You can run the old or new trigger in the TRG. I have both. The older trigger is better for target use as I find it more precise. The newer trigger is probably more robust because they removed a lot of adjustments that people got into trouble with.
Your trigger may in fact be fine if you have not forced it a lot. The way to mess up the old trigger is to make them too light and to not adjust the first and second stage together. As you've seen it's best to leave these triggers alone unless you have some very specific reason to adjust them. Just assume Sako knows what they are doing and don't touch anything.
As it were, there was a post here a while ago but it's broken now. It was written by someone that I suspect worked at Sako and he gave very detailed instructions on how to properly adjust the older Sako TRG trigger. If you follow the instructions exactly you can likely get it working again assuming the internals were not damaged by forcing anything:
Old link (broken):
https://www.snipershide.com/shooting/showthread.php?t=16234&p=237465&viewfull=1#post237465
Original text I saved:
Sako TRG Proper adjustment procedures
1. Check that the weapon is free from cartridges and
remove the bolt
2. Remove trigger unit as described in the user manual and clean it thoroughly.
3. Pull the safety on and adjust first stage pull weight against the safety to approx. 0.5-0.6 kg (small hex screw behind the trigger). Use loktite (222, 243 etc.)to prevent accidental loosening of the screw. When you have the safety pulled on you can feel the actual first stage movement of the trigger before it comes in contact with the connector inside the trigger unit.
4. Assemble the trigger unit to the weapon.
Never have the bolt in the weapon when assembling and tightening the trigger unit in place. If you have it in place it will immediately cause damage to the trigger unit when sear is forced down against the connector by the cocking piece of the firing pin and cocking surfaces/ corners on the sear and connector will break/crack.
5. Adjust the final pull weight to minimum of 1.0kg (hex screw in front of the trigger). Use loktite on the screw.
Always have sufficient gap in between first and second stage pull,
first stage should be approx. half of the final pull weight (0.5/1.0kg, 0.6/1.2kg etc.). If the two weights are set too close the cocking corners will get rounded and the weapon will not stay properly cocked causing hard and long single stage pull like you described.
6. After adjusting the pull weight check the functioning of the safety. Cock the rifle and pull safety on ->bolt handle will not come up and trigger will only move the first stage travel.
Push the safety off and dry fire the rifle. Try pulling the safety on -> safety should bounce back to the front position and not stay on. If the safety engages when rifle is not cocked the safety lever inside the trigger unit is worn or too loose and should be re-tightened properly. Have trained gunsmith do this!
Do not over lubricate the internal parts of the trigger unit; small amount of very thin synthetic oil or dry lube will do the trick.
Two of the most common mistakes causing problems:
Improper adjustment of the first and second stage pull set too close to each other will wear the internal parts.
Assembling of the trigger unit to the rifle with the bolt in place will immediately break the internal parts.
Hope this helps and you'll get the trigger operational again?
If you're not able to adjust the trigger properly have the cocking corners in the trigger unit checked, they may be damaged already and the pull weight will not adjust properly...
When you set the final pull weight to 2lbs you have to adjust the first stage also to 1lb. I they are too close it will wear out and will become that 15-22lbs again.
You shouldn't go below that 1kg on the second stage or the thing you just described will definately happen. OK for some time but after a while will get very heavy.
I've seen some triggers adjusted to less than 1lb but this will require replacement of some internal springs and lots of fine tuning and polishing. NOT recommended unless you're a gunsmith and know exactly what you're doing.
I'd stick with that 1.0kg and over. All of the guns (TRGs)I usually shoot a pretty much set to 1.5kg. Not too light and not too heavy...and remember; If you play with second stage weight always do the same thing with the first stage!
