I have a broken MD 421 that simply doesnt put out any sound/voltage, thus most likely its a ground issue (i think). Now with everything ive been able to access so far the connections seem to be fine, except that the third terminal which i guess should be ground (im not really sure about the wiring of the humbucking coil so maybe its just floating or something else) is not connected to ground. Does anyone here have experience with this?
MD421 Ground Disconnected
- Thread starter mmmmm
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Help Support GroupDIY Audio Forum:
Rob Flinn
Well-known member
Unlikely to be a ground issue, since the ground isn't part of the audio signal. I would verify that the capsule isn't open circuit with a meter. Connections looking good doesn't verify anything, using a meter is the way to go.
I did check the connections with a meter ofc. in what way is ground not part of the audio signal? No voltage without a defined reference
... Except for differential sources / inputs... Ethernet wouldn't work, if that assumption was true, among other things.
Rob Flinn
Well-known member
In a dynamic mic the ground is just to connect to the body of the mic to screen it. It has no connection to the audio signal. Why don't you see if you can download a circuit diagram for the mic, it may help you understand how it's hooked up.
its a bit later now, didnt have any time the past few days. But finding a circuit diagram is essentially why i came here in the first place. Maybe im searching at the wrong places since microphone repair is new to me. I am aware of how differential amplifiers and signals work, but even then they have a relative ground. Maybe there is just something im missing rn
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Rob Flinn
Well-known member
You are basing your understanding of a balanced interface on one circuit with an op amp. The op amp needs the ground reference to work. If you connect 2 pieces of balanced gear together you do not need to have the grounds connected between the two pieces of gear (unless it's a mic) because the hot of the balanced signal references to the cold & vice versa. If you use a transformer to balanced input there is not a definite requirement for a connection from the primary or secondary to ground. If you look at the attached diagram for an MD421 you will notice pin 1 is NOT connected to the audio circuit, it just is a screen. You should research balanced interfacing, not how to make a balanced input with an op amp.
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Rob Flinn
Well-known member
The circuit is the same despite the connector. i.e no connection between ground and the audio.
I don't know if this helps but the mic has an audio +, an audio -, and a shield /ground. The audio signal is extracted differentially from the audio + relative to the audio -.
JR
JR
Depending on the type we have , direct leads from voicecoil to the connector , a filter or a transformer , we have different measurements to the connector.
With direct connection , the voltage or impedance from the voicecoil , can be measured on the connector.
With a broken filter or transformer not...
With direct connection , the voltage or impedance from the voicecoil , can be measured on the connector.
With a broken filter or transformer not...
allright thank you! Ive been wondering about this for a while, but now it makes sense. A bit annoying how it is often wrongly explained in the audio world... Hm now im wondering how that works in the case of active preamplification within the microphone.
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so there is no connection between the two outputs. Voltage wise i just measure some noise... To be honest im not sure how to proceed with the troubleshooting from here on.
"No connection", as in, your multimeter shows "OL" / out-of-range, regardless which pair of pins you're measuring?
no my MM is crappy so i can only measure continuity via the resistance. So the resistance between the pins is either higher then the max value it can measure (i think 10Meg) or open circuit.
garp
Well-known member
In the case of the 421 there could be multiple sources for the trouble (the transformer, the switch for low cut etc ..) the best is to disassemble the mic and measure the coils directly at the two wires of the coil capsule . In case of a good reading, you can simply remove all the stuff (transformer switch) and still have a great mic that will perform perfectly.
panman
Well-known member
Just measure the resistance between the lugs on the side(not the middle, but where the red wire is connected and the "third"). Should be around 210 ohms. Could be more or less. If no reading, the voice-coil is broken. The humbucking - you can separately measure between the middle(blue wire) and the right-lug("the third"), which you obviously did..
