Sennheiser MD431 electronics repair?

[Christof]

The microphone does not output sound.

 

[rock soderstrom]

My hope therefore: The capsule should be fine - but something must be defective in the electronics (probably some of the Tantalum capacitors).

As member @RoadrunnerOZ said before, measure the capsule isolated from the circuit.

The tantals are always suspect but in this application they don't die as fast as in a PSU application. I would still check them all and see if they have continuity. Also test the magnetic switch to see if it still works. I would check the capsule first.

 

[Whoops]

The microphone does not output sound.

Ok,
so the first thing I would do is to test the capsule alone, not connected to the circuit.
If the capsule alone outputs sound, the problem is in the circuit.

If the capsule doesn't output any sound and measures Open, then it's game over,
no need to mess around with the circuit

 

[Christof]

To measure the capsule open I would rather not disconnect the wires directly at the capsule, but the wires on the circuit-board. But again - no access possible without removing the plastic shroud.

 

[RoadrunnerOZ]

I'm not one hundred percent sure, but it's the other way around. You can unscrew the switch completely, then the mic is always on AFAIK.

You may well be right there - best way is to meter the reed and see what it is at “idle”

 

[Whoops]

To measure the capsule open I would rather not disconnect the wires directly at the capsule, but the wires on the circuit-board. But again - no access possible without removing the plastic shroud.

If it’s possible that would be better for sure. The contacts at the capsule where the wires connect might be the same contacts where the thin coil wires are soldered also.
Sonic possible it’s better to remove the wires from the PCB.

Although I’m the past with other mics I unsoldered them at the capsule tabs when I had no other chance

 

[RoadrunnerOZ]

To measure the capsule open I would rather not disconnect the wires directly at the capsule, but the wires on the circuit-board. But again - no access possible without removing the plastic shroud.

Can you post some better photos of the plastic where it fits to the inner casing - that should give a clue as to how it comes apart

 

[Christof]

One earlier reply had to be checked by a moderator: Sennheiser MD431 electronics repair?

In that post I wrote: Having a closer look you can see on the photo that the plastic shroud is clipped on with some kind of "teeth". I tried to loosen those, but they are quite rigid.

I will post some better photos later.

 

[RoadrunnerOZ]

One earlier reply had to be checked by a moderator: Sennheiser MD431 electronics repair?

In that post I wrote: Having a closer look you can see on the photo that the plastic shroud is clipped on with some kind of "teeth". I tried to loosen those, but they are quite rigid.

I will post some better photos later.

From what I can make out it looks like it needs to rotate to clear rather than trying to lift the tabs - they may break - is there a recessed screw - it looks like it in No. 2 photo - in a lot of mics the screw goes in to free the outer casing. Sometimes you need to push in, rotate and then pull - you should be able to see through the acrylic if there’s a keyway for the teeth to follow to remove the casing. I use a head mounted magnifier with a Cree LED light to work on mics and fine SMD components.

 

[RoadrunnerOZ]

To measure the capsule open I would rather not disconnect the wires directly at the capsule, but the wires on the circuit-board. But again - no access possible without removing the plastic shroud.

Just hook a scope across the capsule terminals without disconnecting and see if audio is delivered with the mic sitting on a table and slap the table with a ruler or output a sine wave from a tuner app on your phone next to the mic - if you get a good signal the capsule is ok.

 

[Christof]

Some things to check:

• When measuring the reed-switch-contacts in "default" position (with the magnet unscrewed / housing open) the switch is open.
  As the switch seems to shortcut the output, this should be o.k.
• The magnet provided by the manufacturer was not able not change anything from the outside (only when I did remove the housing).
  Did use a small neodym-magnet to test and you could easily hear the "clicking" sound of the reed-switch.
• With the switch closed, there is no output from the microphone at all. With the switch open, I can hear a hum coming from the mic (without the housing that is to expected; Dave Rat showed how he removed a low level humming by just unscrewing and retightening of the head-basket - so no housing at all is capturing all kind of hums).
• I can't hear anything from the capsule, but though my headphones I could hear myself ticking on different parts of the mic (from the suspension to the output-socket). So I still think, the capsule might not be dead.

I still didn't find out how to get access to the electronic circuit without destroying anything. Sennheiser service replied, they do not have a service-manual available. I can find the service-manual for the 441 and the 421 online - but no source for anything for the 431/531.

The plastic shroud is definately clipped on a small ridge at the capsule side. The wires connecting the capsule to the circuit-board do not have much slack, so I really don't know how to gain access to the circuit board. Would be great, if anybody has already found a solution when repairing another 431/531.

 

[RoadrunnerOZ]

I can't hear anything from the capsule, but though my headphones I could hear myself ticking on different parts of the mic (from the suspension to the output-socket). So I still think, the capsule might not be dead.

If the capsule is open circuit then you will hear contact noise via the circuit. Can you get clip leads onto the capsule terminals and run those to an XLR male plug and plug that into your audio system - that will bypass the internal circuitry and give output straight from the coil - if no output then it would appear to be an open circuit capsule coil.
Can you take a photo of each side at 90deg to the one above?

 

[RoadrunnerOZ]

Any luck?