[Kenwood] 520SE what goes bad?

[radionut]

This is kind of a off the wall question, I have two Kenwood TS-520(one S and
one SE) rigs. Recently I decided to pick up a few of the tubes just to have
on hand incase one went bad. I was able to find a number of good ones on
e-bay and have a stock pile now.

This got me thinking, while I have not had any problems for quite some time
with either rig, what are the most common failures in the TS-520 S and SE
line?

I think it would be wise to collect a few of those components that are the
most common to fail so that they are on hand in case of the need for
emergency repair.

Told you it was kind of off the wall but I think it would be helpful for my
self and others who have these rigs.

Any ideas or suggestions of what history has shown the most common failed is
with these rigs and what parts would would be good to stock?

Mike