Hi Akira san,
thanks for your update:
He said the cause of spurious was the noise of VCC around 19 MHz of 0.15V.
As the result of the modifications of Bitx40 to reduce the noise, such as adding the big condenser and choke coil etc , the spurious level went down below of the criteria.That's good news, congratulations!
This issue may also be related to the strong "birdie" that can be heard at 7199 kHz when the radio is in LSB mode (almost no birdie in USB mode).
It was found that the strength of the birdie greatly depends on the VFO drive level. Default drive level is 4 mA. In the SETTINGS menu, you can try different drive levels (2,4,6,8 mA). The optimum level may be different for each BitX40 radio.
The VFO drive level setting may also have an effect on the spurious signals you observed. Perhaps it is possible to fix this issue just by adjusting the drive level setting (in that case, extra hardware modification such as adding capacitor or choke could be avoided).
Could you try it out?
73 Allard PE1NWL