Electronics > Beginners

Microphone Circuit troubleshooting?

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Rango:
Hi guys. So i picked up Condenser Microphone online from guitar center. Bought xlr cable, hooked it up to audio interface with 48v phantom power and mic is barely working. XLR cable is new but i paid $5 for it. Maybe it's a cable but doubt it. It moved cable around and no changes in noise or anything. The mic inside looks like new and unmolested from any mods.

It's picking up audio but it's like 5-10% of what it should and gain is on 100% and white noise is huge as gain is so high. It is also making high pitch "oscillation noise".

So basically something in circuit is not working. It's actually very good mic. First production was in 2004 so around that time is the date of manufacturer, so it's ~ 15 yrs old. I know electrolytic capacitors go out somewhere around ~20 yrs?

I looked for schematic but i can't find it anywhere. Any idea where should i start with my DMM? Is this worth the hassle or should i return it?

I can probably replace every single part when i order from mouser and it will be less then $10 as most of the time those caps are like 0.30$. Bad Transformer? Thoughts?

Circuit seems simple with not a lot of parts so i'm wondering if there something obvious i can check with DMM?

I can't find anything below $100 that's it's decent. This has same response curve as $300 Rode NT1 circuit and arguably is it's clone and definitely sound like it when check it out on youtube.

http://recordinghacks.com/microphones/M-Audio/Nova

Here is few pics of the circuit. Any help where to start or any advice would be greatly appreciated.









Shock:
The back side of the PCB looks very messy.

First time setup with an old secondhand mic and cheap cable? Why don't you take your mic, cable and mixer/preamp to the nearest professional audio store, ones that do mics/amps, audio hire and make cables etc.

Ask what else you need to get it working and otherwise play dumb. Let them test it out on their proper high gain pricey gear and you will know exactly where your problem is.

Otherwise start testing resistors, caps and wires for continuity etc or send it back. But don't mess with it and then return it, that is not how it works.

Rango:
Well, their store is not far away from me. I can take my gear there and say mic is not working for me. If faulty i will lose $15 shipping which is 1/3 price of the mic which was $30.

I guess that's what i get from buying a cat in the bag. I just didn't expect that from guitar center. I mean that doesn't even happen to me on ebay.

It's probably one of the components which would be cheap fix from mouser. I just dunno where to start.

Other thought process of mine is maybe this mic is not worth all the hassle but the shell itself is probably worth $30. I dunno.

I reached out to the mic guy from California that fixes those mics if he can share schematic but he said he couldn't since he's authorized dealer of them.

ZaphodBeeblebrox:
Looks like a class-A amplifier (only one transistor, probably a FET).  So while it may look as a lot of components, it's probably not a very complicated schematic. The first thing to do is to reverse-engineer the schematic of that board, especially around the transistor. Then you know which component is which and what to measure where. The thing I mostly do with these kinds of things is to "follow" the signal with a scope. First you probe the input (the diaphragm), and then you just go to the "next" component/stage and you expect to see a gain greater or equal than one relative to the previous stage. To have the slightest clue on where to probe: you need that rudimentary schematic :-)

Now, judging from the statement that the mic is old: measure all electrolytic capacitors with a multimeter set to resistance (mind polarity please). Chances are you will find a short (or some low-ohmic similar value). The nice thing is that you do not need to remove the caps from the circuit to do this. If you find a suspicious cap, desolder it and measure it outside of the circuit using the same method. Still a short? You got your faulty component right there. No longer a short: look for other components between the same net and judge (based on the schematic) if this is to expected or not.

Note: if you find one such dried-up capacitor, my advise is to simply replace them all. Save yourself the effort of heving to do the same fix to deifferent components over time :-)

Rango:

--- Quote from: ZaphodBeeblebrox on January 04, 2020, 12:04:55 pm ---Looks like a class-A amplifier (only one transistor, probably a FET).  So while it may look as a lot of components, it's probably not a very complicated schematic. The first thing to do is to reverse-engineer the schematic of that board, especially around the transistor. Then you know which component is which and what to measure where. The thing I mostly do with these kinds of things is to "follow" the signal with a scope. First you probe the input (the diaphragm), and then you just go to the "next" component/stage and you expect to see a gain greater or equal than one relative to the previous stage. To have the slightest clue on where to probe: you need that rudimentary schematic :-)

Now, judging from the statement that the mic is old: measure all electrolytic capacitors with a multimeter set to resistance (mind polarity please). Chances are you will find a short (or some low-ohmic similar value). The nice thing is that you do not need to remove the caps from the circuit to do this. If you find a suspicious cap, desolder it and measure it outside of the circuit using the same method. Still a short? You got your faulty component right there. No longer a short: look for other components between the same net and judge (based on the schematic) if this is to expected or not.

Note: if you find one such dried-up capacitor, my advise is to simply replace them all. Save yourself the effort of heving to do the same fix to deifferent components over time :-)

--- End quote ---

Thank you very much. I appreciate your help.

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