Shure SM81

[Emmathom]

Hi

I have an issue with my Shure SM81...
A great mic I bought in 1990 and which "lived" with me in West Africa for about 10 years in a very humid country (just to inform > that can maybe help to understand the problem). So humidity aera from 1990 to 2000 and in EU since 2000 and without any issue.

The mic works well unless you move it, but as soon as you (gently !) shake it or move it, it starts to generate noise. This noise stops as soon as the mic becomes still again.
I tried to open it but it's not that easy : the XLR connector is plugged into the board and gets out easily but I don't see how to take the board off the body.

If someone already opened a SM81 could he tell me how to proceed ?
Where could this noise come from ?

Here is a sample of the noise when I "shake" the mic :
SM81 noise

Regards

 

[k brown]

 

[jp8]

Happy New Year to everyone!

Apparently, several iterations of the Shure SM81 were made, each with a different schematic. So, there is no such thing as "the" schematic.

Given the age, electrolytic or tantalum caps may have become leaky. But why that would only happen and cause problems when moving the SM81, I don't know. Furthermore, under the influence of prolonged high humidity and high temperatures, dendrites may have formed on the PCB, causing intermittent shorts.

Once you manage to open the microphone, can you share detailed, high-resolution pictures of both sides of the PCBA?

Jan

 

[Emmathom]

Happy New Year to everyone!

Apparently, several iterations of the Shure SM81 were made, each with a different schematic. So, there is no such thing as "the" schematic.

Given the age, electrolytic or tantalum caps may have become leaky. But why that would only happen and cause problems when moving the SM81, I don't know. Furthermore, under the influence of prolonged high humidity and high temperatures, dendrites may have formed on the PCB, causing intermittent shorts.

Once you manage to open the microphone, can you share detailed, high-resolution pictures of both sides of the PCBA?

Jan

Maybe start by cleaning inside the tube (ground contact), XLR's pins (which plug in the pre board)... Well, one more time, cleaning with isoprop

 

[Emmathom]

Hi Folks

I opened the mic, clean all I could. The mic is silent unless you shake it (but what's the goal for shaking a mic ?)
I mean when it's motionless there's no noise...
BUT...
when the high-pass (first level) is engaged there's no more noise even when I shake it !
I should maybe investigate in that way : do you have an idea ?

Here are picts of both sides. A plastic shell protects the board so one cannot see it clearly

 

[pmfalcman]

when the high-pass (first level) is engaged there's no more noise even when I shake it !
I should maybe investigate in that way : do you have an idea ?

First thing that comes to mind - the switch. Internal part (slider?) is loose, wobbly. When you move the mic it makes intermittent/improper electrical contact => circuit goes wild.
When engaged it's under tension = no more wobbliness.

Hold the mic steady (in a firm holder, a vice*) so it cannot move and touch/wiggle the switch. See Listen what happens.


--------
*) A vice meaning a tool

 

[Ike Zimbel]

Is there a suspension for the capsule? If there is, it could have degraded after 30+ years.

 

[sr1200]

Clean off the contacts the pin of the capsule makes with the circuit.
@pmfalcman mentioned the switch and I second that. I've had the switch cause all sorts of issues in mine to the point i put the little locks in place on mine and use the pad on the preamp if needed.

 

[Emmathom]

First thing that comes to mind - the switch. Internal part (slider?) is loose, wobbly. When you move the mic it makes intermittent/improper electrical contact => circuit goes wild.
When engaged it's under tension = no more wobbliness.

Hold the mic steady (in a firm holder, a vice*) so it cannot move and touch/wiggle the switch. See Listen what happens.

Well this rotary switch DOES make a little "pop" when you select high-pass filters (two levels) and that's normal since you engage something (a capacitor ?) in the circuit...

 

[Emmathom]

Clean off the contacts the pin of the capsule makes with the circuit.
@pmfalcman mentioned the switch and I second that. I've had the switch cause all sorts of issues in mine to the point i put the little locks in place on mine and use the pad on the preamp if needed.

Hum... bad news ! because I want to fix it to be able to resell it...
I'v cleaned carefully the contacts of this rotary switch with isoprop. on soldering side : isn't there another solution except lock it in place ? (and except buy a new switch)

 

[sr1200]

The switch makes noise in brand new mics when you go from 0 to -10 (its not a gradual pad, its just 0 or -10, on or off) Not a great design. Theres always a little bit of a pop (nothing major though). You can try some deoxit on the switch. The pin to the capsule was more my concern since your pic made it look like there was some major oxidation going on up there.

 

[Emmathom]

Sorry I mixed up my mind : the pad is not the issue. The issue is about the high-pass filter which operates with a rotary switch (60Hz / 100Hz more or less I guess)

 

[pmfalcman]

Well this rotary switch DOES make a little "pop" when you select high-pass filters (two levels) and that's normal since you engage something (a capacitor ?) in the circuit...

You mean an audible POP, right? Understandable and it shouldn't be the problem.
What I meant is: there are contact springs (flat pieces of steel) in the switch. In the disengaged position there is no contact (no tension on the spring) so the slider or the rotary pin is loose. It may make the contacts touch but they shouldn't. This messes up the circuit currents or creates wrong connections. When engaged the spring exerts pressure, closes the circuit properly, holds the rotary pin tight and everything's fine.

Quite common ocurrence in simple, low quality switches. Even new ones.

If the switch can be disassebled (I doubt it) this flat spring could be bent out a bit so as to exert pressure on the outer casing and hold the pin tight.
But I suspect the switch is too small.

 

[jp8]

I still suspect an electrolytic or tantalum coupling capacitor to be leaky. When you turn on the high pass, the electrolytic cap is detached from the circuit and audio only passes a small film or ceramic coupling cap. That's how many HPFs are made in condenser mics.

 

[Emmathom]

You mean an audible POP, right? Understandable and it shouldn't be the problem.
What I meant is: there are contact springs (flat pieces of steel) in the switch. In the disengaged position there is no contact (no tension on the spring) so the slider or the rotary pin is loose. It may make the contacts touch but they shouldn't. This messes up the circuit currents or creates wrong connections. When engaged the spring exerts pressure, closes the circuit properly, holds the rotary pin tight and everything's fine.

Quite common ocurrence in simple, low quality switches. Even new ones.

If the switch can be disassebled (I doubt it) this flat spring could be bent out a bit so as to exert pressure on the outer casing and hold the pin tight.
But I suspect the switch is too small.

Moving the switch (plastic hood appart), pressuring it does not fix the issue.
So I would follow @jp8 opnion and suspect a capacitor which is bypassed when the high-pass is on...
I Must open again the mic tomorrow and see if I can do something, but a leaking cap is a **** issue (desolder, get the right one for replacement, resolder...)

 

[jp8]

Good luck!

 

[pmfalcman]

Moving the switch (plastic hood appart), pressuring it does not fix the issue.

Good! One probable cause eliminated
I guess it's easier to find a cap replacement than such a switch

 

[emrr]

I have an MKH435 that generates noise with motion, no switches on it. Haven’t had a look yet. Also fine when still.

 

[jp8]

I have an MKH435 that generates noise with motion, no switches on it. Haven’t had a look yet. Also fine when still.

Same noise as posted by @Emmathom in his first post?

“Noise” is such a broad concept that simply mentioning the word is not enough to determine the root cause.

 

[Emmathom]

Here is a sample of the noise when I "shake" the mic :
SM81 noise