Production BP1 is Microphonic

[Don Allen]

I have a Production Intercom BP1 beltpack that is microphonic. If you touch it or wiggle the headset connector it makes scratching and other loud noises. I have tried different headsets, same result. I have replaced the single and dual opamps, checked for dry solder joints, replaced XLR rivets with m3 screws so I can use star washers for good case earthing, cannot find cause ? Any suggestions ? I thought the microphonics might be semiconductor shot noise ? I have the BP1 schematic from a previous post, but it is not accurate for the pcb No.90005 Rev3 I have, TR1 is J174 P channel mosfet, TR2-4 are @N3417.

[DrV]

I'm not familiar with the particular unit - is it surface mount? Ceramic caps are a strong candidate. Also contamination trapped under SMT components can be a problem. If you can operate the device with the PCB exposed, very careful and precise prodding with a plastic knitting needle or similar is one way of narrowing down the area of interest.

 

Edit: had a problem a bit like that on a wireless belt pack once which turned out to be an invisible break in one of the wires to the jack. It had been soldered over the insulation and only had about one strand making contact.

Edited November 1, 2021 by DrV

[Don Allen]

The components are through hole. I replaced all of the electrolytic capacitors and the open circuit board is finally quiet to touch, but when I put the circuit board back into it's case, it was unstable, so that is tomorrows problem to fix. What is curious is that all of the capacitors tested close to nominal value using my Peak LCR tester, but that is testing at a low voltage, not the 24V working voltage of the BP1.