I found a good deal on a pair of used Sontronics STC-S1's and now it turns out that maybe the deal was not so sweet

One of the mic's has extreme self noise. Here is an example of the same (silent) audio recorded though identical settings on both mic's. The gain is turned up to max on my Audient ID14 sound card.

The good mic: https://drive.google.com/file/d/1UwD...ew?usp=sharing

The bad mic:
https://drive.google.com/file/d/1mHY...ew?usp=sharing

Supposedly the mic's had just been lying dorment the last 10 years and had not been used much before that.

I've tried to switch the capsules but the hissing was the same on the same mic. So the capsule seems ok to me.

All the switches seem to work fine and do what they should.

What could be wrong, and do you think whatever is wrong can be fixed? Or is it just a write off and a reminder to be more vigilant testing a used mic before purchasing it?

High vibrational greetings
Vilhelm from Denmark

If the capsules are indentical and ok
then I would suspect either a FET , capacitor or a bad joint. In that order.

You could start by swapping the FETs ( one by one ) from one mics' electronics to the other and see if the problem moves too.

Thanks. That a great idea. I will open them up and see if I believe I can do anything like that attempt that the next time I encounter this problem. The guy I bought them from has agreed to take them back.

I have been in contact with Sontronics, and they offered to inspect both microphones at their facility in the UK.

They have offered to switch the faulty PCB if that is indeed where the problem lies, and give them both a cleaning. All free of charge. I only have to pay for shipping.

That is pretty good service on a pair of 15 yo mics that I bought 2. hand.