Help troubleshooting mic build

[skyy7]

Hello,

I am currently building a km84 type mic from https://halfshavedyaks.xyz/pcb/rect-tx.php

I have modified the circuit by changing the C7 decoupling capacitor to 47u. I am also using an electret capsule so I have removed C5, C6, R10, R11, and R12. I have used a 0.47u PP film capacitor as the coupling capacitor at C1. I have also used a 6.8p capacitor as the charge capacitor at C4. There is also a mistake on the schematics where the jfet intended to be used is a J305.

I have an issue where there is 0 volts at the source, 2.35v at the gate, and 28v at the drain. The bias trim does not change the voltage at the drain or source. Otherwise, the voltages are correct at every other point in the circuit. I am getting 29v before R1.

When injecting a test signal into any point in the citcuit I do get output. I do not get an output with a microphone capsule.

I have rebuilt the circuit multiple times, checked for shorts to ground, removed and tested every component individually, swapped out the jfet multiple times, and tried different capsules. This has led me to believe that there is something wrong with the modifications I have made or the original schematic.

Any help would be greatly appreciated and please let me know if there is any other information I need to provide.

-Sky

 

[dmp]

I have an issue where there is 0 volts at the source, 2.35v at the gate, and 28v at the drain. The bias trim does not change the voltage at the drain or source.

On the attached schematic you are showing 0v at the gate and 2.4v at the source. With a 1G resistor to the gate, your multimeter will pull down any voltage at the gate and you won't be able to measure it. Try changing R8 to a smaller value and see if the FET biases and the circuit starts passing signal

 

[Khron]

Do you only have one single electret capsule to test this with? The capsule itself could be a dud?

 

[LevinGuitar]

Not the same circuit as yours, but in an original KM84 circuit build I made yesterday, I had similar voltages around my fet 2N3819: 0v on source, 2.8v on gate and full (24v in my case) on drain. Same result with two different fets from same source, then changed for a different brand fet from a different source, and it work as expeted. In my case it was bad (or fake?) fets.

 

[skyy7]

Not the same circuit as yours, but in an original KM84 circuit build I made yesterday, I had similar voltages around my fet 2N3819: 0v on source, 2.8v on gate and full (24v in my case) on drain. Same result with two different fets from same source, then changed for a different brand fet from a different source, and it work as expeted. In my case it was bad (or fake?) fets.

Bought mine from interFET on mouser.

 

[skyy7]

Do you only have one single electret capsule to test this with? The capsule itself could be a dud?

I got 2 capsules one of which i know is good cause it worked fine on another mic... both didnt work.

 

[skyy7]

On the attached schematic you are showing 0v at the gate and 2.4v at the source. With a 1G resistor to the gate, your multimeter will pull down any voltage at the gate and you won't be able to measure it. Try changing R8 to a smaller value and see if the FET biases and the circuit starts passing signal

Ah i forgot that the impedance is too high at the gate... i also got a few 2.7k resistors i can try instead of 6.8k...

 

[skyy7]

On the attached schematic you are showing 0v at the gate and 2.4v at the source. With a 1G resistor to the gate, your multimeter will pull down any voltage at the gate and you won't be able to measure it. Try changing R8 to a smaller value and see if the FET biases and the circuit starts passing signal

So I tried a 2.2k resistor, and the fet still won't bias... It still passes a good signal when injecting a 1khz 0.1v AC signal but won't work with a capsule...

 

[Khron]

Is the trimmer ok? What voltage(s) can you measure on the wiper? And might be worth replacing the JFET too, "just in case".

 

[Voyager10]

Sorry if this is an obvious question but - you're aware that the pins on the J304 are in a different order to those on the 2N3819 originally used in the KM84?

 

[skyy7]

Sorry if this is an obvious question but - you're aware that the pins on the J304 are in a different order to those on the 2N3819 originally used in the KM84?

View attachment 127929

Sorry if this is an obvious question but - you're aware that the pins on the J304 are in a different order to those on the 2N3819 originally used in the KM84?

View attachment 127929

im using the J305 by interFET and going off this diagram...

 

[skyy7]

Is the trimmer ok? What voltage(s) can you measure on the wiper? And might be worth replacing the JFET too, "just in case

When I tested the resistance between the terminals outside the build, it seemed fine as it would go from ~0 ohms to 1MOhm. I am also getting 0 volts at the wiper.

 

[Khron]

When I tested the resistance between the terminals outside the build, it seemed fine as it would go from ~0 ohms to 1MOhm. I am also getting 0 volts at the wiper.

... But in-circuit, you're still measuring 2.4V on the source pin of the JFET?

im using the J305 by interFET and going off this diagram... View attachment 127940

Especially if you have more than one of those J305's, might be worth turning it around, "just in case"? Fumbles in datasheets are not necessarily unheard of. And what would be the point of messing with the pinout of a known part number, on purpose, despite calling it the same, y'know?

 

[skyy7]

... But in-circuit, you're still measuring 2.4V on the source pin of the JFET?



Especially if you have more than one of those J305's, might be worth turning it around, "just in case"? Fumbles in datasheets are not necessarily unheard of. And what would be the point of messing with the pinout of a known part number, on purpose, despite calling it the same, y'know?

Ive used the J305 in the same configuration in another build and it was fine but ill try flipping it and see what happens...

Im measuring 2.4V at the gate of the JFET (im guessing this value is not accurate cause of the high impedance and the load of the voltmeter dragging down the voltage). Im measuring 0 at the source and ~29V at the drain.

One intresting issue ive noticed is that when checking for continuitiy between ground and the source pin I get a quick little beep that won't return everytime I touch the probe onto the source pin. Could this mean I may have a ground short?

 

[jp8]

One intresting issue ive noticed is that when checking for continuitiy between ground and the source pin I get a quick little beep that won't return everytime I touch the probe onto the source pin. Could this mean I may have a ground short?

Nope, no real short. It's C7 being charged by the meter, so briefly "shorting" the meter.

Jan

 

[jp8]

What kind of electret capsule are you using? With or without built-in JFET?

Jan

 

[Khron]

Im measuring 2.4V at the gate of the JFET (im guessing this value is not accurate cause of the high impedance and the load of the voltmeter dragging down the voltage). Im measuring 0 at the source and ~29V at the drain.

I have an issue where there is 0 volts at the source, 2.35v at the gate, and 28v at the drain. The bias trim does not change the voltage at the drain or source. Otherwise, the voltages are correct at every other point in the circuit. I am getting 29v before R1.

Pick one; both can't be simultaneously true, so which is it?

 

[skyy7]

What kind of electret capsule are you using? With or without built-in JFET?

Jan

Without a built in FET... the JLI3412

Pick one; both can't be simultaneously true, so which is it?

I believe I said the same thing in both posts... just to confirm i am getting 2.35v at the gate, 0v at the source, and 28v at the drain. The first one was more of an estimate from my memory while the second post I was measuring the fet.

 

[ccaudle]

i am getting 2.35v at the gate, 0v at the source,

That should forward bias the gate-source junction, so it should not be possible to get the gate higher than the source by more than about 0.6V or 0.7V. Something is wrong in that circuit, either the pinout is not what you think, or the device is damaged with an open gate-source junction.

 

[skyy7]

That should forward bias the gate-source junction, so it should not be possible to get the gate higher than the source by more than about 0.6V or 0.7V. Something is wrong in that circuit, either the pinout is not what you think, or the device is damaged with an open gate-source junction.

Ill try flipping the jfet and see what happens...

Update: Flipping the JFET causes the output to be just noise. After the drain resistor, it reads 1.6V. And it reads 1.4v at the source resistor.

Also tried a new J305 and it did nothing...