1

MP573 Microphone preamplifier / Re: DI not working, noise

« on: June 21, 2022, 04:39:51 AM »

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!

2

MP573 Microphone preamplifier / Re: DI not working, noise

« on: June 18, 2022, 11:40:10 PM »

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.

3

MP573 Microphone preamplifier / Re: DI not working, noise

« on: June 16, 2022, 02:54:35 PM »

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

4

MP573 Microphone preamplifier / Re: DI not working, noise

« on: June 16, 2022, 03:49:37 AM »

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?

5

MP573 Microphone preamplifier / DI not working, noise

« on: June 12, 2022, 07:49:01 PM »

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

6

LA502 Optical Compressor / Re: 1vac on output

« on: June 10, 2022, 01:46:02 AM »

I did in fact swap the pots. Good to go now. Thank you.

7

LA502 Optical Compressor / Re: 1vac on output

« on: June 09, 2022, 04:12:18 AM »

P3, P4 & P5 CCW 20 turns: yes.

U6 removed AC voltage 0v to tp2:
0~10.93vac

U6 inserted AC voltage 0v to tp2:
0~1.79vac

DC voltage 0v to tp3: 0~1.13v

8

LA502 Optical Compressor / Re: 1vac on output

« on: June 08, 2022, 04:12:39 PM »

The output voltage will only go down to 2.3vac with the peak knob all the way up.

9

LA502 Optical Compressor / Re: 1vac on output

« on: June 07, 2022, 02:20:46 PM »

Update: I had my led’s backwards. After reversing, able to dial in the -3db led, but due to not being able to reduce the voltage down to 1vac or less, the -20db led cannot be calibrated. 

10

LA502 Optical Compressor / Re: 1vac on output

« on: June 07, 2022, 03:44:44 AM »

You need to separate the issues.
First, without connecting the LA502 yet, setup a sine source on one output XLR of your rack. Check that you can measure it with your AC voltmeter.
When done, you can plug the LA502 in (with the extender) and connect the working source to the analog input of the compressor.

Pin 2&3 of my xlr source read 1.0vac, & .98vac when running through the LA502. Gain fully clockwise reads 26vac, and when set to 15vac the peak knob will not reduce down to less than 1vac. It will only reduce the voltage down to 2.3vac with the peak knob fully clockwise. I tried to calibrate the led’s anyway (revision 1.4) and no -3db light after gain @ 15vac and peak set to 10.6 vac. Could this be because of the peak setting being unable to reduce the current to less than 1vac & if so, where do I start?