Uher M538 and microphone maintenance concepts

[monsieurpr]

Hello
One of the nicest thing with vintage microphones for DIY enthusiasts is that they tend to fail.
And when it occurs you can notice different manufacturers approaches for maintenance.
Here for example, a lovely Uher M538.

This very nice and balanced microphone suddenly sounds like it has a bandpass filter.
For those who don’t know this microphone, it was manufactured by AKG for Uher, using a dual capsule design borrowed from the AKG D200. Visually, the capsule is identical but the M538 lacks the humbucking coil.

Failure may come from one of the capsules or from the cross-over filter (my hope)
I need to open the microphone to check this. That’s where it starts to be interesting.

As on other specimens I could see on internet, I expected to be able to open the casing by removing the metal strips on the sides which I supposed would reveal mounting screws.
On the first picture one can notice the edge of the metal strip on the side of the casing. It covers the junction line of the two-halves of the body.

On the second picture, my M538: capsule is now glued, and the body is a single piece die-cast !
Obviously, at some time, someone decided to cut production costs at the expense of maintenance!
In addition, the high frequencies capsule casing is plastic instead of metal.
As capsules are now unrepairable, I am not that bothered about the glued capsule, but I still wonder about the cross-over filter and have no idea how to remove the big Tuchel connector to tests the electrical components.
Does anyone have an idea? It may be the same as on M537 the body of which looks similar.

And as can be seen on the third picture, there is scoop on the side of my M538 which may host the bass-cut filter switch when the body was used for the M537 (but this is just assumption on my side).
Can anyone help?

For reference, the last picture shows a TD200 capsule (from Reverb). ANother difference is that the TD200 has a larger resonator cavity as on the M538 (from what I can see through the scoop).

 

[monsieurpr]

Hello.
I will answer myself for other M537 or M538 owners. It appears that actually, the capsule is not glued but sealed with wax. Once the wax removed, the capsule could be extracted by firmly pulling. Unfortunately, during this operation, the connectors wires were ripped off as there was little slack.
The picture shows the connector from the inside. No way to remove it, I didn't manage to soften the glue with hot water. If I ever manage to repair the microphone, I will have to cut the connector to install the XLR socket.

 

[monsieurpr]

Here is the M538 capsule. Actually, it is very similar to the AKG D200 but I was a little disconcerted that no cross over filter can be seen. Must be inside the resonator ...
I also particularly appreciate the glued wires at the basis of the resonator...
When slightly taking away the capsule from the resonator, one can only see a small hole (5 mm diameter max) with no possible access.
If I want to access the cross-over filter, I will have to cut the resonator in 2 to see what's inside.
Definitely, This M538 has a no maintenance concept.

 

[volker]

Thanks for the photos. I have one (or two?) of these microphones as well. This will be helpful should I ever have to open them.

 

[panman]

This should help.
https://www.coutant.org/akgd200e/index.html

 

[monsieurpr]

Thank you for the link. Interestingly, the datasheet clearly shows the cross-over filter but it doesn't appear on the part list.
So it is included in a subpart, most probably hidden in the resonator. We need to perform surgery ...

 

[monsieurpr]

Surgical instruments ready...

 

[panman]

Hold on your horses! Check out the capsules first, because there is likely to be the culprit. The capsules on your mic are glued and removing and softening the glue you are likely to get the access to the bass-chamber and the electronics inside. Try vinegar! I don`t see any need for sawing and destroying the bass-chamber. More and better pics from the rear end might help too.

 

[monsieurpr]

You are right: one is never too cautious! I will make sonic tests again before cutting the resonator chamber.
The issue may either come from:
- the capsules,
- the cross-over filter
- or the wiring.
Indeed, when thinking again, I am not totally sure this microphone was fully tested after I converted the tuchel connector to XLR (as I converted several ones at the same time and have many microphones).
I measured the resistance of the capsules and they are in the 200-240 ohm range. As there is the C2 capacitor between the 2 capsules, I would measure an infinite resistance on the capsule if its coil was broken.
The third picture shows the low-frequency capsule membrane (and the back of the high frequency capsule). It doesn't look bad but my experience is that a dynamic capsule can sound horrible with no visual damage.
I will resolder the capsules directly to the XLR connector and test them again. If it still sounds bad I will check what is inside the resonator chamber. anyway, If one capsule is faulty (because of aging of the membrane) it will not be fixable.