I think the first thing to do is to determine whether this is a supply or return issue. Connect a long jumper lead to the NON-banded end of one of the weak coils, say the popper coil. Lower the playfield, put the game in test mode. Not solenoid test, just any other test. The idea here is to prevent the game from energizing the popper coil. Put a ball in the popper, then momentarily touch the jumper wire to ground. I use the lockdown bar receiver, but any ground point will work. If the ball exits the popper, then the supply(claims to be 50, but always reads 70V) is good. If it's too weak and the ball stays in the popper, then you know the supply is bad. You can also do this test in the backbox by grounding the corresponding TIP 102 transistor tab. Don't ground the larger TIP36C tabs. However, I like to connect directly to the coil in case a connector is bad.

If the supply is good, then I would look at the zero crossing detector circuit on the power driver board.

I always approach repairs this way. Divide the problem into areas (Playfield or backbox? Supply or return? etc) then you can focus on the problem area instead of just jumping here and there.

I've worked on lots of these, and I haven't seen any cracked solder joints at the header pins. I don't know if it's because they're not old enough, or they just fit better in the board. Of course, the 5 volt input and GI connectors burn, but that's different from what some refer to as "cold" solder joints.

Since they are still weak, then this rules out the returns. The problem must be in the supply. There is no need to check zero crossing now. Measure DC volts between ground and the tabs of the transistors controlling the weak coils. If it's lower than 70v (say, 20v), then recheck your connections at J106 and J107. That's where the power leaves the board to the playfield and backbox. If it's 70v, then the coil should fire full strength when grounded. I think if there was a problem with the bridge or cap, then all coils would be weak instead of just a few.

The key here is that the transistors are the final step. That is, power leaves the board at J106 / J107, goes out to the coils / flashers, then the returns come back from the coils to the transistor tabs (collectors). U4 provides the signal to turn the transistor on and complete the path to ground. So it really looks like you have something wrong at J106 / J107.

Zaza, your drawings are excellent!