Hammer spring problems can be subtle.  If the piano is well regulated
but the springs are too strong, the jacks may not get back under the
butts, resulting in poor repetition and uneven key height.  Also, it
may just feel hard to play.

I check stiffness by slowly pushing down on a key until the hammer
reaches let-off.  Then I watch the speed of the hammer as it goes
through let-off and falls back.  This is done so gently that the
hammers don't strike the string.

If the hammer definitely falls back, that is good.  If it falls back
quickly, that is bad.  It should not act like a sling shot, for
example, neither should it just stay at let-off.  If that happens,
the spring may be too weak or the hammer flange too tight.

I ran across this problem when I was just learning the tuning business.
I tuned a lot of brand new pianos for a store and one brand in particular
had this problem.  The way to check if the hammers are too strong is
this:  Pull back on the hammer rest rail -- all the hammers should
follow.  This indicates that there is enough room for the jacks to get
under the butts.  Adjust key capstans of those that don't follow.

Now, gently push all the way down on each key.  Gently release, try
to make the jack hang up on the butt.  Assuming there is no friction
problem elsewhere, the hammer jack may be too stiff.  The hammer butt
flies back faster than the jack can get back under.  Also, as you are
gently pushing on the key, you may feel a grinding sensation as the
action goes through let-off.

The cure is to reach down between the hammers and the strings and
carefully bend the springs toward the strings.  You will need to reach
down as close to the spring coil as you can and use a gentle touch.
It's a subtle feel but you can learn it quickly.  Check and repeat as
necessary.

A word of caution: some manufacturers used poor quality springs and
trying to bend springs invariably leads to breaking them.  I have never
had a problem with brand new replacement springs, but you may have a
problem in a piano with 80+ year old springs.

Jeff Davis
in sunny Seattle, WA