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MP573 Microphone preamplifier / Re: MP573 low signal level

« on: April 21, 2024, 11:17:51 PM »

What I've done (in order):
- resoldered the transformer wires
- reflowed the switches
- reflowed the output transformer connector (CN2)
- tested if it works
- reflowed the gain pot
- reflowed the trim pot

Sound went from normal to a few seconds to cutting out but coming back when I played with the rotary/gain switch. After that I had stable volume and quality for about an hour but after I shut the whole lunchbox down for ~25 minutes it's back at silent levels untill I give it a super loud level at around +60 db gain.

Im gonna look at the transformer values/dc resistance tomorrow if I have the time.

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MP573 Microphone preamplifier / Re: MP573 low signal level

« on: April 19, 2024, 11:00:31 PM »

A 2,58
B 4,09
C 22,3
D 13,3
E 1,99
F 12,7
G 0,92
H 4,14
I 0,338
J 3,56
K 22,8

signal trace
input transformer pin 7: 0,88
tp1: 4,795
output transformer yel: multimeter cant properly read the value and changes between 114v, 0v, 7,4v and 14v

also for the signal trace test i set the volume to reach 0,88vac at input transformer pin 7, incase that is wrong please clarify for me (as you might have guessed im new to this "magnitude" of a project). at this level the front plate led was constantly red. i was able to hear the signal dignificantly more silent than before and especially with less low end

for the test i disconnected the DI board and put on the jumper at jmp3

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MP573 Microphone preamplifier / Re: MP573 low signal level

« on: April 19, 2024, 07:55:04 PM »

"Did you connect a device to the input which could have some DC volts on its outputs ?"

I have two possible ideas:
- I could have been stupid enough to spill some solder on it by accident and not notice
- I could have plugged in a cable with phantom power on in the "out" of the connected XLR

Pins are all cut short and I did the initial measuring "short circuit test" of the setup guide without finding any, did it again before posting here.

I'll ofcourse look again after disassembling it.

Thank you in advance!

4

SK25/SK99 Discrete Op-Amp / Re: R10 Burnt on

« on: April 19, 2024, 07:00:29 PM »

Sorry for posting in this old thread. 

Exactly the same has happened to me... except the resistors were fine and I was able to swap the transistors only. Nevertheless I'm going to build a new SK25 and swap it out plus the transistors so they are sure to be fine. This was completely on me aswell. The documentation literally says which transistor to put in but well... my only additional suggestion would be to print it in bold and red?

Thank you JPK!

5

MP573 Microphone preamplifier / MP573 low signal level

« on: April 19, 2024, 03:54:46 PM »

Hi there,

I've just recently built myself an MP573 and it has been working fine for about two weeks. For the last 1-2 days the sound level kept changing at the start but stabilized short after that (like 2-3 mins). But since after I pulled out the module and reinstalled it I have constant low volume and close to no low end.

I've searched for other topics to "no low end" and therefore checked the Input transformer resistance on the 2/3 pin of the XLR and my values are in the kOhms - way to high for the expected 48 ohms that should be expected afaik. At the same time I noticed, that one of the PCB-connection pads seems to be pretty damaged. Could this be the reason? If so is this repairable or have I damaged it beyond repair?

VTB9045 is at:
2-4: 23,3 ohm
3-5: 23,3 ohm
7-9: 130 ohm
8-10: 130 ohm

VTB1845
yellow-orange: 6,4 ohm
red-brown: 5,2 ohm
green-blue: 19,8 ohm
violet-black: 19,8 ohm

Would have measured the dc voltages aswell but the referred image of the post https://www.soundskulptor.com/forum/index.php?topic=5.0 is missing.

Again - it worked for 2 weeks flawlessly and sounded awesome. the only thing i redid was the solders to from the vtb1845 wires to the little connector board since my solder job wasn't that good.

Best,
Thomas