I bought this thing for $10.. knowing fully well that it may work fine.... but it may not. That is exactly what I wanted it for.
I am now looking at it on the bench. (By the way, I am a hobbyist at this... not a pro that is doing it for customers. Just me having fun!)
Receive is excellent.
Carrier is excellent.
Receive AUDIO is excellent (I emphasized this for a reason).
Modulation is "in the toilet".
It is THERE.... but it looks like it is only about 5%... maybe 10% modulation.
Working the AMC adjustment from end to end brings it up a tiny bit.....BUT.... the point in the schematic where the AMC sink transistor ties in would kill the receive audio as well if the circuit was killing the microphone audio.
This is going to sound like I am repeating things..... but...... the AMC circuit would be killing the RCV AUDIO TOO .... IF IT were killing the mic audio.
This is going to sound hokey.... but it works GIVEN the circuit in the schematic: If I pipe an external 1 Khz into the audio pin on the mic connector... it goes through clean as a whistle, it scopes clean as can be at the output of the audio chip. So this suggests that WHATEVER I pump in to the audio pin gets all the way out of the audio chip and is CLEARLY heard in the speaker. HOWEVER..... if I plug the mic in and key it up WHILE I am adding the external 1 Khz tone...... THE TONE FROM THE GENERATOR GETS SQUASHED. As if the MIC ELEMENT was loading it down.
Note that this radio has NO microphone amplifier. The pin from the mic jack is wired through a resistor and cap and ties right together with the output of the volume pot. This is the same place that the AMC sink transistor ties in.
Repeating again:
The RCV AUDIO screams in this thing! Loud and clear.
When I inject a 1 Khz tone into the mic jack audio pin (leaving the mic unkeyed) the external 1 Khz screams in this thing!!! Loud and clear.
If I key the microphone.... the 1 Khz external tone at the injection point squashes down to nothing.
Now.... let me back up.........
If I key this radio... and put the mic near a tone generator... you can definitely SEE modulation.... and it is clean!!!! Sinusoidal!!!!!
It is just ONLY ABOUT 5% modulation!!!!!!! Maybe 10% at the most. But the actual modulated waveform is clean!!!!
This is the original mic, it has a 500 ohm element in it. However I note with interest that there is no MIC AMP in this radio. The mic jack audio pin is mixed with the receive audio just by tying them together and feeding that to the audio chip. RCV audio.... screams. TX audio... whispers!!!!
That is why I am asking..... have I missed something????
Were these GE radios KNOWN to be lousy modulators and they just failed to notify ME???????
This is the weirdest thing I've seen. It is like it was DESIGNED to modulate this low.........
I need a drink.......
I am now looking at it on the bench. (By the way, I am a hobbyist at this... not a pro that is doing it for customers. Just me having fun!)
Receive is excellent.
Carrier is excellent.
Receive AUDIO is excellent (I emphasized this for a reason).
Modulation is "in the toilet".
It is THERE.... but it looks like it is only about 5%... maybe 10% modulation.
Working the AMC adjustment from end to end brings it up a tiny bit.....BUT.... the point in the schematic where the AMC sink transistor ties in would kill the receive audio as well if the circuit was killing the microphone audio.
This is going to sound like I am repeating things..... but...... the AMC circuit would be killing the RCV AUDIO TOO .... IF IT were killing the mic audio.
This is going to sound hokey.... but it works GIVEN the circuit in the schematic: If I pipe an external 1 Khz into the audio pin on the mic connector... it goes through clean as a whistle, it scopes clean as can be at the output of the audio chip. So this suggests that WHATEVER I pump in to the audio pin gets all the way out of the audio chip and is CLEARLY heard in the speaker. HOWEVER..... if I plug the mic in and key it up WHILE I am adding the external 1 Khz tone...... THE TONE FROM THE GENERATOR GETS SQUASHED. As if the MIC ELEMENT was loading it down.
Note that this radio has NO microphone amplifier. The pin from the mic jack is wired through a resistor and cap and ties right together with the output of the volume pot. This is the same place that the AMC sink transistor ties in.
Repeating again:
The RCV AUDIO screams in this thing! Loud and clear.
When I inject a 1 Khz tone into the mic jack audio pin (leaving the mic unkeyed) the external 1 Khz screams in this thing!!! Loud and clear.
If I key the microphone.... the 1 Khz external tone at the injection point squashes down to nothing.
Now.... let me back up.........
If I key this radio... and put the mic near a tone generator... you can definitely SEE modulation.... and it is clean!!!! Sinusoidal!!!!!
It is just ONLY ABOUT 5% modulation!!!!!!! Maybe 10% at the most. But the actual modulated waveform is clean!!!!
This is the original mic, it has a 500 ohm element in it. However I note with interest that there is no MIC AMP in this radio. The mic jack audio pin is mixed with the receive audio just by tying them together and feeding that to the audio chip. RCV audio.... screams. TX audio... whispers!!!!
That is why I am asking..... have I missed something????
Were these GE radios KNOWN to be lousy modulators and they just failed to notify ME???????
This is the weirdest thing I've seen. It is like it was DESIGNED to modulate this low.........
I need a drink.......
