Hello,

A while back I purchased a used MTG MV203 measurement mic to use as comparison/testing for any of my future projects. After researching the MTG documentation I built a rack-mounted power supply/preamp to go with the mic. Everything was working fine (PSU functioned nicely & super clean/healthy output from the mic). I moved the PSU into a new enclosure to make things more portable and started having issues with the mic. Sometimes I would get no output, sometimes very distorted, sometimes functioning perfectly. After thoroughly troubleshooting my PSU I found no issues with it at all, which led me down the harrowing road of cracking the mic body open and prodding around with my DMM.

Here are pictures of the internals and my best shot at the schematic. Output was disconnected to verify the cable was not causing issues. Had to replace a 6k8 resistor that took flight when I was desoldering the adjacent mystery component.?

I used the component codes, DMM, and educated guesses to determine most of the components and I plan on measuring the unlabeled caps.

I'm fairly confident the High Z section under the resin is two 10G resistors in series and a 0.2 pF cap (input impedance is specified at 20G and 0.2 pF). However, I am not sure what the black mystery components are. Intuition tells me they are resistors, but they must be on the order of >>10M because my DMM cannot read them. I will have to carefully remove them and employ something like to measure them on a separate board.?

Luckily I had also taken a look inside while it was working. I know from my first measurements that, when functioning, the output sits at around 65V DC offset. It now sits around 40V DC (sometimes as low as 24V) and I measure no signal regardless of sound source.?

Prime suspect is the 1uF tantalum cap as I understand they are prone to damage and considered somewhat unreliable. The dual diodes, MOSFET body diodes (not sure I understand why they are used here), and Both BJTs measure ok with the DMM diode mode.

I'm posting this here mostly for general interest, but if anyone has suggestions on troubleshooting the circuit I would greatly appreciate any insight.?


? ?

Just got done measuring the blank resistors. They form a simple 2x100Mohm voltage divider with 10Gohm feeding the divided voltage to the rest of the circuit. Here is the updated schematic with all component values.