[Zane219]

Hi there,

I bought a pair of MP573's for my first ever build, 1st one worked perfectly after completion (and I LOVE it!), 2nd one I am having some issues with.
The followed the setup guide and the preamp passed all the setup tests I performed, but when I went to plug it in and do a mic test there is a loud noise every time I click the gain switch above 20, and a constant noise (sort of heartbeat like) that is repeating.
I did a pretty thorough inspection of the board and didn't see anything that looked wrong (although this is my first build so I may have missed something).
I've attached a video of the issue as well.
Any help would be appreciated 

https://youtu.be/i3mMxraCNIg

[JPK]

This sounds a lot like a reversed electrolytic or tantalum cap (in particular C14, C20, C22, C24).
Also check again without the first MP573 plugged, in case the lunch box has problems powering 2 units.

[Zane219]

Thanks for the reply JPK.
I had checked the unit without the first MP573 initially, and in a different lunchbox and still had the issues.
All the electrolytic caps are in the right way after a check, I am not sure exactly how to check the tantalum caps because they are soldered in so no longer have the longer leads indicating the positive lead?

[Zane219]

I think I may have figured it out, when googling the 22u-16V tantalum caps (c20, c22) they look something like this (pictured below), and although the + sign is pointing down towards the shorter leg, the longer leg is the positive side if I am not mistaken? So I may have these 2 reversed.
If this is the case my next question is, are the caps salvageable or having run voltage through them in reverse do I need to replace them both?

Cheers,
Zane 
 

[Zane219]

Actually I just checked all the orientation's vs. the first preamp that is fully functional and everything in the 2nd pre has the correct orientation! very confusing...

[JPK]

although the + sign is pointing down towards the shorter leg, the longer leg is the positive side if I am not mistaken?
This picture is wrong. I have always considered that the longest lead is the positive and I will continue. I don't think our caps have this ambiguity.

The pop noises when switching are always caused by DC voltage in the signal path.
Check if you can measure any DC (millivolts) on T1 pin 7 or TP1, against TP2, at high gain.

[Zane219]

The DC voltage sits stable at 10 and 20 db gain but any above that (which is where the noise starts happening) the voltage fluctuates up and down pretty wildly.
Here's a video again to show what's happening

https://youtube.com/shorts/z2TgfdUHMdQ?feature=share

[JPK]

The DC voltage sits stable at 10 and 20 db gain
Where are you measuring this voltage?

on T1 pin 7 or TP1, against TP2 it should be 0VDC.

[Zane219]

I was measuring from TP1 to TP2, definitely getting more than 0VDC

[JPK]

There seems to be an issue with either C22 or C24. One or both might be reversed.
Does the DC change at different positions of the +/-10dB gain pot?

[Zane219]

I've attached a picture of the 1st MP573 that worked perfectly against the 2nd MP573 that is having issues, both C22 and C24 are in the same orientation on each board.

There is a change in DC when the +/-10dB gain pot is moved.

[JPK]

Do you have a 10 to 100uF, 16V minimum, electrolytic capacitor laying around (no need for a tantalum) to substitute to C24 then to C22 to check if it makes a difference?

[Zane219]

I don’t but could grab a couple from my local electronics store. 
Would these do the trick?
https://www.jaycar.co.nz/100uf-16vdc-low-esr-electrolytic-capacitor/p/RE6310?pos=7&queryId=4524723df3b51ce50a983cbf7318aff3&sort=relevance

[JPK]

This one would be even closer :
https://www.jaycar.co.nz/22uf-25vdc-electrolytic-rb-capacitor/p/RE6090

[Zane219]

Thanks, I will grab a couple tomorrow morning. 
So should I replace C22 with one of these new caps, test, and then replace C24and test again?

[JPK]

Exactly. see if it makes a change. In such a case the tantalum cap would be defective.

[Zane219]

Ok so I grabbed 2 new Electrolytic Caps, (ended up getting what you said but 22uF but 50VDC instead of 22uF 25VDC, cos they've didn't have the latter in stock but the guy said they would be interchangeable).
Still getting the same issues! 
Still getting more than 0VDC between TP1 and TP2, and still getting that cycling popping noise with the switch at 30db and above....
Very confused!

[JPK]

The issue was not caused by the tantalum caps.
If you remove C24 completely, do you still have DC on TP1 (against TP2) in the 10 and 20dB positions?
Do the DC voltage change with the gain trim position?

[Zane219]

Still DC voltage in the 10 and 20 positions with C24 removed, but the trim knob doesn’t change the DC

[JPK]

This is not normal.
Look for an unwanted solder bridge somewhere. Look on the rotary switch pins, on the input transformer daughterboard pins.
Or maybe a forgotten solder joint?

With the gain switch on 10, remove the jumper on CN1 and test the center pin and the right pin for DC against TP2. On which pin is the DC?
Is the DC value changing when switching between 10 and 20?

[thewisestudio@yahoo.fr]

Hi,

I have exactly the same problem on one of 2 MP573 that I just finished to build.
I ordered an XT500 to check the DC voltage like "In case your MP573 doesn't work as expected
" with more flexibility than inside my lunchbox (Cranborne)
I tested transfos, there are ok. All seems in the good place when I compare with one that work.
How did you manage to solve the problem?
thanks

[Zane219]

Hi,

I have exactly the same problem on one of 2 MP573 that I just finished to build.
I ordered an XT500 to check the DC voltage like "In case your MP573 doesn't work as expected
" with more flexibility than inside my lunchbox (Cranborne)
I tested transfos, there are ok. All seems in the good place when I compare with one that work.
How did you manage to solve the problem?
thanks

Hi, I haven't actually solved the problem. Have been crazy busy so haven't got around to more testing but hopefully will soon.

[thewisestudio@yahoo.fr]

Hi Zane219, Thank's for your answer, I wish you to find the issue.