From reading the post, it sounds like you guys are really qualified mechanics and I don't want to come accross as a "know it all". Also did not want to try to help if no one was listening. From what I have read here, you two mentioned the amplitude circuit breifly and somewhere mentioned that you though it changed polarity of the circuit(I am not good at putting quotes in threads). I just wanted to let both of you know (in case you didn't) that the amplitude switch changes the direction of the hydraulic motor that spins the eccentric weights that make the drum vibrate. So there is (at least on the ones I have worked on) a 3 way valve - thumb button on FNR lever turns off - high amplitude switch postition should pull valve one way when energized, and on low pulls the other side of the valve. There are 2 weights on the shaft in large housings on each side of the drum, running in gear oil. 1 wieight is keyed to the shaft, the other can move about 100 degrees (maybe less - it has been a long time since I went into one of these). In the low amplitude direction the moving weight spreads away from the keyed weight and comes close to balancing the assembly, thus it has a lower up and down motion - kind of like a tire that is mildly out of balance. In the Hi amplitude rotation direction, the wieghts stay together creating one big weight thus shaking the machine way more violently.
The first one I worked on had the full operators manual and the information in there, regarding properly compacting soil and soil types, is very extensive. Yet I have talked to experienced road building supervisors who only know - wet it, grade it, pack it, grade it, pave it. There are places around here where new paving jobs have speed bumps in them because the guy running the vibratory compactor was on high amplitude when he should have been on low, and everytime he switched direction, he over compacted a spot in the subgrade. Running on high on already compacted surfaces is probably the main reason these machines get shaken to peices.
Just thought I would try to help since there are probably very few people who get to rebuild one of these drums and see what they do. Hope I did not insult anyone's intelligence.
BTW the reason I rebuilt the drum was because the old dirt contractor bought a SD120D to pack key trenches in tank dams, installed pad foot plates on the smooth wheel and went to climbing in an out of a short but 40 ft deep trench in wet clay. The operator said it kind of stalled out one time going up the grade and never really climbed as good after that. The complaint years later was that the machines tires would loose traction on the least incline. I was green, and took some bad advice from a dealer technician and had the drum drive motor reman at a hydraulic shop. Did not change a thing. Then the same technicion said must be a bearing problem in the main drum bearings - that is what is causing the drum to seemingly bind up and make the back wheels spin out. So I pulled the drum and figured reseal everything since we have it tore down this far. So I got to go into the weight cases and drive system. It was a pretty involved job by myself. While I had the torque hub planetary off (the one the drive motor is attached to) I went to change the oil in it and found mostly metal - it was like a coffee grinder on the inside. Don't know why I followed that guy's direction, all I had to do was drain the oil in the torque hub and found the metal. What I learned was that the "D" models are made for smooth wheel and have a much smaller torque hub, the "F" has a larger torque hub and is made for pad feet. I replace the torque hub with and "F" model hub and had to replace the plate it mounts to as well. We compared part numbers to every relavant hydrualic component to make sure the "F" model did not need more parts, which it didn't and as far as I know he still has that machine today (this was in about 2011).
