Sennheiser MD431 electronics repair?

[Whoops]

Hi Whoopsy ,
If those two mics broken mics need a new home I'd be happy to take them for experimentation .

They’re on their way to Panman in Switzerland to be fixed.
He is the only person in the World that I trust Dynamic capsules to be repaired

But thank you so much anyway to offer to help in cleaning my house from audio gadgets,
my wife would be quite happy if I got rid of most of it

 

[MicMaven]

So addressing your question of course by a "known good" mic you need to have a reason to trust it's a good item to do the comparison with. .... Hope this helps MicMaven

Ah ... yes. Excellent reply. All very sensible and logical.

And yet . . . ahem . . . er . . . (he typed with a wry smile) - none of that would be admissible in Court !

FUN further reply:

And yet all of that STILL begs the question of what ACTUALLY IS a valid and proper "reference" microphone to compare the suspect models to. Without a true and confirmed reference unit we can only guess. Perhaps we would have to send them ALL to Sennheiser to compared them ALL to the MASTER MODEL it keeps in a hermetically sealed jar in a secret underground vault at a Swiss bank to really verify whether ANY of them sound the way they are supposed to sound!


OK . . . OK . . . enough fun - In truth I DO accept your reply as sensible and authoritative, even if it would not be admissible in Court ! I all seriousness, thank you for the uptake and story about your experience. - James -

 

[Whoops]

It's easier of course with a mic model that you know very well how it sounds.
I don't know the 431 that well, but I know the 421 very very well, on certain instruments it's quite easy for me to know if the mic is good or not without even any comparison/reference.

Extreme Low end loss is also very easy to notice, it's not a subtle thing, and this happens a lot in the MD421 and the MD409/BF509 (AKG D12 also)

I know you wrote with humor involved, it's cool mate.

But comparing mics it's a thing that as an Engineer I have to do many times, sometimes when I go into a new studio, other times with my own mics because I notice something is off in one of them, so I just wanted to share my experience in case it's useful for someone.

 

[MicMaven]

I know you wrote with humor involved, it's cool mate.

But comparing mics it's a thing that as an Engineer I have to do many times, sometimes when I go into a new studio, other times with my own mics because I notice something is off in one of them, so I just wanted to share my experience in case it's useful for someone.

Right - on all counts. It really IS about the extent of one's experience.

With less experience than many, I am more likely to misjudge such matters and occasionally require assistance assessing microphones. For example, over the past few months I purchased three condenser microphones, none of which seemed quite right because they each had very low signal levels. With guidance form several members of this group, including some one-on-one assistance from metalockpick and Chrisfromthepast, I determined a Shure KSM44 and KSM27 were both physically ill, requiring surgical intervention beyond my skill set. Based on that experience I was able to determine the third microphone, a less expensive Sanson CL7 acquired for just $15, is, in fact, OK and works as it should - it simply has much lower output than other models - which is, I suppose, why my mixer has both GAIN and LEVEL controls!

As you observe, while I am still learning how these things work, you and the other guys would have diagnosed the case for each model right away without discussion. (Fortunately, I do not feel terribly inadequate, as I have my own field of expertise!)

Parenthetically, I appreciate all of the helpful guidance I have received you and the others in this group. James /K8JHR

 

[raketenmann]

Out of curiosity, can you measure the DC resistance of the capsule and verify the value of the resistor across the XLR. I read 10k for the latter but I'm not sure.

Well well - so since you're all going crazy about offtopic stuff around that mic, I put mine back on to the workbench. Well, as I said I can't do scientific tests, but the mic with or without electronics actually sound pretty damn the same Of course, it seems to be a little differenc somewhere in the midrange, but definitely no crazy gain, lowend or highend difference.

But - in the end this actually doesn't help much, since I can't tell if this is the way it needs to be or if the electronics doesn't work correctly, hah!

 

[RoadrunnerOZ]

Yes true, this needs to be corrected. Where is the switch/ reed relay?

Maybe that shunt is the switch in the mic off position - might make a hell of a pop when engaged/disengaged Definitely something wrong in that schematic.
Also I’d be looking at how good those capacitors are - they look like tantalums - may have died from old age.

 

[RoadrunnerOZ]

The On/Off switch it's just completely stupid,
it makes no sense in a Live Sound situation to have a switch that can be turned off by mistake or fail in the middle of the show.
At least they could do 2 versions, with and without the switch like Shure does.

The switch is lockable in the on positron

 

[Whoops]

For example, over the past few months I purchased three condenser microphones, none of which seemed quite right because they each had very low signal levels.

As you observe, while I am still learning how these things work, you and the other guys would have diagnosed the case for each model right away without discussion. (Fortunately, I do not feel terribly inadequate, as I have my own field of expertise!)

In that scenario that you describe I don't think I would be able to "diagnose" the problem right away as those are mics that I have no previous experience with.
I can do it with mics that I know well, but if it's a mic I never used then I don't know it's stock frequency response or output level, I have no idea
So in that case I needed to have other mics from the same model to compare, otherwise I couldn't know. (a "known good mic"... )

 

[RoadrunnerOZ]

I have used these mics before for live, not in the studio. As a good all-rounder with a different response curve to a 58 it was a fallback for vocalists who didn’t suit a 58 but needed a handheld. Stores still sell these so it would be worth taking your mic(s) and comparing to a new stock mic.
I’m wondering looking at the hand drawn schematic if someone did a trace on the board and drew it up from that - the short circuit across one of the chokes would possibly have been the switch in the off position if that’s how the off works. Also looking at double wound chokes maybe they’re out of phase to prevent pickup of noise/hum from overhead lighting etc - like a humbucker pickup.

 

[Tubetec]

Measuring the response of the filter in the 431 might be instructive Whoops ,
or anyone else with the same mic might be interested in doing it .

Its does seem likely the inductors are in a symetrical arrangement ,
they look very much like common mode RF chokes ,
theres lots of off the shelf options with those components nowadays ,

Applying the MD431 technique to a Shure 58 makes a lot of sense ,
, creating a tiny sub pcb to fit inside the mic itself looks tricky ,
, keeping the mic stock in any case is more useful ,
Id envisage everything mounted up in a DI box type enclosure ,
switchable HPF, attenuation and mute switch ,
a broad cut to the mids ,like Rossi did, could likewise be arranged on a switch ,

In another thread I mentioned the Sennheiser MZA-14 powersupply ,
trimming out the phantom power part of the circuit leaves you with a nice active LPF and pad ,
the only extra tweak is getting that circuit to run off externally supplied phantom power .

 

[Tubetec]

https://coutant.org/md421u4/
A few extra details of the Md421 ,
some versions had a selectable roll off filter , via a switch on the base .
Its does indeed state that all the components are symetrical , along with a humbucking compensation coil .

 

[Khron]

Its does indeed state that all the components are symetrical

It "only" says it's "symmetrically wired", which i'm pretty sure means balanced output (as opposed to single-ended).

 

[Whoops]

A few extra details of the Md421 ,
some versions had a selectable roll off filter , via a switch on the base .

All MD421 versions had and have a Low Cut filter, except one, it’s the 421-2 with the big Tuchel connector.

Then there’s the Blackfire version the BF521 which is the same as the MD421 but without low cut filter also.

The first thing I do in a 421 microphone is to completely remove the Lowcut circuitry and bin it. I have no use for it and it’s an hassle when it’s activated by mistake or by normal handling

 

[Philbo King]

Eddie Ciletti (out of Minneapolis, I believe) does restoration & repair of vintage mics. I'm not sure if it makes economic sense; depends on where you are.

 

[RoadrunnerOZ]

It "only" says it's "symmetrically wired", which i'm pretty sure means balanced output (as opposed to single-ended).

Yes - symmetrical wiring is another term for balanced - opposite and equal on the output pins hence symmetrical.
The 431, on further reading, does incorporate hum bucking coils as does the 421.

 

[Christof]

Need some help with electronics repair for a BF 531 (which is the "Blackfire" version of the MD 431). I measured the resistance at the capsule. At first, a fairly high resistance is displayed briefly, then the resistance stabilizes for a moment at about 200-250 ohms and quickly drops to 0 ohms afterwards.

My hope therefore: The capsule should be fine - but something must be defective in the electronics (probably some of the Tantalum capacitors). I would like to repair this myself, since the repair fee at Sennheiser is not exactly low. However, I could not find a service manual and have not yet come across any instruction video or the like on how to access the circuit board.

How to get to it is not clear to me (and I don't want to pull on anything by force, if that is possibly the wrong way). The metal shielding can be slided around, but is so tight that it cannot be easily pulled off. The transparent plastic surround runs over the entire board with its components and I don't know at all how to get that off without damaging anything. I would have liked to replace the four tantalum capacitors, but I just can't get to them.

So I'm searching for any helpful advice from experienced users, a reference to the service manual or anything else that can help with the repair.

 

[RoadrunnerOZ]

Need some help with electronics repair for a BF 531 (which is the "Blackfire" version of the MD 431). I measured the resistance at the capsule. At first, a fairly high resistance is displayed briefly, then the resistance stabilizes for a moment at about 200-250 ohms and quickly drops to 0 ohms afterwards.

My hope therefore: The capsule should be fine - but something must be defective in the electronics (probably some of the Tantalum capacitors). I would like to repair this myself, since the repair fee at Sennheiser is not exactly low. However, I could not find a service manual and have not yet come across any instruction video or the like on how to access the circuit board.

How to get to it is not clear to me (and I don't want to pull on anything by force, if that is possibly the wrong way). The metal shielding can be slided around, but is so tight that it cannot be easily pulled off. The transparent plastic surround runs over the entire board with its components and I don't know at all how to get that off without damaging anything. I would have liked to replace the four tantalum capacitors, but I just can't get to them.

So I'm searching for any helpful advice from experienced users, a reference to the service manual or anything else that can help with the repair.

From the look of the photo the plastic shroud rotates anticlockwise as you pull it away - some better photos of the tabs on the shroud would help. If the mic has an On/Off switch this operates a magnetic Reed switch which shorts out pins 2 and 3 to mute the mic - I think with the case and switch mech off the Reed switch will be closed, meaning off.
1: where are you measuring the capsule
2: are you sure your meter is not showing 0L (zero load = open circuit) as opposed to 0Ω = short circuit.
You need to measure the capsule out of circuit. Or try putting a magnet next to the Reed switch (right at the front of your second photo embedded in the plastic) then measure.

 

[Christof]

Thanks for the tipp about the plastic shroud. 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. The part above went kind of loose when trying that (where the elastic mount of the capsule is fitted). It seems to be just pressed together loosely.

I've been measuring the capsule directly at the contacts of the red and blue wires at the capsule according to the advice found in another forum: With the basket unscrewed, measure the resistance on the red and blue wires leading to the microphone head. The capsule has about 250 ohms. If you measure nothing, i.e. open, the capsule is defective. If you measure short circuit, i.e. 0 Ohm, the switch is closed (mic off). If you measure about 250 ohms, the defect is between the capsule and the connector.

This advice does only make sense without the necessity to diconnect the capsule from the circuit.

The meter has not been showing zero load / open circuit. The reed switch is inactive when no magnet is close as the mic has definately been constructed to be ON when the external magnet-switch is unscrewed from the microphone.

 

[rock soderstrom]

If the mic has an On/Off switch this operates a magnetic Reed switch which shorts out pins 2 and 3 to mute the mic - I think with the case and switch mech off the Reed switch will be closed, meaning off.

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.

 

[Whoops]

Need some help with electronics repair for a BF 531

So I'm searching for any helpful advice from experienced users, a reference to the service manual or anything else that can help with the repair.

In your post you didn't described what is the actual problem of the microphone?

Does it work? Does it output sound? is it sounding bad?