The Yaesu FT-450 is a HF/50 MHz 100 W transceiver. It has a general purpose receiver that starts at 100 kHz. It performs very well on 160 to 6 m, but unfortunately it has a permanent 20 dB attenuator on frequencies below 1.7 MHz. As it can be seen below, the attenuator consist of resistors R2004, R2005 and R2006. Next to it, there is a high pass filter to block anything below 1.7 MHz. It appears that when the high pass filter is selected (relay RL2001), the attenuator is bypassed; when the attenuator is selected, the high pass filter is bypassed. The attenuator switches on automatically when frequency drops below 1.7 MHz.
Clearly this is going to be a problem is this radio is to be used for receiving 136 kHz and 500 kHz signals. Fortunately, the attenuator resistors are very easily accessible by simply removing the bottom cover of the transceiver. If R2004 and R2005 are removed and R2006 is replaced with a 0 Ohm 0805 SMD resistor, the attenuator is permanently removed. Obviously, by removing the attenuator we are risking interefence from the MW broadcasters. This can be mitigated by using an appropriate external low pass filter.
Perhaps, a three pole low pass filter can be placed in the position of R2004, R2005 and R2006 - this is something to consider in the future.
The attenuator can be easily accessed by removing the bottom cover. There are four screws at the bottom side and another four (two on each side) on the sides.
Locate the relay RL2001. The attenuator resistors are on the side of the relay as seen in the (unfocused!) photo below:
As it has been pointed out to me by Bob ZL2CA, the removal of the 20 dB attenuator will not necessarily make the receiver in the FT-450 perform as well as it does on 160 m or higher as there are other components (for example RF transformers and coupling capacitors) that will affect performance on LF. Bob identified T2003 in the FT450 circuit as a transformer that should be checked for its performance on LF, but that's something that I have not done yet - when I get a chance to test that, I will report it here.