1

MP573 Microphone preamplifier / Re: MP573 weird behavior (no volume after 20)

« 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.

2

MP573 Microphone preamplifier / Re: MP573 weird behavior (no volume after 20)

« on: August 27, 2025, 02:47:45 PM »

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

3

MP573 Microphone preamplifier / Re: MP573 weird behavior (no volume after 20)

« 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.

4

MP573 Microphone preamplifier / Re: MP573 weird behavior (no volume after 20)

« 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)

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MP573 Microphone preamplifier / Re: MP573 weird behavior (no volume after 20)

« 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

6

MP573 Microphone preamplifier / MP573 weird behavior (no volume after 20)

« 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.

7

MP573 Microphone preamplifier / Repairing a MP573 with no sound

« on: July 15, 2025, 04:14:33 PM »

Hi, 

I have a MP573 on my bench (version 1.3). It worked good for years and now it powers up but doesnt have sound.

Voltages are correct as per the setup and assembly instructions.
No shorts.

I notice the Relay was acting weird and somewhat faulty and ordered a new one (not installed yet). Resistance on the coil is varying a lot.

Meanwhile I injected a signal into the pin 7 of the input transformer and get sound normally, but if i inject into pin 2 I get no sound. 

Resistances on the transformer (installed):
2-4: 24 ohms
3-5: 22 MEG (definetely weird)
7-9: 126 ohms
8-10: 127 ohms


Resistances on the transformer (not installed):
2-4: 24 ohms
3-5: open (dead for sure)
7-9: 136 ohms
8-10: 137 ohms

I think this transformer is cooked. Is there another way to be 100% sure?