cracked plate?

[Dave Nereson]

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    Heard (and felt, through my tuning hammer and the floor, as though =
someone had dropped an armchair or sofa) a loud bang from a 1903 =
Rembrandt upright I was tuning today.  I had cleaned it out, tightened =
the plate screws, seated strings, filed hammers, regulated, and did a =
half-step pitch raise.  No audible protests from the piano.  I then =
started over to do my fine tuning.  Set the temperament, tuned octaves =
to the top, then started in on the bass, and while tuning bass unisons, =
I heard "the big bang".  Now the first octave of the plain wire section =
was suddenly a quarter-step flat, but the bass and rest of the treble =
were still up to A=3D440 level.  So I took the action out and with a =
bright light inspected the plate very closely and could find no cracks.  =
There was one hitch pin that had two very small  hairline cracks around =
it, but someone (maybe the factory?) had relocated the hitch pin about 2 =
inches lower, out of the cracked area.  I searched and searched and =
could find no evidence of plate cracking, rib separation, bridge =
splitting, pinblock separation or anything else having given way.  The =
dampers and hammers were still in proper relation to strings, still =
regulated.  It's a 3/4 plate, if that matters.  I re-tuned it a =
quarter-step low, lowering the previously-raised sections back down, for =
fear of ruining the piano, and it seems to be holding.  Still don't know =
if the plate cracked or what gave or shifted or settled.  Sure scared =
me, even though I've had a plate break on me before.  The other time, =
which sounded like a rifle shot,  was a Baldwin console, and it broke =
not across a strut, but across the lower web, below the mid-treble.  =
Part of the plate jutted forward, jamming against wippen tails.  And =
exactly under the break, on the bottom board, was a little pile of sand. =
 Someone knocked sand into the molten iron, causing a weak spot in the =
casting, I guess.  Fortunately, it was still under warranty and they =
replaced the plate and restrung. =20
    But this is the first time I'm not absolutely sure of what caused =
the big bang.   No, there was nobody moving furniture, no construction =
outside, no sonic booms;  it came from the piano.   Weird.  Wot =
hoppinked?
    --Dave Nereson, RPT, Denver

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