Microphone Circuit troubleshooting?  - Page 1
Forum section: electronics / beginners

author: Rango
author_rank: Contributor
timestamp: January 03, 2020, 11:13:36 pm
content: 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/NovaHere is few pics of the circuit. Any help where to start or any advice would be greatly appreciated.

author: Shock
author_rank: Super Contributor
timestamp: January 04, 2020, 03:49:20 am
content: 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.

author: Rango
author_rank: Contributor
timestamp: January 04, 2020, 09:28:09 am
content: 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.

author: ZaphodBeeblebrox
author_rank: Contributor
timestamp: January 04, 2020, 12:04:55 pm
content: 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 :-)

author: Rango
author_rank: Contributor
timestamp: January 05, 2020, 05:52:27 pm
content: Quote from: ZaphodBeeblebrox on January 04, 2020, 12:04:55 pmThank you very much. I appreciate your help.

author: StillTrying
author_rank: Super Contributor
timestamp: January 05, 2020, 06:20:28 pm
content: The images aren't large enough.

author: Rango
author_rank: Contributor
timestamp: January 05, 2020, 06:59:46 pm
content: Quote from: StillTrying on January 05, 2020, 06:20:28 pmHere is 16MP res, my camera max. Was little hard to do macro pic too.fronthttps://flic.kr/p/2iayHoQbackhttps://flic.kr/p/2iaAP3nI just noticed something. D2 and D3 in middle bottom. Are those burned? Are they suppose to look like that?What is that green stuff on D1 by middle top?R4, bottom left looks like it's missing soldering blob but from back it looks soldered. On the back looks few are missing solder blobs but those may be empty slot on the board. Didn't check that but it's assumption.I wouldn't notice ANY of that if i didn't take max size picture. That's crazy.

author: StillTrying
author_rank: Super Contributor
timestamp: January 05, 2020, 08:21:16 pm
content: "D2 and D3 in middle bottom. Are those burned? Are they suppose to look like that?"They look OK-ish to me, measure them with a multimeter."What is that green stuff on D1 by middle top?"Can only see the end of it, do you have any 20,000 x 15,000 120MB images.Any silicon that's electrically close to inputs or outputs such as Q2 has to be suspect because of static discharges getting to them when connecting up. A circuit diagram and some measurements would help.

author: Shock
author_rank: Super Contributor
timestamp: January 05, 2020, 08:51:09 pm
content: Quote from: Rango on January 05, 2020, 06:59:46 pmGermanium diode it's the polarity marking.

author: ciccio
author_rank: Frequent Contributor
timestamp: January 05, 2020, 09:31:06 pm
content: Other posters have given the most simple solution: search for a competent help.In any case, some simple checks can be done...If you have a multimeter, check:1) XLR cable : pin 1 on female goes to pin 1 on male? Pin2 on female goes to pin 2 on male? Pin 3 of Female goes to pin 3 on male ? No pin is shorted to any other?If any of these tests fail, then the cable is defective.2) measure that phantom power is working : you must have a nominal 48 V between pin 2 and pin 3 respect to pin 1 without the mic connected., then connect the mic and measure again, you must have a (generally lower voltage, the same value between pin 2 and pin 1 and pin 3 and pin 1. Make all the measurements on the female end of the cable, directly on the connector's pins (remove the shell).These voltages must not be very lower that the ones you meaured without the mic connected. It they are really lower (e.g. lower than 10-12 V ) then the mic is defective. If one of the two voltages measured with the mic connected is very lower thw the other then the mic is defective.Are you sure that the phantom power you are supplying is correct (48 V nominal) and can source enough current for you mic?Best regards

author: Rango
author_rank: Contributor
timestamp: January 05, 2020, 09:32:57 pm
content: I googled schematic for M-Audio Nova and Sterling ST-51 as it's same circuit to no luck. Guy from Cali wouldn't share it due to affiliation.Not sure where you guys get your schematics from but google was a miss.http://recordinghacks.com/microphones/Sterling/ST51

author: Gregg
author_rank: Super Contributor
timestamp: January 06, 2020, 03:30:27 am
content: Hard to say, but the solder joint on the longer leg of R4 looks dodgy. I would start by resoldering (with good flux and leaded solder) all joints that look suspicious and give the board a good cleaning.

author: konohimawm
author_rank: Newbie
timestamp: January 08, 2020, 03:39:05 pm
content: Cardioid Condenser Microphone need highly regulated power source. It's critical. I think first you should check your power source.Afterwards you should check capacitors and diodes on board.

author: Rango
author_rank: Contributor
timestamp: January 10, 2020, 11:23:33 pm
content: Quote from: konohimawm on January 08, 2020, 03:39:05 pmI have audio interface umc202hd with phantom power 48v and it's on. Do you or any mic expert know what is the function of that gold/copper plating on the capsule itself, right by the crew?The plating itself is missing and it looks like it was there as i see under magnification lamp traces of that plating by the screw but 98% of it it's gone. I wonder maybe that is why this mic it sounds bad? It's working but sounds so bad that its basically unusable.I googled capsules and they have that plating by my mic does not, at least not in any significant amount.Thanks guys for the help.From aliexpress, cheap k87/k47 capsule

author: Shock
author_rank: Super Contributor
timestamp: January 11, 2020, 01:10:23 am
content: Quote from: Rango on January 10, 2020, 11:23:33 pmIf that was the diaphragm then yes it looks completely missing. It's hard to tell in your washed out photo if that was the front or the back. Different models mount the wires either front, back or on the side. But if those screws are on the front, it needs a diaphragm there or it won't work at all. What is on the back, is it just a plate?

author: Rango
author_rank: Contributor
timestamp: January 11, 2020, 03:04:45 am
content: Above is stock photo where u can see gold plating on capsule, where on my mic it is missing. This below is my mic pic where u can see traces of gold plating but it is gone mostly.Question is if capsule/mic will work without that gold/copper plating? And function of this plating?

author: Shock
author_rank: Super Contributor
timestamp: January 11, 2020, 06:44:56 am
content: It's the diaphragm the part you are talking about, I don't know if they are all gold coated or if your one was, but it needs a diaphragm.

author: Rango
author_rank: Contributor
timestamp: January 11, 2020, 05:33:44 pm
content: The diaphragm is there. Its just not gold plated. If u look at the screw you can see bit of gold plating on diaphragm. Thanks for clarification.I can't find the schematic for this mic so i'm not sure what the values should be for what im measuring with DMM and i don't wanna guess as that's not going to get me anywhere, especially me, the beginner.I also touched up with solder that R4 resistor on the leg and after i cleaned board with iso alcohol and toothbrush to get rid of the fiber and any nasty residue on back and front too.No change. This thing still buzzes and very little gain at max audio interface gain. Maybe transformer is out, maybe bad grounding, maybe lost grounding somewhere.For $45 shipped i'm not sure this thing is worth my time but thought maybe it was simple and obvious fix that someone on the board would know. Obviously without schem it is not and lots of guessing which is time wasting.Now if this was a Neumann mic it would be worth the hassle for sure. I actually contemplated on using shell for Neumann build out but ppl are crazy and charge $300 for parts for this mic which i can get form Mouser for $20. Problem i found is i can't buy circuit board, they won't sell as they i will order parts from mouser for $20, literally. If i could buy circuit board for $20-40 this build would be worth it but i can't and i won't pay $300 for clone. hehe. I'm cheap. lol.I've ordered the same mic in excellent condition from reverb, $60 shipped. I made sure with the seller that it's in perfect working condition. Aesthetically looked amazing.This mic sounds like Rode NT1 and has low self noise at fraction of the price so it's excellent mic for the money. I actually found this Nova mic as well as Rode NT1 better sounding then any Neuman mic so there is that too. I don't need anything super expensive for what i will do with it but was interested in cheap good mic i can use for podcast, interviews etc. I'm not a singer. lol.For this faulty one i opened a case with paypal for full refund, including shipping so i guess i'll see what happens with that. Not buying anything again from guitar center.This is really disappointing that they ship broken merchandise. I lose 1/3 of value due to shipping. What's crazy, stuff like that doesn't even happen to me on ebay, let alone huge retailer. I didn't expect that from retailer.