[davidfloegel]

Hi Jean-Pierre,

I've just finished my second MP573 and it went together a lot smoother than the first!

The only issue I can't seem to work out is my LED. At first I had it the wrong way around (or so I think), so it only ever started glowing red but only at really high gains.

Now, it seems to at least go green first and then red, but I need quite a bit of input gain + trim to get it to come on.

I've checked all resistors that I thought are relevant (R48 - R48) and they have the right values.
I've checked the Diodes D6, D7, D9 for orientation.
I also checked the solder points for the U3 (I believe this is controlling the LEDs?) and they all look good too.

Would love some pointers on where to look.

Thanks
David

[JPK]

Can you report all the DC voltages on the 14 pins of U3, against 0V, with no input signal?

[davidfloegel]

Yes, see attached image! This is looking down onto the component.

Had my red lead on the V0 test pin and used black to go through the pins

[JPK]

U3 pin 1 : -1.31V...-2V   Doesn't seem correct.
Check the direction and solder joints of D6.
Check the solder joints of U3 pins 1, 2, 3.
Check D6 with the DMM.

[davidfloegel]

U3 pin 1 : -1.31V...-2V  Doesn't seem correct.
Check the direction and solder joints of D6.
Check the solder joints of U3 pins 1, 2, 3.
Check D6 with the DMM.

checked solder joints - all good but resoldered anyways
direction of d6 - all good
value of D6 with DMM in diode test mode - 680

Retested values but Pin 1 still goes up and down in-between the previous volt ranges

[JPK]

Compare the diode test of D6 with D7. Are they the same?
Try replacing D6.

[davidfloegel]

Tested all of the diodes Diodes and compared them to my working unit - the readings are the same. Is there a point replacing them if the readings are the same?

[JPK]

Is there a point replacing them if the readings are the same?
No.

With an input signal of 100mVAC, 1kHz, Gain=20dB, Trim=+10, Z=1200.
What AC voltage do you get on R50, against 0V ?
R50 left = 0.14VAC
R50 right = 0.57VAC

[davidfloegel]

I will test this next week as I'm traveling now.

What's the best way to generate the signal? I do not have a signal generator other than my phone. Could I do
Phone -> Microphone -> Preamp and measure?

[JPK]

There are plugins for that.
Also https://www.soundskulptor.com/en/content/12-download-useful-links

[davidfloegel]

Ok I've just tried testing it..

I've played the sine wave on a loop and put an Audix I5 in front of it. I can see the LED light up green and I'm getting:

~0.1 on the left side (towards the controls)
~0.4 on the right side (towards the back side)


Side note: 
My Q6 is getting pretty warm right away, I rested my finger on it while measuring. It's not so hot I can't touch it, just very warm.

[JPK]

With an input signal of 1kHz, Gain=10dB, Trim=-10, Z=1200.
What AC input voltage do you need to light up the green LED?
- Normally about 140mVAC (measured with the preamp connected)

With an input signal of 1kHz, Gain=10dB, Trim=+10, Z=1200.
What AC input voltage do you need to light up the red LED?
- Normally about 1.4VAC (measured with the preamp connected)


It is normal that Q6 gets warm. It dissipates around 1.25W.

[davidfloegel]

Hmm I'm not sure how to best measure this.

With an input signal of 1kHz, Gain=10dB, Trim=-10, Z=1200.
What AC input voltage do you need to light up the green LED?
- Normally about 140mVAC (measured with the preamp connected)

I can't actually get the signal loud enough through my speakers to trigger the LED at all.

With an input signal of 1kHz, Gain=10dB, Trim=+10, Z=1200.
What AC input voltage do you need to light up the red LED?
- Normally about 1.4VAC (measured with the preamp connected)

I can get it to go green, but I can't get the signal loud enough through my speakers for it to go red.


Are there other ways I can feed in a signal?

[JPK]

You must not go through a mic but instead connect a balanced cable between an output of your converter and the preamp mic input.
Then send a sine signal to this output and measure the amplitude with the DMM on AC Volts connected between pins 2&3 of the XLR. The measurement must be done while the output is connected to the pre input.

[davidfloegel]

Ah I see, that makes more sense. 

I’m sorry if this is a dumb question but at which points do I measure the voltage? If the XLR is plugged in to the preamp while sending a signal, I’m not sure where to measure. 

I hope that makes sense!

Thanks for your patience!

[JPK]

It does make sense!
What we do is remove the cover of the XLR plug to access the pins and connect the meter to them.