Sound Skulptor forums

Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: camarada78 on August 21, 2025, 10:04:16 PM

Title: MP573 weird behavior (no volume after 20)
Post by: camarada78 on August 21, 2025, 10:04:16 PM

I have a MP573 on my bench that used to work normally.

It then got no sound at all. It was the transformer that had an open tap. I got a new one and that was fixed. 

But now i have a weird problem:

It works normally and with beautiful sound with main knob at 10 or 20 volume, after that it has no sound. From 30 and above it has no sound. if i return to 20, it works normally.

Innitially i thought it was some open resistors, but they appear to be fine. 

Does someone had this issue before?

Thanks,

Have a good day.

Title: Re: MP573 weird behavior (no volume after 20)
Post by: JPK on August 22, 2025, 09:23:31 AM

The problem is located in the first preamp stage, around Q1, Q2, Q3.
Can you report the voltages A to F shown on the image here:
https://www.soundskulptor.com/forum/index.php?topic=5.msg5#msg5

Title: Re: MP573 weird behavior (no volume after 20)
Post by: camarada78 on August 26, 2025, 03:11:56 PM

Thanks for the reply.

My version of the preamp is a little different than the one of the picture.

its the rev 1.3

But, if i measured correct, the voltages are

F = 12v9
A = -9v

Title: Re: MP573 weird behavior (no volume after 20)
Post by: camarada78 on August 26, 2025, 03:21:35 PM

I measured the three transistors with a component tester and they are working fine (q1, q2 and q3)

Title: Re: MP573 weird behavior (no volume after 20)
Post by: camarada78 on August 26, 2025, 06:15:10 PM

I think the TL074 is weird.

Pins 3 and 12 are both with -9v and i dont think this is right.

Title: Re: MP573 weird behavior (no volume after 20)
Post by: JPK on August 27, 2025, 09:50:24 AM

Here are the voltages for the REV 1.3
The voltages are measured against TP3.

Pins 3 and 12 are both with -9v and i dont think this is right.
I think this is not measured against TP3?

Title: Re: MP573 weird behavior (no volume after 20)
Post by: camarada78 on August 27, 2025, 02:47:45 PM

My voltages, in reference to TP3, are as follows:

(https://i.imgur.com/8PA8IEL.png)

Title: Re: MP573 weird behavior (no volume after 20)
Post by: camarada78 on August 27, 2025, 06:05:20 PM

FIXED - it was a bridge solder from a previous fix i've made. 

Thanks a lot for the help. 

have a good day.

Title: Re: MP573 weird behavior (no volume after 20)
Post by: JPK on August 28, 2025, 08:38:56 AM

(https://fonts.gstatic.com/s/e/notoemoji/16.0/1f44d/32.png)