Well, the "lowering of the noise floor" is from the parts swap.
To take advantage of it, you need to make the AGC less aggressive in "Gain" when you have noise present - meaning, the signal will be in the noise - when you upgrade to a Low-Noise amp part - say you want to try that 2999 mod, or maybe the Schottky diode addition, then remember these parts affect how quickly (that Aggressive part) the AGC responds..
Many think AGC just keeps the needle steady, well - yes it does, but it also prevents the Receiver from damages cause by excessive signal strength as well as noise present in the Radio the Antenna sends it.
In a way, it "limits" the "listening level" your receiver has. The PIN diodes in the front end section of many radios rely on AGC bias, the power behind to being their effort at RF attenuators - they begin to "back off" the receivers input levels BEFORE it arrives to the RF amp - so the RF amp doesn't overload - causing you to hear distorted signal because the amp overproduced and now it's clipping and has saturated output and crappy audio performance is heard as the result.
So, if you choose your parts carefully, you can then reduce the action AGC does against the input side of the RF amp. In the Galaxy line - you can change R44 to a higher value - so it (AGC) doesn't "see" as much energy the S/RF does - so the AGC does NOT begin to act until a threshold is reached.
This is a process of the "tickler" resistor - an 8 volt regulated power voltage - "feeder" - think of it as a pilot light, and you "fan the flames" using the S/RF meters' output "tap" as the fuel to make AGC "Flare up" and punch down the gain the receiver has when the Needle of the S/RF goes past a specific point.
So the "tickler (R43 a 270K or so resistor)" places a steady voltage on the AGC sense input of IC1 - when the signals arrive, the S/RF meter pushes the needle upwards in scale - eventually the level gets past R44 and into the Diode/Resistor combo and summed in with power present the value R43 is chosen for - that tiny amount of "trickle charge" to start the fire - when the two sum together - the input Pin on IC1 "triggers" and starts to amplify that signal GREATLY - and this process ignites AGC into a control voltage to the RF Front End when signals arrive.
You'll see in the schematic of the Galaxy circuit - there is a Diode and Resistor "combo" placed in parallel to each other TOWARDS the input pin for AGC, but BEFORE the "tickler" from the Reference voltage - this lets the circuit use the diodes INTRINSIC ability to "avalanche into conduction" and make AGC act quickly to keep stronger RF signals from exceeding the "overhead" the designers built into the gain of the RF amp, IF strips and AF detectors - so they have enough room in signal level to work with, and add very little distortion - or at least less distortion than what could have happened if it had not been accounted for..
The Resistor (R274) part of this circuit gives the ability of the S/RF meters output - some Linearity, to handle flutter, fade and other aspects of signal degradation you may hear as you listen to distant stations. The diode is a one way valve - once it conducts - it's ALL IN - so it acts like a CLAMP to keep strong signals from overpowering the receiver and you also can hear both the weaker signals as well as the stronger ones within a similar volume LEVEL - so AGC does something to protect the amplifier stages, but also lets you hear weaker and stronger stations together within a specific range of volume level so your ears don't get too fatigued or your forced to ride the volume control excessively (Read: A LOT - you get tired of listening)
Resistor R274 can go DOWN in value - but this makes AGC stronger in reaction to wildy swinging radios - which may not work well for the listener. The value is pretty high, so it allows the weaker signals some ability to "swing" the needle - when you reduce the value of the Resistor - more power from the S/RF meter can get by the Diode and make AGC act more as a "Limiter" and "Regulator" - which can help in noisy traffic areas like cities and highway stops, but too much, everyone starts to sound the same and you lose your ability to hear the distant stations as "distant" versus the close-by stronger ones - can play tricks on your hearing. It can confuse you while you're driving - so careful with this resistor value.
This AGC trickery and action only affects the NB indirectly - caused from the lack of "pushing back" the power of all the signals and noise embedded in all of that - so NB is more of a SECONDARY benefit of the AGC action - The NB can work harder but then - remember too - it is "inverting" the impulse noises it detects, so this may work AGAINST the lowering of the Nosie Floor and make the receiver even more noisy.
Ok, how do you fix that? You have other factors discussed in this thread to consider - one of them being the bandwidth of what you want to let in. So the Noise blanker is only going to do so much to REDUCE the initial noise you have in the signal "stream" arriving to the IF. - your NB is processing this in the 1st IF stream (or Strip)
So in your 1St IF, you have your Mixer, but before that, you have a PRE-SELECTOR - some call the Band Pass Filter or B.P.F. Some radios have 3 coil design, (HR2510) while others have just one or two (Galaxy L7 and L8). More coils can help "pare down" the bandwidth to help pull out your signal you want, but the price you pay is the LOSSES get quite high, to the extremely high losses when you arrive to the edges of the band (your bandwidth "skirt" losses are higher)
Galaxy uses a L6 (or L5) coil - open air coil - to help the RF amp in capturing signal. Roughly tuned to the band you want to receive - but you pay for it, the price is from this kind of open end capture is the noise level the signal may have around it is also getting amplified. So one signal you want, may get overpowered by another close by on another frequency that L6 allows thru - so this goes back to why AGC and PIN diodes are so important to have in these radios - to protect the receivr from those signals that would otherwise overpower and possibly damamge the receiver by strength of signal alone can overloand and induce distortion.
The 1st IF has several coils much like the ones AFTER the RF amp but before the Mixer for your 1st IF. So one stage is working pretty hard as it is trying to push RF into being sorted out - but there are even more AFTERWARDS being used too - once you get the signal "bandwidth" range you want. So this extra filtering helps, but then DEGRADES the ability of the receiver to perform well in all conditions.
So, there are a lot of aspects to adjusting the RF front end of ANY receiver to accept these RX Mods - the effort comes from the laws of diminishing returns on your investment. DROI -
So, to put is simply, - if you have the time and the curiosity to try it, that's fine, but when you APPLY the mods, you need to take in many different variables into account, one being the Time invested, the AGC action you want to have in processing - and the limitations each design has; when you try to implement a mod that you have to figure out the requirements. Then apply those of the radio and the part and what they need to work together to accomplish what you want to have happen.
It does not always turn out the best for both the Radio, parts and even the Operator.
