions while the unit is running and the drive output is on the scope to see if it causes any glitches. If it does, which it will, there are still bad joints that need sorted or an issue will develop down the road.
This is a unit where best practices need adhered to - don't mix old and new alloy, clear all conformal coating to avoid contamination, proper solder filets to prevent pad/trace cracks etc.
I can refurb 2.5 SP-10MK3 in the time it takes me to do a single TT-101 right.
http://fidelisanalog.com/wp-content/uploads/sites/3/2018/12/LM-TT101-Crack1.png
Once the mask was scraped away you can see that the scratch actually cut clean through the trace, and that the little corner of solder which cracked was the only piece making electrical contact.
http://fidelisanalog.com/wp-content/uploads/sites/3/2018/12/LM-TT101-Crack2.png
The boards on these units are very hygroscopic and haven't stood the test of time, nor has the solder. With the 101 I now vacuum clear every single joint and re-solder fresh. The riveted vias take a bit of skill as they trap gas and bubble so you have to get the temperatures right and have patience to get a joint that hasn't blown out, and also doesn't have excess solder.
For testing I flex the board approx 30deg in all directions while the unit is running and the drive output is on the scope to see if it causes any glitches. If it does, which it will, there are still bad joints that need sorted or an issue will develop down the road.
This is a unit where best practices need adhered to - don't mix old and new alloy, clear all conformal coating to avoid contamination, proper solder filets to prevent pad/trace cracks etc.
I can refurb 2.5 SP-10MK3 in the time it takes me to do a single TT-101 right.