[Nate08]

Setup (0v to V+ : 3.45 megaohms, 0v to V- : 0 ohms) dialed in p2 to 3v, sound check noisy, DI zero signal.

In case your MP573 doesn't work as expected:
Jumper CN1: yes.
Polarity switch: left.
Solder joints: checked with large magnifying glass and hot soldering iron, check.
Reversed components: double checked (as best as I could see with the capacitors)
Flipped IC's or transistors: no
Nothing touching chassis plate

DC Voltages:
A: 2.59
B: 4.08
C: 22.46
D: 13.44
E: 2.01
F: 12.81
G: 0.91
H: 4.14
I: .34
J: 3.56
K: *0.0
L: 2.96

DI Input: Not working

Following the signal with 1vac sine wave:
Input transformer pin#7: .96
TP1: 171.4 mv
Output transformer yellow wire: 0

Verify the transformers DC resistance:

VTB1847:
yellow-orange: 7.8
red-brown: 6.7
blue-green: 21.2
violet-black: 21.2

VTB9045:
2-4: 25.5
3-5: 24.8
7-9: *0.4
8-10: 139.8

[JPK]

Do you confirme this?
VTB9045:
7-9: *0.4
If confirmed,you must look for a short circuit between pins 3&4 of the daughter PCB, near C5.
If there is nothing here, you will need to remove the input transformer, following these instructions, and check again with the transformer disconnected.

DI Input: Not working
Do you hear the DI relay click when you start inserting the DI jack?
Same question when you insert the jumper JMP1?
Check you didn't swap Q7-Q8.

Output transformer yellow wire: 0
Do you get any sound at the pre output?

[Nate08]

I found what appeared to be a short between 9&10 on the daughter board and fixed that. 

Do you confirme this?
VTB9045:
7-9: *0.4 (now 33.2)


DI input clicks and works with high levels of noise. 

Q7 & Q8 are not swapped

Yellow wire is now @ 22.6 

I get noisy signal through XLR, +48v and DI, polarity works and green led lights up.

It seems all is functioning except the input transformer 7-9 resistance is very low. I didn’t think to test the resistance with it removed. I only checked for shorts. Should I go ahead and remove it again and check it?

[JPK]

Yes, 7-9 resistance doesn't look good. It should be similar to 8-10.

[Nate08]

With input transformer removed:
2-4: 26.1
3-5: 25.5
7-9: 150.0
8-10: 146.7

[JPK]

The transformer is good.
The short circuit must be on the connector that links the transformer daughterboard to the main PCB.
Check the resistance between pins 3 and 4 (from left, near C5).

[Nate08]

Resistance dropped there. Ended up finding a solder blob between 3&4 behind the plastic spacer of the right angle header. Replacement header arriving Monday. I will follow up. I feel confident this is the issue even though you had to tell me twice.

[Nate08]

Victory! Everything checks out, sounds great! Next time I’ll be using a very very small tip on my iron. Thank you for walking me through this! I learned so much!

[JPK]

Congratulations!