Did I miss a memo or something??? (GE 3-5811B related)

[guitar_199]

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.......

 

[danmcclain]

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.......

Is that mic 4 pin or 5 pin din?

 

[danmcclain]

Is that mic 4 pin or 5 pin din?

I don't

Is that mic 4 pin or 5 pin din?

I don't see any of the wires tied together....

 

[danmcclain]

I don't

I don't see any of the wires tied together....

This is what I pulled from the all-in-one mic guide

4 pin 1 audio 2 shield 3 receive 4 transmit

5 pin din 1 shield 2, N/C 3. transmit 4. audio 5 receive

 

[guitar_199]

Def

Is that mic 4 pin or 5 pin din?

Definitely 5 pin DIN. That is what the SAMS 148 says, and that is indeed what it is.

 

[guitar_199]

I don't

I don't see any of the wires tied together....

Dan, most likely I haven't done a good job in describing what I meant. I'm going to try to write in support of the pic I uploaded:

If you will, look right in the middle of CKT TRACES 19, 80 and 81.... where R57, R59 and C83 meet together. That is the "tie point" I was trying to describe.

From the left... R57 comes from the Volume pot wiper.
From the top... R59 and C81 come straight off of the audio pin on the mic connector.
From the bottom... that comes from the AMC.... the collector that acts like the throttle for the modulation control.

With this in mind... what I am saying is... the audio from the LEFT is the RCV AUDIO and it comes screaming through, loud.... oh Lordy LOUD.

When I pump a signal generator (1KHz tone) into the audio pin on the mic jack... THAT comes screaming through loud!

IT is not like anything is wrong down in the AMC (at least I dont think) for two reasons:
- it isn't killing the test tone that I shove in...and it is hotter than the mic element, and

- if that AMC limiting transistor was shorted it would be killing the RCV AUDIO as well... and that is not happening.

When I use the microphone though..... the mic audio waveform is good! It is clean!!!! It is just.... SMALL. It is like the little mic element doesn't have enough ooomph.

And I did try another mic from a Realistic radio (same pinout). Exact same results And THAT mic works fine in the realistic...

I am stymied!!!!!

 

[guitar_199]

This is what I pulled from the all-in-one mic guide

4 pin 1 audio 2 shield 3 receive 4 transmit

5 pin din 1 shield 2, N/C 3. transmit 4. audio 5 receive

I am definitely the 5 pin DIN and I confirm that the pinout you supplied is correct on this radio.

 

[guitar_199]

To make things more clear, I am posting two pics more!!!! The area of concern is on a page split... so I have screen shot each area to try to get the most of what is involved here.

Part One shows the microphone and the mic jack with the wiring trailing off to the right.

Part Two Shows the Volume Control, the AMC circuit and the path in to the audio chip, the mic wiring is trailing in from the left.

This pretty much covers the area.

But the mic audio and the receive audio all meet right there where R57, R59 and C83 tie together. Receive audio blasts through fine. Mic Audio is WAY WAY low.... so of course... modulation of the carrier is low. The only thing between the mic jack audio pin and that junction is C81 (.0022 uF) and R59 (1K).

And again....

1. when I push a signal generator into the mic jack pin.... the audio is LOUD ... AND the output pin of the audio chip scopes clean and pure sinusoidal with a great amplitude, and

2. when I take a working microphone (works on the TRC-421A!!!) and substitute it.... I get the same low mic audio/low modulation result.

 

[TM86]

Not saying I know what's going on here, more like too stupid to keep quiet.

Since both microphones behave the same and yet the audio generator behaves differently maybe look at what the difference is in how they're connected? IOW, are the mic plugs stressing the DIN jack in a way that the audio generator doesn't? Could suggest an intermittently bad connection if there's a solder joint somewhere between that connector and the main board. Put stress in the right direction and it works great. Stress in the wrong direction and, well, not so great.

Anyways, that's my 5 cents. Would have been 2, but we don't make pennies anymore.

 

[danmcclain]

Not saying I know what's going on here, more like too stupid to keep quiet.

Since both microphones behave the same and yet the audio generator behaves differently maybe look at what the difference is in how they're connected? IOW, are the mic plugs stressing the DIN jack in a way that the audio generator doesn't? Could suggest an intermittently bad connection if there's a solder joint somewhere between that connector and the main board. Put stress in the right direction and it works great. Stress in the wrong direction and, well, not so great.

Anyways, that's my 5 cents. Would have been 2, but we don't make pennies anymore.

I'm not a tech either.... I just thought maybe a mic was wired wrong...