Can you check out my analog mic pre and repair?

Lakeland MN

Description

Hi. I have a Fredenstein F609 500 Series Tube Microphone Preamp that I just sold and buyer says is now not working. Quite the inconvenience for both of us. Anyway, I used it for ribbon mics and it worked fine when I had it. I plan to give buyer's money back tomorrow but I want to know what happened to my mic pre. Looking for someone who knows how to solve these riddles. Mic pre is valued at $690. If it's an easy fix, and you are interested in it, I will sell it to you at a far better price or if you repair it but don't want it I can just pay that as well if it's reasonable.