Yamaha BK-20C Series Manual page 17

aid)
4. ..... the Electone reproduces radio or TV signals.
If there is a powerful transmitter such as a radio station
in the vicinity this can occur. Contact your Yamaha
dealer.
By ices pedal notes sound too high, upper keyboard's notes
too low. This is especially noticeable when comparing
the Electone with piano. Piano tones are combinations of
harmonics and must interact with their surroundings,
while Electone harmonics are simpler (multiples of the
fundamental tone), requiring the Electone to be adjusted
differently at the assembly stages.
Ge. cis rattling (sympathetic vibration) occurs.
AIl materials have critical resonance frequencies at
which they vibrate. The Electone's continuous tones
will naturally cause other objects (windows, shelves, etc.)
to vibrate.