Exxxxcellent, as Mr. Burns would say
FYI that 1Ω multimeter reading is just the resistance of the multimeter probe leads and the meter's internal circuitry. If you touch the probes together you'll still measure 1Ω. So you can ignore it; if the meter shows 1Ω then there is full continuity between the points you're probing.
I think you'll be able to solder the wires to the insides of the "vias" (the holes that connect the copper on the top side to the bottom side). If not, you can scrape off the green solder mask that covers the copper, using a small sharp blade held near the end (for best control, to avoid having it slip off and leaving a big messy scratch on the board).
Now we have to connect the signals between the boards, and match the levels.
The signal flow in the amplifier unit is like this:
Microphone --> Mic socket --> mic preamp --> volume control --> TEA2025B power amp --> speaker.
We will be inserting the voice processor board into the signal path of the amplifier board, in between the output of the mic preamp, and the input of the volume control.
In the first picture in post #12 I marked the two points we need to connect to. The one on the top side, on the west end of R8, is the output of the mic preamp, and the one on the bottom, on the actual pin of the volume control, is obviously the input to the volume control.
Currently these points are connected together by the track that's marked with 'X' on the photo below. You need to cut that track somewhere, and the only accessible place is at the 'X'.
Cut it as close as possible to R8 so that there's plenty of track sticking out from under the volume control, so that in future, if you want to return the amp unit to normal operation, you can scrape some solder mask off the track and connect it back to R8 with a blob of solder.
Then we need to make two connections from this board to the voice processor board:
1. Feed the signal from the output of the amp unit's mic preamp (on R8) into the input of the voice processor, and
2. Feed the signal from the voice processor's output back to the amp board, at the volume control.
The second connection is pretty straightforward. The last photo in post #12 shows the input and output points on the voice processor. You need to run a wire from the OUT point on the voice processor to the volume control pin. You MAY be able to connect it to the cut track, if there's enough copper, if you scrape off some solder mask. Otherwise you'll have to connect it to the pin of the volume control where it comes through on the underside.
The first connection is a bit trickier.
The output from the mic preamp is indicated on the photo. This point has the audio from the amp unit's mic preamp, which we will need to feed into the voice processor.
The signal path on the voice processor board is like this:
Microphone --> mic preamp and voice processor chip --> crappy power amp --> speaker.
So we will have to remove the microphone from the voice processor, and feed the signal from the amp unit's microphone preamp circuit (at R8) into the input of the voice processor (marked "IN" on the last photo in post #12).
However, that input is designed to take a signal straight from the little "electret condenser" microphone in the voice processor, which doesn't generate a very strong signal, so the voice processor's input will be quite sensitive.
The signal from the amp unit's microphone preamp will probably overload it and cause distortion. We will need to attenuate that signal. We can't change the input sensitivity because the "mic preamp and voice processor chip" section of the voice processor is all implemented in a single chip that is buried underneath the big blob of tarry stuff, and we can't get into it at all.
That input is also fed with a positive voltage, which is needed by the electret microphone. This requires a capacitor to be connected in series with the signal, otherwise that voltage could affect the microphone preamp in the amp unit.
My first suggestion is to insert a capacitor and a resistor in series, into the signal, like this:
Code:
Amp board R -C+ Voice changer
Mic pre out (R8 west) ----------------------------------\/\/\/\/-----| |------ IN pad
The capacitor can be any electrolytic between 1 µF and 100 µF (1 µF is good) and the resistor value... well, you could start with 100 kΩ.
Then, test the circuit by talking into the microphone. As you adjust the volume, and talk louder and softer into the microphone, it should become clear when the voice changer is reaching its maximum input level - the signal will get badly distorted, and talking louder won't cause a louder signal in the speakers. If this is happening when you speak normally into the microphone, you'll need to attenuate the signal more, so increase the 100 kΩ resistor. If the distortion only occurs when you're very close to the microphone, you should attenuate the signal less, so reduce the 100 kΩ resistor.
You could also try a potentiometer instead of the resistor, and adjust it until you get distortion only when the sound at the microphone is louder than it would normally be in regular use.
You may discover that the voice processor's sound quality is not good. Some distortion may be audible at all times. Experiment with it to get the best performance you can.
If you have to set the amp unit's volume control near the bottom of its range, that means that the signal from the voice processor's output is too strong. You can attenuate it by inserting a resistor into the wire that feeds the top of the volume control (no capacitor should be needed here).