500 series mic preamp repair help wanted (Lakeland)

500 series mic preamp repair help wanted 1

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Hey there. I have a Fredenstein F609 500 Series Tube Microphone Preamp that acted wonky when I bought it used and now only partially works. The DI on the front of the pre-amp works fine, but using the XLR port on the back of my 500 series lunchbox does not work with it - it just emits a high squeal when turned up. It's not the lunchbox itself though.
My little brother, who lives in Germany, recently visited and he knows how to check things with the meter and even do the solder work if needed for fixes. He looked at it for me but didn't want to get into it more than checking things with a meter because there is a lot of glue holding things together and he has no schematics for understanding what is supposed to go to what.

He had this to share:
There are 4 diodes in it (D3, D4, D5, and D6) as they are behaving differently and it may be possible that the two middle ones need replacing. D3 and D6 have resistance in one direction (opposing directions), but D4 and D5 show no resistance in either direction. He's not sure if this normal or if this indicates that D4 and D5 need to be replaced.
Anyway - if you are someone who can tackle something like this, please let me know.

post id: 7881758472

posted: 2025-09-15 16:34

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500 series mic preamp repair help wanted - wanted - by owner - sale...

Hey there. I have a Fredenstein F609 500 Series Tube Microphone Preamp that acted wonky when I bought it used and now only partially works. The DI on the front of the pre-amp works fine, but using...