the t. Bone sc 400 - broken

[front247]

Hello
I have been using this microphone for 3 years without any failures, the sound quality is quite good for my needs. Well, 2 days ago there was a microphone failure but I can't locate it. The symptoms are that when I turn on the 48V phantom voltage, the microphone works perfectly for about 2-3 minutes, after which it slowly quiets down by 20-30 decibels and a constant noise increases by about 55dB... the microphone then still transmits the signal but the quality is terrible and it is very quiet... after disconnecting it from the power supply for a few minutes, the situation repeats itself in the same way. As a preventive measure, I replaced all the capacitors on the boards in the hope that one of them had fallen, but it didn't change anything except maybe the sound which, when the mic works within 2-3 minutes, is better and the microphone is quieter. I have no experience with such circuits... what could be the reason, does anyone have any ideas on how to check? Thanks

 

[cyrano]

I'd guess the capsule could be faulty?

Disconnect it and inspect it closely. Replace it with a small cap (50-100 pF) and test again. If the problem is gone, it's the capsule. Obviously, there will be no sound, but you can usually tell the circuit is OK by the absence of noise.

 

[NoisyIndividual]

Try breathing gently onto the capsule through the grille. A faulty capsule will straight away make all sorts of noises and/or go dead.

A good capsule will pass this test unaffected.

 

[front247]

Well, I'll do the test today, I understand that I should solder a 100pF capacitor instead of the capsule? I opened the basket and the capsule looks fine, I didn't notice anything strange, but there is another symptom: a knocking sound and a single clicking sound when the microphone goes silent. I thought maybe one of the transistors might be damaged? There are 5551 5401 and fet 2sk170BL... I could buy them and replace them, but I don't know if it makes sense because it's a very cheap microphone. Ok, I'll test it as you wrote. |thank you

 

[front247]

Try breathing gently onto the capsule through the grille. A faulty capsule will straight away make all sorts of noises and/or go dead.

A good capsule will pass this test unaffected.

I did the test 3 times. Each time you can hear beautiful breathing, clear and noise-free for 2-3 minutes, then the volume drops and loud noises appear.

 

[Khron]

I'd rather start suspecting the oscillator board, in that case.

 

[front247]

I soldered a 100pF capacitor instead of the capsule and... there is still noise and crackling from the system.So replace all transistors? The only problem is getting the sk170bl, I can buy the rest.

 

[micolas]

I soldered a 100pF capacitor instead of the capsule and... there is still noise and crackling from the system.So replace all transistors? The only problem is getting the sk170bl, I can buy the rest.

Did you clean the high impedance areas very well?
Especially in the jFET gate area.
Over time, oxides and dirt may appear

 

[cyrano]

I soldered a 100pF capacitor instead of the capsule and... there is still noise and crackling from the system.So replace all transistors? The only problem is getting the sk170bl, I can buy the rest.

That should rule out the capsule as the source of your problem. Like Micolas said, cleaning the board should be the next step. You can leave the 100 pF in place so you don't ruin the capsule.