How clean are the posts and how good of a connection from batteries to engine. Had an old Axial flow this summer that wouldn't start after I changed from two batteries hooked together to one big battery used a combination of old wires and wasn't getting good connection between engine and wire. This is not your situation, but illustrated to me how good post, wire, to engine connection must be. .
You likely had the battery cover off since you say they are charged. I did have a battery cable short and melt wrecking both on my 9230, similar symptoms. I do agree it could well be the converter, I have had them go on my tractors. Steigerman has the procedure there for you.
We had a 9240 combine not start this spring. Fuse was blown. We replaced it and did our normal servicing to get it ready for harvest over the last week. Last night we pulled it out of the shed and parked it by the other combine. 2 hours later while working on the one next to it, we heard a wired torch like noise....then there was smoke. I run up to the cab to find the key off but monitor was on with warnings. Run down to turn off main disconnect. Found the battery equalizer was completely fried with melted connector. Nearly burnt our machine down!
I would agree with you on the equalizer not being up to snuff. The story I got was Case thought they were warranting out too many swap relays so they went to the battery equalizers on the 30 series. Then during the 30 series run they realized what kind of mistake they made and changed back to the swap relay on the 40 series. The reason I asked if it was the swap relay is because from what I know all of the 40 series have the swap relay's. Good catch on the fire, I've seen quite a few equalizers short out but never catch fire it's a good thing you weren't going down the field.
It would not be a difficult job to do if the 8230 was going to be deleted for the rest of it's life. The 30 series use a 24 volt DEF supply module where the 40 use a 12 volt module so if that system was removed it would just be the swap relay and wiring.