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Help with Audio Chain - M1 and M2 Are Ground? #ubitx-help #v5 #v4 #v6 #audiocircuit

AndyH
 

I'm trying to understand the flow of the signal through the audio amp and pop-fix FET (Q74 in at least the V6).  I *think* I get that in the amplifier stage around Q70 (V6) has a bypass cap to ground, as indicated by flag M1.  The 2N7000 Q74 (V6) appears to have both terminals 2 and 3 grounded (pin 3 via flag M2).  Is that correct?  Am I reading the schematic correctly that flags M1 through M4 are ground?  If so, it appears that grounds the CW tone input and VOL-H via the 1K resistor R70.  

Thanks!
Andy  KG5RKP

AndyH
 

Put another way, maybe:  I can't figure out how the audio signal makes the trip from the audio pre-amp to the LM386 final amp.


On Tue, Jan 21, 2020 at 12:26 PM, AndyH wrote:
I'm trying to understand the flow of the signal through the audio amp and pop-fix FET (Q74 in at least the V6).  I *think* I get that in the amplifier stage around Q70 (V6) has a bypass cap to ground, as indicated by flag M1.  The 2N7000 Q74 (V6) appears to have both terminals 2 and 3 grounded (pin 3 via flag M2).  Is that correct?  Am I reading the schematic correctly that flags M1 through M4 are ground?  If so, it appears that grounds the CW tone input and VOL-H via the 1K resistor R70.  

Thanks!
Andy  KG5RKP

Don, ND6T
 

Andy,
Perhaps you're thinking of the MOSFET as being like a vacuum tube (Fleming valve) and requiring a bias voltage at the drain (corresponding to a tube plate). It does not.
Consider the device to be a resistor between the source and drain terminals and the resistance of that is controlled by the gate terminal. The important bias in a MOSFET is the voltage between source and drain only.
The drain isn't grounded directly. Rather it is connected through the body of the volume control. No DC voltage but the audio is still there.
So just concentrate upon the audio signal, instead. It passes from the pre-amplifier (Q70 collector) through a DC blocking capacitor (C51), through  R70 (which allows some side-tone audio to survive muting) and is applied to the volume control (RV4) which selects how much audio signal is to be applied to the input of the audio output amplifier (U1).
If the gate of Q74 is turned on by a positive bias then it presents a short circuit to that audio stream, routing it right to ground and reducing it so that effectively nothing reaches the volume control (except for side-tone, if it's there).
Does that help?
73,
Don

AndyH
 

Hi Don,

   I appreciate the walk through!  Ultimately, I'm finding that I'm unable to trace the audio signal because it appears to go to flag M1, while the input for the final amp seems to have to come from flag M2.  What I don't understand is how the signal jumps from flag to flag, since the schematic says that flags M1, M2, M3, and M4 go to ground.  I figure I must be missing something simple (mostly 'cause I'm not an EE and didn't stay in a Holiday Inn Express last night :) ).

73, Andy

   


On Tue, Jan 21, 2020 at 03:40 PM, Don, ND6T wrote:
Andy,
Perhaps you're thinking of the MOSFET as being like a vacuum tube (Fleming valve) and requiring a bias voltage at the drain (corresponding to a tube plate). It does not.
Consider the device to be a resistor between the source and drain terminals and the resistance of that is controlled by the gate terminal. The important bias in a MOSFET is the voltage between source and drain only.
The drain isn't grounded directly. Rather it is connected through the body of the volume control. No DC voltage but the audio is still there.
So just concentrate upon the audio signal, instead. It passes from the pre-amplifier (Q70 collector) through a DC blocking capacitor (C51), through  R70 (which allows some side-tone audio to survive muting) and is applied to the volume control (RV4) which selects how much audio signal is to be applied to the input of the audio output amplifier (U1).
If the gate of Q74 is turned on by a positive bias then it presents a short circuit to that audio stream, routing it right to ground and reducing it so that effectively nothing reaches the volume control (except for side-tone, if it's there).
Does that help?
73,
Don

AndyH
 

Crud.  There's an M1/M2 flag on the T/R relay, K3, as well as the M1-M4 flags to ground on the top of schematic.  I guess that's how the signal jumps the M1 to M2 flags.  Sorry for the noise, folks.

Don - thanks again for your walk through - it helped me see how the pop fix works.  Now...maybe...finally...I can get on with my scratch built V3 V4 V5 V6 radio. LOL

Best to the group,
Andy

Don, ND6T
 

Hah! I forgot to include the relay. Also I meant to say "source and gate" but wrote "source and drain"!  So yes,  I guess I shoulda stayed at Holiday In Express last night!