Welcome! I have a Logan 820 as well. It is a good machine, and I enjoy it a lot.
There was one gear in the QCGB that is almost always stripped. My guess is that someone removed the box to fix it, and never got around to putting it back together. I have seen whole QCGB's for 820's for sale on eBay. The prices have varied, but the good news is that there are a lot of these machines out there, and so also a lot of spare parts. There were several issues with mine when I got it, but they were all repairable.
If you need any info or help, let me know, or ask here. This forum has an enormous number of very talented and friendly people!
GG