Beyerdynamic mc930 SDC repair

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Lowfreq

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May 28, 2005
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570
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Christchurch, New Zealand
Hey all,

I've had a pair of mc930s that I've used for the best part of a decade now, and one of them has started to lose volume compared to the other. I've attached a pic from tests of sending pink noise to the mics to show the difference.

They've been well looked after, with no real knocks that I can recall, and stored in a nice dry environment.
Tonewise, both mics still sound the same, so I don't know if I'm safe in assuming the capsule is still good.

I checked that I was getting signal on both pin 2 & 3 of the mic going into my preamps, thinking that I could've had a bad sold joint losing half of the balanced signal, but I'm still getting signal on both pin 2 & 3, but still considerably less than the good mic.
I even swapped the pcb into the other mic, and the problem followed the pcb.

I gave the connector to the capsule, and the hi impedance are a good clean with isopropyl, and that suddenly made the mic work again perfectly, but for only 10 minutes.
It then made a popping, rumbling sound and then I was back to where I started with low output on the mic again. 

Maybe I could send it to be fixed somewhere, but here in New Zealand our options are limited (and expensive).

I have the service manual which has the schematic, but it looks like a lot of the parts they used are now obsolete.
I've built a few tube mics, but I haven't really dived into sdc or fets at all yet (let alone smd components), since what I already had worked until recently.

Has anyone dealt with one of these before? It's the working for 10 mins and then popping and rumbling back into a broken mic that's baffling me.

Steve
 

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emrr

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Apr 12, 2006
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NC, USA
That sounds like a connection problem to me.  I think of a mic I have with a body that completes the ground path, some of it screws together and it sometimes requires adjustment of the screw on grill to get full level without noise.  Consider any areas that might occur. 
 

Khron

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Apr 8, 2010
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Makes me wonder how different the circuitry is from the MC830 / Opus 83. I bought one with some issues (iirc) a while ago, which came good after a couple of capacitor replacements.
 

Lowfreq

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May 28, 2005
Messages
570
Location
Christchurch, New Zealand
Apart from cleaning it, I’ve focused on the receptacle that connects to the capsule. (Pic attached) It seems quite flexible, which could be hindering it making a good connection. It could be flexing before it locks in to the capsule pin.

The capsule seems fixed with the mc930 so I can’t unscrew it or attack the problem from the top, so it’s been more about guestimate of lining up the receptacle before sliding the pcb back in.

I’ve shifted it’s positions a little bit with a small screwdriver a couple of times. At one stage we seemed to be fading between full volume and broken volume every 20 seconds or so.

At the moment I’ve got it where I think the connection is good.
It’s been on all afternoon, and it’s had no volume drops at all. I don’t know if I’m confident enough to use it on a session yet though.

I’ve double checked screws to the body, and I’m getting good continuity there.

It’ll be interesting to turn it back on tomorrow and see if both mics are still behaving themselves.
 

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phonofranz

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Nov 5, 2013
Messages
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Location
Italy
Did you solve the noise in the end?
My experience with mc930s is quite frustrating and related to similar issues.

I've got a couple from Thomann in 2017 and was really pleased with sound and lack of noise. One out of two suddenly started crackling/popping and being noisy after a year or so. Thomann asked for some recording proving the issue and then Beyerdynamic swapped it with a new one under warranty and re-tested the other one too (got two new frequency plots and sens tests).

In 2019 I got the same issues again with the new microphone, the one that has been swapped. Again: they've swapped it with a new one and re-tested the other. This time they didn't asked me for any proof of the issue, they just approved the RMA and send me the pre-paid post in no time.
I was pissed for the double fault but pleased with the service from Thomann and Beyerdynamic, even if they made me think they had a few returns.

Now: yesterday again the other microphone, the one that never acted up before, got extremely noisy, a pink/brown broad noise without crackling. Two years from the last swap has passed and I guess they won't respond this time (I will try tomorrow tho).

I've opened the mic and checked for screws/chassis contact with pin 1. Checked/cleaned capsule connection. Noise still there unchanged.

EDIT: I've just noticed that the noise floor keep the same level switching the pad.

3 GOhm resistor? Capacitors? Any component except bigger caps is under a thick coating.
PCB, capsule connector and bigger capacitors looks very different than the drawings in the service manual. Definetely a newer revision.
Capsule receptacle is soldered directly on the pcb (no stand-offs, no flexibility). Capacitors are on small plastic bases and looks similar to those used on motherboards....

I will try to swap the pcb between the two microphones too. Any other tip in troubleshooting?

Note: microphones have been used in studio and classical theatres, never in opened air and never been dropped.... bad batch from Beyrdynamic?
 
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