MCA SP1 (MXL603 circuit) noisy (static storm) while "warming up". Where to look?

[777funk]

I have a couple of these and both work pretty well. But one seems to take about 30 minutes of being on with Phantom Power to lose this static storm noise (at least as loud as the signal). It doesn't go away by knocking the mic into my hand like a bad solder joint sometimes may. My guess is a bad electrolytic cap somewhere. Am I on the right track?

 

[Khron]

Got a voltmeter / multimeter?

 

[micolas]

I have a couple of these and both work pretty well. But one seems to take about 30 minutes of being on with Phantom Power to lose this static storm noise (at least as loud as the signal). It doesn't go away by knocking the mic into my hand like a bad solder joint sometimes may. My guess is a bad electrolytic cap somewhere. Am I on the right track?

When you blow into the microphone, do you speak loudly up close, does the signal break?
You should thoroughly clean the pcb and the components at the jFET input, with isopropyl alcohol, but not with cotton wool, lint remains, thin threads stuck, impregnated with residues. Over time, oxides appear that compromise proper operation.
If the cleaning doesn't work, only then move on with other investigations.
Depending on the test with high SPL you can appreciate if the capsule is defective.

 

[777funk]

Got a voltmeter / multimeter?

Yes, I do.

When you blow into the microphone, do you speak loudly up close, does the signal break?
You should thoroughly clean the pcb and the components at the jFET input, with isopropyl alcohol, but not with cotton wool, lint remains, thin threads stuck, impregnated with residues. Over time, oxides appear that compromise proper operation.
If the cleaning doesn't work, only then move on with other investigations.
Depending on the test with high SPL you can appreciate if the capsule is defective.

It doesn't seem to change it by speaking loudly into the mic. I do have another capsule. That may be something to try out as a test.

 

[Khron]

Yes, I do.

You could check the voltage between ground (chassis) and the D3 / C11 / R9 node, see if, by any chance, it ramps up unreasonably slowly (correlated with the low/distorted signal period).

 

[777funk]

You could check the voltage between ground (chassis) and the D3 / C11 / R9 node, see if, by any chance, it ramps up unreasonably slowly (correlated with the low/distorted signal period).

In this case it's actually like a static storm in the background (as opposed to low signal and distorted). I was thinking I had a recording of it and I do somewhere, but can't find it. But sort of like pink noise as loud as the mic signal (vocal, guitar, etc).

 

[Walrus]

I've had that before, turned out to be a contaminated capsule.
After it had stopped, you could make it happen again by breathing on the capsule.

 

[Khron]

Worth sticking the capsule in a Ziploc with a whole bunch of (freshly baked) silicagel for a day or two, then.

Photos of the front of the capsule might be interesting, though?

 

[777funk]

You could check the voltage between ground (chassis) and the D3 / C11 / R9 node, see if, by any chance, it ramps up unreasonably slowly (correlated with the low/distorted signal period).

Interesting. I have two of these mics which makes it easy to swap boards, parts, etc. I put the polarization voltage supply board from a good mic on the problem mic and it fixed the issue. I then tried the transistor from one to the other. Still no good. I swapped the electrolytic cap. Still no good. And what I should have done first instead of guessing... I measured across R9 on a good working unit vs the bad unit. There's about a 4-5V drop on R9. On the bad unit, R9 has about a 20-22V drop. I'm going to measure the resistor itself to see if it's in spec. I suppose further down the line more stuff could be a voltage divider that isn't working correctly.

EDIT: Oddly, it was the 22nF cap C9 that was the issue. It measured OL with a multimeter on resistance. But... I disconnected the wire running to the capsule and it didn't change anything. The resistor measured ok (within 10% of 1M ohms), so I replaced C9 with a new 0.022uF and no more problems.

I even changed the capsule in the process. I had one I had ordered several months back and wanted to try anyways, so that wasn't entirely a waste. I'm very surprised the little film cap failed.

Thanks Khron for pointing me in the right direction on the polarization supply!

 

[Khron]

I even changed the capsule in the process. I had one I had ordered several months back and wanted to try anyways, so that wasn't entirely a waste. I'm very surprised the little film cap failed.

Thanks Khron for pointing me in the right direction on the polarization supply!

That's pretty rare indeed, for a film cap to fail. But great that you got it working again

 

[777funk]

The old and the new. It's actually C12 and R12 on this mic (same circuit different markings).

 

[Khron]

The old and the new.

That's actually a ceramic cap, and that's a lot LESS surprising to have failed.

 

[777funk]

That's actually a ceramic cap, and that's a lot LESS surprising to have failed.

Ahh OK! I'm used to those looking like the other ceramic caps (left side of the picture). Didn't know this was a ceramic. Thanks for the correction! What I replaced it with is a film cap. Hopefully it won't fail. Pretty sure it's a 50v Panasonic ecq. May be a little close to the polarizrion voltage.

 

[777funk]

It didn't show any leakage with a meter on ohms. I'm guessing that the only way to check the cap would be leakage at the voltage it was running at?

This would make it a little difficult to troubleshoot on the PCB. Are there any other tricks of the trade for a case like this?

 

[Khron]

Are there any other tricks of the trade for a case like this?

First commandment of troubleshooting: Thou Shalt Measure Voltages

Other than that, ceramic capacitors, as you've just discovered, can be iffy - surface-mount ones more often than leaded, so depending on the value and role in the circuit, some can be "replace on sight".