[Co]

Hi JP,
My MP573 started to develop some strange behavior. After working normally for a couple of minutes, the gain starts to become unstable, going down and up again a few dB. Then after ca. 46min the gain drops considerably. The gain then very slowly goes up again.
When I use the gain switch (48min), the preamp starts working normally again, but the problem accurs again after a couple of minutes.
This problem alway occurs, no matter, what gain I set and no matter, if the EQ is in or out.
The file from the screenshot was recorded with a digital 100Hz sine.
It would be great, if you could help me with this.

[Co]

I just the following:
I also have a second MP573/EQ573 combo in another rack, that workes without problems. I switched them out, so the combo, that was in the IGS rack before is now in the API rack and vice versa, so I can rule out, if the power supply might cause the problem.
I repeated the test and now bothe units work perfectly. I have no explanation for this.
Do you have any clue, what could have caused this?

[JPK]

This sounds like a weak solder joint. I would first check things around the power transistor because the changes in time could be caused by the device getting warm.