Sennheiser MKH416T repair

[The Documentary Sound Guy]

I'm trying to repair a Sennheiser MKH416T.

I've been tinkering with it on and off for a few years, and it seems to keep getting worse.
Originally, I stopped using it because turning it upside down (or in certain orientations) would create a loud bass thump in the mic. After checking for loose components and not finding any, I came to the conclusion that what sounded like an acoustic problem was actually electrical. I *think* what was happening is that the wire connecting the capsule had gotten loose. In any case, the wire pulled out extremely easily when I disconnected the capsule. I sent it in to a shop, and it came back without any wire connection at all!

That's all back story. Today I tried to revisit that repair. The back of the capsule looks like this:


The capsule wire originally came out of the tiny hole in the centre. The black plastic "sheath" rotates and screws in and out about 1/8" of an inch ... it stops at both ends and can't be removed by unscrewing. I came to the conclusion that this was a "clamp down" mechanism for creating electrical contact to the capsule without soldering, and my repair attempt reflects this. So, I found suitable bit of wire, stripped the end, inserted it into the hole, and screwed it down. This does appear to create a connection of some kind. Not a strong one, but presumable enough to create electrical connection. I soldered the other end of the wire to what I believe is the correct trace on the circuit board (labelled "1" on the schematic). It seems odd to me that there's no second wire to complete the circuit, but I assume it must come through ground via the chassis. The schematic shows it floating... This image shows the repair; the red wire at the far left is attached to the capsule. Forgive my soldering job ... but electrically I believe it is in the right spot.


So far, so good. I powered it on and got ... nothing. Well, not nothing ... a whole lot of noise. And maybe I can hear my voice very faintly in the background if I raise my voice.

So, maybe I've botched the repair. Maybe I've damaged the capsule in the process. However, I'm inclined to think that a component (or components) on the PCB have gone bad. Why? The schematic shows expected voltages at several points on the circuit board, and they don't measure right. Furthermore, they measure wrong very early in the circuit.

Specifically, the schematic shows these voltages:

• +9.8V @ C15 (measured 11.8V, which is probably unchanged T-Power voltage)
• +7V near solder point 8 (measured 9.43V)
• +0.85V / 0.7VHF @ R11 (measured -.0028VDC, which may be normal; I'm not sure how to interpret the dual voltage spec (I assume HF means high frequency, but I don't understand why a second number is listed), but it's clearly in the AC part of the circuit, so a DC measurement could be meaningless)

Given that the voltages measure high and incorrect early in the circuit, I'm inclined to think something (probably a capacitor) is wrong with the power circuitry.
I'm wondering where I should go from here. I've started reading up on testing capacitors, and every guide I've read says I need to remove the capacitor from the circuit to test it properly. I'm willing to do that if necessary, but sure there are easier and faster ways to begin troubleshooting? I've done simple solder / wiring jobs in the past, but troubleshooting a PCB is new for me. Is there a quick way of identifying a bad component without desoldering each capacitor individually?

I'm also wondering if I should try cleaning the PCB. It's kind of gross looking, with dark, oily blobs / stains on some sections. It's not super obvious in the photos, but it certainly doesn't look like a fresh PCB. Other than the grime, I don't see anything visually wrong, though things are packed pretty tight. I did notice that the wire leads for R9 & R10 seemed to be in contact, so I bent them out so they were no longer touching. I suppose this could point to a cause of failure?

Anyway, I would love some help on what would be a logical next step to narrow down a point of failure. Here's a couple full size shots of each size of the PCB:

 

[The Documentary Sound Guy]

Here's a recording of what the mic sounds like in its current state. I've cranked up the gain to make the noise floor audible. It doesn't sound normal to me ... it's quite a variable floor, with lots of crackle & what sounds like wind noise.

https://download.documentarysoundguy.ca/index.php/s/FAMpxbrDmmJ3sZx

 

[Voyager10]

If the capsule isn't working, the DC levels in the rest of the circuit may be thrown out. You could check by replacing the capsule with a 27pF capacitor and see if the DC levels look normal.

There's a 416T service manual in the Technical Documents forum which gives an adjustment procedure - measure the DC voltage between two test points, which goes +/- 100mV or so as you adjust the coil. The aim is to end up as close to zero as possible.

If this process works with a capacitor in place of the capsule, the electronics are probably fine. If not, you might need a scope to track it down - the 'HF' voltage is an 8MHz RF signal.

 

[The Documentary Sound Guy]

Thanks! That's super helpful. I've got the technical manual, and I was able to look at the test points across the coil. It does appear to be calibrated correctly with the capsule installed. I'll report back what I find using a capacitor in place of the capsule.
One question I have with the capacitor ... the schematic shows the connection to the capsule floating, which confuses me. I assume I'll need to connect the negative leg of the capacitor to ground to get a proper reading?

 

[The Documentary Sound Guy]

Also: Should I assume a broken capsule is unfixable? It's KS15-41 capsule, which I assume I could find in a 'for parts' mic on eBay, but even the parts mics seem to go for a pretty penny, and there's no guaranteeing the capsule would be good either.

 

[Icantthinkofaname]

Also: Should I assume a broken capsule is unfixable? It's KS15-41 capsule, which I assume I could find in a 'for parts' mic on eBay, but even the parts mics seem to go for a pretty penny, and there's no guaranteeing the capsule would be good either.

I believe Sennheiser still services the MKH 416T anyway, so you should have no issues getting a replacement capsule .

 

[The Documentary Sound Guy]

Sadly, they don't. I started messing with it after Sennheiser sent it back to me because it was too old for them to look at. I know they still service their newer 416s, but for the T version, I'm out of luck.

 

[Voyager10]

Also: Should I assume a broken capsule is unfixable?

Funny you should ask... I have an MKH406 with a broken capsule, which has resisted my (so far gentle) attempts to fix it. It certainly isn't easy to disassemble.

(This thread: Reviving Sennheiser MKH capsule?)

If I ever manage to revive it I'll certainly let you know.

 

[Voyager10]

Regarding the capsule connection: the capsule is connected between lead 1 on L1 (where the wire goes) and the mic chassis. On the T-power mics most of the circuit 'floats' between the + and - signal leads, and has no connection to the chassis (i.e. 'ground').

There's a ground trace on the PCB which is connected to the chassis by the brass fastening strips which hold the PCB in place. You can solder the 27pF cap between that trace and the terminal where the capsule wire went.

 

[The Documentary Sound Guy]

Looks like you got farther in dissembling it than I did. I think could get it apart with special tooling; there are pincer holes on the inner section that need to be immobilized so the end section can be screwed off. I tried MacGyvering it with some tweezers, but it's too tight to unscrew.
Given how it broke (by tearing off the wire), I think a short due to metallic debris seems a likely cause, so if I can get it open, I bet I can clean it out.

 

[The Documentary Sound Guy]

Ok, I've had a chance to re-test the DC voltages on the PCB with a 27pF capacitor instead of the capsule.

The new voltages that I measured are:

• +8.94V @ C15 (nominal +9.8V)
• +6.61V near solder point 8 (nominal +7V)

These aren't correct, but perhaps they are correct enough compared to the previous test? They have at least gone in the right direction, and are closer to nominal than before. I think @Voyager10 is correct that the capsule is toast.

My next step would be to measure capacitance of the capsule, but I don't have a tool suitable for that small a value.

I suspect I've damaged the capsule with my tinkering. Now that I've read the repair manual, I've learned what the rotating plastic part is for, and it's not for maintaining electrical connection:

"Take care not to turn the black slotted plastic part at the rear centre of the capsule since it sets the damping of the capsule." Ooops.

Also: "The inner tube must not be turned; the row of holes must always be aligned with the slots of the outer tube" Ooops again ... I did this while trying to figure out how to disassemble the capsule. I did realign the holes.

I think my next step is to see if I can disassemble the capsule further, since if I have any hope of fixing it, I need to resolder it properly.

The learning continues...

 

[The Documentary Sound Guy]

What would be a suitable method for calibrating the damping that I've screwed up? Am I right in assuming this would be a matter of testing the frequency response and adjusting until it's in spec?

 

[Voyager10]

Are you able to get 0V between the A and B test points (by adjusting the coil)?

From memory, with a working capsule it adjusts between +/- 100mV or so, but with my broken capsule it was over 1V at any position.

If you do disassemble the capsule, post pictures, I'd love to see inside.

 

[The Documentary Sound Guy]

When I started, it was at 0V without any calibration needed. I'll check it again once I resolder it
Could be a while though.

 

[wyseacre]

Two things:

Regarding the crackling and thumping... my 816T was worse than your audio sample, and it turned out to be the capsule having a poor connection to the chassis. This was surprising since the capsule seemed very tight, and it's so much metal-to-metal contact, it would appear to be the best electrical connection in the unit. Trying to tighten it further had zero effect. Yet it was sensitive to sideways pressure against the capsule. To verify it, I separately shorted the capsule to the chassis and the crackling vanished. To "fix" it, I put a modest amount of Deoxit D100 on a cotton swab and worked that around the perimeter of the seam between the capsule and chassis, hoping a tiny trace of the fluid might seep in via capillary action. It worked. Then I tried to tighten it further following the Deoxit, and it didn't move perceptibly, but I suspect it might have slipped microscopically. Now I can't cause it to crackle by any means. The problem is gone. I'm just guessing that the cast metal chassis and the machined metal capsule are dissimilar, and that over the decades enough humidity found its way in along the screw threads of the capsule-to-chassis joint to create a semiconductor.

Regarding the hiss... what are you powering the mic with? If it converts P48 to T12 with a single 12V zener diode (as is found in popular circuits on the net) then what you have could effectively be an inline noise injector. A zener diode above a certain voltage (6.3V?) apparently clamps in avalanche mode, forming a prolific noise source. It would be better to have four 3V zeners in series. This could be tamed with filtering, but is it? There is unlikely to be a 1000uF electrolytic inside a cheap P48 to 12T commercially made barrel adapter. Possibly there is a cap with a value that is too low, and a corresponding impedance which is too high, making it inherently defective since the primary reason for getting the adapter would be to use affordable old Sennheisers with their low noise floor.

As a test, see whether the hiss presents when you simply insert your power adapter into the audio chain without any mic attached. I recently ordered a Sennheiser 12TP48 barrel adapter from Adorama, and what I received was a less expensive PSC unit (with the PSC sticker) which turned out to be nothing but a huge noise source. They cheerfully replaced it ("wrong item shipped") but the replacement unit was identical, even to the PSC sticker. The unverified rumor is that Sennheiser USA has been shipping PSC units without even rebadging them (implying this was not Adorama's fault.)

My solution to the hiss was a Sennheiser BP-2 found on eBay for $40, which is powered by two 9V batteries. I haven't looked at its circuit, or taken measurements, but the ridiculously loud hiss has vanished after swapping the BP-2 in.

 

[The Documentary Sound Guy]

Good to know. I was using that PSC barrel adapter, so it's good to know it's not great quality. I'll do future tests using the T-power in my Sound Devices 442. That's a known good source of T-power for me.

 

[wyseacre]

Also... the positional sensitivity you mentioned, holding it in certain orientations causing the cracking and thumping, was exactly how my problem first presented. I'm convinced this was a result of the interference tube sleeve moving and making a varying electrical contact with the mic capsule. This wouldn't matter if the capsule already had a good connection with the chassis.

 

[The Documentary Sound Guy]

I'll definitely clean the threads for electrical connection if I get the capsule working again. In my case, I think the thumping was a symptom of a breaking solder connection causing electrical disconnection, since the wire ultimately disconnected from the capsule, leaving me in the current situation where I have no way to attach the capsule electrically.
I'm crossing my fingers I haven't damaged the capsule more seriously by trying to repair it, but I won't know until I figure out how to open it up and get inside.

 

[Icantthinkofaname]

Two things:

Regarding the crackling and thumping... my 816T was worse than your audio sample, and it turned out to be the capsule having a poor connection to the chassis. This was surprising since the capsule seemed very tight, and it's so much metal-to-metal contact, it would appear to be the best electrical connection in the unit. Trying to tighten it further had zero effect. Yet it was sensitive to sideways pressure against the capsule. To verify it, I separately shorted the capsule to the chassis and the crackling vanished. To "fix" it, I put a modest amount of Deoxit D100 on a cotton swab and worked that around the perimeter of the seam between the capsule and chassis, hoping a tiny trace of the fluid might seep in via capillary action. It worked. Then I tried to tighten it further following the Deoxit, and it didn't move perceptibly, but I suspect it might have slipped microscopically. Now I can't cause it to crackle by any means. The problem is gone. I'm just guessing that the cast metal chassis and the machined metal capsule are dissimilar, and that over the decades enough humidity found its way in along the screw threads of the capsule-to-chassis joint to create a semiconductor.

Regarding the hiss... what are you powering the mic with? If it converts P48 to T12 with a single 12V zener diode (as is found in popular circuits on the net) then what you have could effectively be an inline noise injector. A zener diode above a certain voltage (6.3V?) apparently clamps in avalanche mode, forming a prolific noise source. It would be better to have four 3V zeners in series. This could be tamed with filtering, but is it? There is unlikely to be a 1000uF electrolytic inside a cheap P48 to 12T commercially made barrel adapter. Possibly there is a cap with a value that is too low, and a corresponding impedance which is too high, making it inherently defective since the primary reason for getting the adapter would be to use affordable old Sennheisers with their low noise floor.

As a test, see whether the hiss presents when you simply insert your power adapter into the audio chain without any mic attached. I recently ordered a Sennheiser 12TP48 barrel adapter from Adorama, and what I received was a less expensive PSC unit (with the PSC sticker) which turned out to be nothing but a huge noise source. They cheerfully replaced it ("wrong item shipped") but the replacement unit was identical, even to the PSC sticker. The unverified rumor is that Sennheiser USA has been shipping PSC units without even rebadging them (implying this was not Adorama's fault.)

My solution to the hiss was a Sennheiser BP-2 found on eBay for $40, which is powered by two 9V batteries. I haven't looked at its circuit, or taken measurements, but the ridiculously loud hiss has vanished after swapping the BP-2 in.

I wonder if that's the issue with my 415. Cause I also have the PSC adapter. Cause outside that hiss everything is normal. Sounds phenomenal and it has pretty healthy output and doesn't clip any sooner tham my other location mics.

 

[Icantthinkofaname]

Two things:

Regarding the crackling and thumping... my 816T was worse than your audio sample, and it turned out to be the capsule having a poor connection to the chassis. This was surprising since the capsule seemed very tight, and it's so much metal-to-metal contact, it would appear to be the best electrical connection in the unit. Trying to tighten it further had zero effect. Yet it was sensitive to sideways pressure against the capsule. To verify it, I separately shorted the capsule to the chassis and the crackling vanished. To "fix" it, I put a modest amount of Deoxit D100 on a cotton swab and worked that around the perimeter of the seam between the capsule and chassis, hoping a tiny trace of the fluid might seep in via capillary action. It worked. Then I tried to tighten it further following the Deoxit, and it didn't move perceptibly, but I suspect it might have slipped microscopically. Now I can't cause it to crackle by any means. The problem is gone. I'm just guessing that the cast metal chassis and the machined metal capsule are dissimilar, and that over the decades enough humidity found its way in along the screw threads of the capsule-to-chassis joint to create a semiconductor.

Regarding the hiss... what are you powering the mic with? If it converts P48 to T12 with a single 12V zener diode (as is found in popular circuits on the net) then what you have could effectively be an inline noise injector. A zener diode above a certain voltage (6.3V?) apparently clamps in avalanche mode, forming a prolific noise source. It would be better to have four 3V zeners in series. This could be tamed with filtering, but is it? There is unlikely to be a 1000uF electrolytic inside a cheap P48 to 12T commercially made barrel adapter. Possibly there is a cap with a value that is too low, and a corresponding impedance which is too high, making it inherently defective since the primary reason for getting the adapter would be to use affordable old Sennheisers with their low noise floor.

As a test, see whether the hiss presents when you simply insert your power adapter into the audio chain without any mic attached. I recently ordered a Sennheiser 12TP48 barrel adapter from Adorama, and what I received was a less expensive PSC unit (with the PSC sticker) which turned out to be nothing but a huge noise source. They cheerfully replaced it ("wrong item shipped") but the replacement unit was identical, even to the PSC sticker. The unverified rumor is that Sennheiser USA has been shipping PSC units without even rebadging them (implying this was not Adorama's fault.)

My solution to the hiss was a Sennheiser BP-2 found on eBay for $40, which is powered by two 9V batteries. I haven't looked at its circuit, or taken measurements, but the ridiculously loud hiss has vanished after swapping the BP-2 in.

Can you recommend another good adapter? I definitely want to test my 415 with a good one, if I had the U version I'd just get it converted to P48 as I know someone who does that, but I have the Tuchel version (weird since it was presumably used by a Canadian TV station).