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Author Topic: Sennheiser 421-N Hum Induction Coil and other issues  (Read 524 times)

iipeacefrog

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Sennheiser 421-N Hum Induction Coil and other issues
« on: December 06, 2022, 03:34:06 AM »

Hi all. I'm repairing a 421-N that had an odd rattling sound. When I opened up the mic, I found that entropy had worked it's magic and the hum induction coil had come completely unwound and turned into a big ball of chaos. I tried unknotting it for about an hour, thinking I could rewind it around the resonator, but eventually the wire broke. Does anyone know what a viable replacement may be? I've talked to Sennheiser support and they have no specs on the microhenry amount of the coil, but it looks like it's about 28 gauge copper wire.

Also, the wiring on the tuchel connector has snapped off, and the wires coming from the PCB are too short, so I have to replace the whole run (from the bottom PCB to the connector area), but I can't seem to figure out how to get in that PCB area to see where the wires are soldered. Does anyone have any advice there?

Thank you!

See attached photos for more info...
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RuudNL

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Re: Sennheiser 421-N Hum Induction Coil and other issues
« Reply #1 on: December 06, 2022, 12:27:10 PM »

It happens more often...

A friend of mine once had a MD421 with a damaged hum compensation coil.
Since there was no information available, I decided to remove the compensation coil and connect the capsule wires directly to the output connector.

As long as you don't use the microphone near transformers (which is often the case) this shouldn't cause problems.

Maybe not an award winning solution, but the microphone worked again and he had never problems after this 'repair'...
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Kai

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Re: Sennheiser 421-N Hum Induction Coil and other issues
« Reply #2 on: December 07, 2022, 01:05:50 PM »

For the coil only the number of turns is important.
The uH or wire gauge doesnít matter.

Maybe you can untangle the ratís nest to determine the wire length.
Cut it here and there and stick it together afterwards.

The screw for the back assembly is located under the little name plate in the standmount slot.

The Large Tuchel insert can be pulled out if itís two back screws are removed from the outside.

Hereís the MD 421 service manual:
https://www.manualscenter.com/manuals/sennheiser/md421-service-manual.html#.Y5DUOy3GJOk

Iíve attached itís 2 pages either.
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iipeacefrog

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Re: Sennheiser 421-N Hum Induction Coil and other issues
« Reply #3 on: December 07, 2022, 01:26:08 PM »

Thank you!
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panman

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Re: Sennheiser 421-N Hum Induction Coil and other issues
« Reply #4 on: December 08, 2022, 02:29:34 AM »

Late for the show. You already got all you need. I only want to add, that it is advisable to connect the capsule without the humbucking-coil and test if it is ok with full bass and reproduction of all frequencies. also be careful not to use too much heat when soldering the terminations. The plastic top of the capsule melts easily and in the worst case the soldering-lugs can melt the diaphram or the silk cover or both.
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Esa Tervala

iipeacefrog

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Re: Sennheiser 421-N Hum Induction Coil and other issues
« Reply #5 on: December 08, 2022, 04:43:04 AM »

So, I ended up just resoldering all of the connectors, and adding a ground wire from the 3rd lug down to the low cut PCB and it seems to work just fine with no hum...it's gonna live on a floor tom here, so I think it's ok..
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panman

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Re: Sennheiser 421-N Hum Induction Coil and other issues
« Reply #6 on: December 08, 2022, 10:51:11 AM »

.....and adding a ground wire from the 3rd lug down to the low cut PCB and it seems to work just fine with no hum...
Yes, it works just fine, but grounding the third lug is totally unnecessary. Instead, you should connect the added wire to the grill to keep the hum and other disturbances away in most situations.
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Esa Tervala

Kai

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Re: Sennheiser 421-N Hum Induction Coil and other issues
« Reply #7 on: December 09, 2022, 08:38:28 AM »

So, I ended up just resoldering all of the connectors, and adding a ground wire from the 3rd lug down to the low cut PCB and it seems to work just fine with no hum...it's gonna live on a floor tom here, so I think it's ok..
Watch out for all the advice on sealings and damping materials in the service manual to preserve the micís famous bass response.

Iíve attached a cutout view that shows some.

The 3rd lug probably is just a soldering post for connecting the hum compensation coil, grounding it (with the coil removed) doesnít do anything good or bad.
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