Sound Skulptor forums

Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: mbogner on March 08, 2021, 07:27:33 PM

Title: Input gain problem
Post by: mbogner on March 08, 2021, 07:27:33 PM

Hey,

I just finished building the MP573 and all the testing went well.
When I came to the part of pluging the mix there is one problem:

I only get signal when the gain knob is on 10 or 20 dB. For all the other settings there's nothing.
Alls the other switches work though.
I checked all the solder joints of the rotary switch and reworked some, but still have the same problem.

Any idea what it could be?

Thanks, Malte

Title: Re: Input gain problem
Post by: JPK on March 10, 2021, 03:12:16 PM

Something wrong in the first amp stage around Q1, Q2, Q3.
Check the voltages A to F : https://www.soundskulptor.com/forum/index.php?topic=5.msg5#msg5

Title: Re: Input gain problem
Post by: mbogner on March 17, 2021, 05:37:20 PM

Thanks for the reply.

Here's the voltages I checked:

A: 0V
B: 0,63V
C: 22V
D: 21,92V
E: 3,2V
F: 21,25V


As I see it only C is correct.

Title: Re: Input gain problem
Post by: JPK on March 18, 2021, 11:29:08 AM

You must check the values, solder joints and direction (if relevant) of R29...R38, C14...C23, Q1...Q3.
There must be something wrong here.