[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