MD441 Repair Help

[MicDaddy]

Long story short:

Saw an 'As-Is' non-working MD441 on ebay for too low of a price to stay away

Sure enough it showed up dead, no audio at the XLR, dc resistance between pins 2 and 3 showed open.

With some TLC and a tad bit of PFM I've managed to get inside the microphone and tear down the element somewhat.

Here's where I'm at... The voice-coil/humbucker series are showing ~160Ohms. If I wire the two leads (voice-coil/humbucking coil) to XLR pins 2 & 3 it passes audio

Now, what is the best way to get inside of this resonating cavity?

I've emailed Sennheiser and this is what they told me (repair costs holy $^#^)

"Inside the suspension canister is a filter assembly for which we have no diagram. Beware there may be capacitors in the circuit which may fool you as "open" circuits.
Does any audio come out of the red, blue, yellow, green - at all?
The containment does net need to be airtight, so you may ope it any way you can to investigate. .......however I must tell you that in 25 years' experience here I have yet to find an open filter in an MD441.

The capsule / filter / suspension is part 037126 at $536.77"

So, what sonic characteristics am I missing by omitting what looks like a transformer and a passive eq/filter network.....

The element hardwired to the XLR sounds pretty dang good as is, though I'd love to get into that silver can and make this baby fully functional.

Has anyone ever opened the silver can and what did you find?

Thanks for looking
Best Regards



Ben

 

[MicDaddy]

....here's the capsule naked without the passive circuitry  (actually sounds pretty dang good like this)

 

[MicDaddy]

...mysterious (resonator/damping) cavity that I cannot get into with kid gloves.

 

[MicDaddy]

I specifically asked Sennheiser if the cavity/element assembly needed special attention to remain 'airtight' and they responded surprisingly "no".  (I don't know squat about this Senny mic, but my experience with dynamic microphones tells me otherwise) 

There is a tube coming out of the bottom of the element inside of the plastic threaded part, that feeds into the silver can, this had some cotton fiber around it.  I'm tempted to measure resistance with the magnehelic but I don't have a fixture fabricated at this time...

Thanks for looking and for any insight.

 

[Sredna]

If you´re careful, you can separate the aluminum tube and the black plastic part by
twisting and slowly pulling it out.

Be also careful with the PCB that's attatched to the black plug, it's fragile, check it for cracks.
On the PCB you´ll find some tantatums as well, could be a good idea to switch them out.

The filter assembly has with the "bright" switch to do.

The silicone paste on the copper spirals are part of the damping system but annoying to get on finger, tools etc, some plasic foil to cover them is useful.

Hope this helps,

Anders

 

[MicDaddy]

Thanks Anders, I was thinking that was the end to attack from but you have given me some confidence.

I have tried (to no avail), with my hands to pull apart the plastic plug from the aluminum tube, I will apply some warm air and see if it gives any.

Any chance you know what that transformer in there is doing, or is that a large inductor?

Thanks again for the help!!

 

[Sredna]

It could be some glue there but I just carefully twisted the black plug a 1/8 turn or so and then
pulled it out.

Sorry, I was too lazy to trace the circuit and measure the transformers/inductors.

It´s funny that Sennheiser claim not to have the circuit diagram for a product still in production...

My bet is that there´s a crack in the PCB.

 

[MicDaddy]

Nice call Anders, 

You were dead on about the pcb being broken.  How these break inside this assembly is quite the mystery.  (I got it disassembled using the boiling water technique from drtechno over at gearslutz)

C4 is toast, has completely fallen apart, tantalum looks like 33uF 35V.  

 

[MicDaddy]

Here you can see how the pcb has broken into two pieces held together only by 2 resistors and the broken tantalum legs on either side of the broken pcb.

 

[MicDaddy]

..another view of the broken pcb, you can see the tantalum that fell apart. 

 

[kazper]

Since you have it all apart, do some reverse engineering for educational purposes.

To fix it, it may be just as easy to proto a small board. Seems like a common problem in those mics.

I'm guessing they can't handle the drop, and just crack all to hell.

 

[clintrubber]

Sounds like you got the 441 back up & running, right ? Nice! 

Did you happen to compare the 'direct' usage w.r.t. the 'intended' original version with all filtering inserted ?
Or is there not just filtering but also a signal-TX inserted ?

 

 

[MicDaddy]

Currently waiting on another MD441 (dead) to show up and hopefully again the element is in good shape (if not it's coil time)

I'm very buried in projects and children at the moment, figured I'd wait for the second mic just do them both as a batch 'Lean manufacturing' 

On a side note, not having access to pads or acad (only an old orcad running on XP)what (if any) freeware are folks using (OSX) for board layout?  Is there a 'good/friendly' freeware that will create/open gerbers?  Shot in the dark, circuit simulator, monte carlo analysis? 

 

[clintrubber]

Prices of dead MD441 seem to go up these days 

W.r.t. circuit-simulators, SIMETRIX Intro (free) does Monte Carlo:

http://www.simetrix.co.uk/site/demo.html

 

[MicDaddy]

..side note, I was able to pick up SIMetrix and it is running via crossover on my '06 imac 2.16 duo, (smc fancontrol for life support damn GPU) Thanks for that!

I have not been able to compare the response of the element 'hardwired' to the XLR vs with the filter in the signal path.  I should be able to take some sweeps and print some curves showing the differences.  (if I get the filter up and running)

 

[Rossi]

I bought a non-working MD441 a while ago, and I can't get it to sound like it should. It's not the filter, it's the capsule itself. It sounds nasty, very nasal and lacks in bass response. So beware, a non-working MD441 may just be a waste of money - you never know.

On a happier note, I scored a perfectly working, pristine looking MD441 for about 20 Euros on a fleamarket.  ;D

 

[MicDaddy]

Rossi, 20Euros is disgusting  ;D

Do you still have your nasal 441?  Care to venture in dissecting the element?  

Do you have the capacity to capture a power/frequency curve, and if so can you sweep both your nasal and working mics and post or mail the response curves?

I've been digging about for an old patent hoping to find a print or exploded view which would give me a better idea on how to attack the element. (hint hint anyone out there)

 

[Rossi]

I still have the nasal one. I dismantled it again after I saw this thread, seeing some details that were not clear to me before.

I also took the capsule apart. You when you unsolder the moving coil wires and unscrew two tiny screws, you can take apart the cartridge. I was able to make basic experiments, resoldering the wires,and listening while moving the two parts a bit. It did get to sound a bit fuller, when there was a small gap between the two parts. So it seem to be a mechanical problem, but I don't know how to fix it. Apparently the part that goes into the coil is too close to the diaphragm. Another possibility would be that the coil rubbs against the magnets.

I could make measurements, but I don't think it's worth the trouble (my measuring method is quite time consuming). It's not just a little off, the mic sounds way out of spec.

 

[MicDaddy]

Was there a good bounce to the diaphragm?  Check if it has a good range of motion by gently 'bouncing' it.  If you own a microscope take a look, sometimes you can verify if the coil is rubbing within the gap.  If the coil is rubbing or if the diaphragm is stuck or its motion is inhibited it will lose low end.  The coil may have fallen apart as well, I'm curious if it is a self-adhering or if it is wound on a bobbin.

I do not know these elements, would you be so kind to share photos should you tinker around with it again?  Thanks for your input!

 

[Rossi]

I'll take pictues when I open the element again.

It is kind of weird: when you take it apart the way I described, you look at the diaphragm and coil *from below*. The other part is a kind of dome that fits below the center dome of the diaphragm and is very magnetic, of course.