(ET) E20 now dead

Hello, this group is a god send and I hope that someone can shed some light on my current problem. I purchased an E20 from a listing in this group, a few months ago. Long story, but tractor was supposed to be in running condition when I had it shipped to me from out-of-state location. Owner of tractor was in Bolivia, tractor was at daughters house in Colorado - many emails back and forth related to the purchase, etc. Tractor came with snow blower, tiller and mower deck, etc. Owner stated that he last used tractor some 8 months ago and all was functioning, minus 1 mower motor, at that time. Needless to say the tractor did not work when it arrived at my location. I tried a charge on the existing batteries which proved to be bad. Replaced them with some sealed agm's that I had from a solar array project - batteries are good ones. Tractor would seem to function with the charger connected to a/c, but not with the batteries. Took me some time to figure out that the power disconnect needed to be pushed in - not just turned to up posistion as manual states, for circuit to be completed. First time I pushed in disconnect I heard front battery (1 - 12volt) starting to churn and I figured it was probably from the 24volts being delivered from the 4 - 6volt rear series batteries. Power on the tractor still did not work with batteries on system, no drive motor or direction switch showing power. Next thing I did was plug it back into charger unit and heard a loud pop sound from what I thought was below tractor. Now when I try to bring tractor back on line with the disconnect engaged or disunengaged and charger plugged in - I have no activity at all, tractor is completely dead. I have checked all the obvious things, fuses and cb2 breaker, but everything seems normal. Can anyone point me in a troubleshooting mode that I am overlooking?