No, not normal.
Check the position of that "Balance Check" switch on top right of the front panel -- even it if
is in the "normal" (unmarked) position (as opposed to "ON"), -- you
might still want to try it in the
other position (i.e., "ON"); or just 'fiddle' with it a bit.
Any and all switches on a five-plus decade old amplifier may be dirty and/or have very worn contacts at this late date.
Also...
as my late father would always remind me when I was trying, frustrated, to troubleshoot an old kit-built hifi component that had found its way to me, "That's the problem with a kit. You don't know if it
ever worked right!" :- P
In other words, when issues present themselves, sometimes one just has to check
every single connection, point to point wire, and solder joint -- against the original assembly manual.
Yes, they are a little quirky -- not to mention ugly (well, at least homely). They have some well-documented idiosyncracies that can, and should, be mitigated.
Pretty good iron in 'em, though.
DSC_0198 by
Mark Hardy, on Flickr
EDIT: If you don't have an ST-40 manual, it is readily available on teh webs. I think it can even be found on AK's own "Digital Doc Database".