Ah...Thanks for the link to that thread.
Now I know what your problems have been.
You can find the point to inject at, but can't seem to develop the "power" to get into the circuit.
Ok, remember that caps BLOCK DC, but easily pass AC or Audio...
Well, if you understand that, then you also need to know they, caps, can be used as limiters.
How?
Let's do up a quick diagram to show what I'm referring to...
A Basic Ripple Filter
Ok the above is to help you understand why your audio is so low.
The 4558 Mic amp, is your radios' Mic Amp to AMPLIFY a small signal from a dynamic element or power mic - into a VOLTAGE signal - meaning that it has a greater voltage amplification than current amplification.
So if you know Caps pass AC VOLTAGE - but cannot pass DC VOLTAGE - look at the circuit above - for the TOP RED LINE is similar to what your AM Regulator uses to mix a DC voltage into your Audio Signal arriving from the Mic amp.
- That red line represents a power level in both voltage and current - but resistors and capacitors can utilize their characteristics to pass DC and AC at the same time. It's when you don't have enough resistive elements in the circuit - properly placed, that let the small signal level and power behind it, pass across into the next stage of an amplifier - the DC power and the current it has available as a reserve - prevents the flow of power across into the next section.
- In the diagram above, it is called a ripple filter, it can be used for many things but mostly for setting a given level of signal input by letting capacitance take some of the audio signal passing thru this circuit out at a given rate and voltage spikes are taken away to prevent the next stages' input levels to be exceeded and potentially (SIC) cause damage.
That DC level used in AM Power for your carrier is about 4.5 to as much 6 volts DC - but if your INJECTION signal level is much lower in voltage and in current or wattage power even, for it to pass - why? Because of the DC presence on the other side of the cap (upper part) - that level of voltage can blot out any weak small signal you're trying to pass into or thru this circuit.
So to avoid that - they use a resistor to limit the current power but keep voltage presence and the cap passes AC but keeps DC signal from backfeeding into another section - so this combination provides a working element that allows DC and AC to be present at the same time.
How does this apply to the radio you're trying to "HI-FI"?
Well, when you decide to inject, you'll need a means to change the current present in a simple power amplifier that pumps into a set of speakers - into a voltage signal so it can move or jump over or punch thru the DC voltage present on the other side.
Where you inject is just as important as to what you use and how you inject. So you know...
I hope you can see what I'm getting at otherwise the explanation will eventually go beyond the scope of this thread...
In basics' - you'll need to convert a small signal in both power and current, into a voltage signal - with enough voltage to force itself across the capacitors that will easily take the radios own voltage amplified signal - so your effort needs to be focused on converting current power with low-voltage push, into a voltage signal with a high enough Peak to Peak signal level in VOLTAGE to cross over the "blocking" DC Bias it also uses to mix with and send to the AM Regulator to set Carrier power as well as Audio signal so it can be converted into a power Signal that the Driver and Final can use.
