My last inquiry about an AO 820 repair ended up with IMEB used equipment
dealer, who had these instruments (a bunch, if I recall) sitting in a back
room.  I don't think parts are made anymore, a repair can be done,
not cheap!and a considerable down payment on a new microtome!) and I think
parts may be scavenged off the older machines in some cases.  I don't think
Leica even bothers with them anymore.      This was about
to happen with my one good 820 and an old 820 no longer usable on my shelf,
I opted for a new microtome.  Was difficult to give up the old workhorse,
but I had to put it out to pasture for a new thoroughbred!

Gayle Callis