AKG P420 grounding issue.

[SoundsOfJill]

Hello everyone,
I recently acquired a loose P420 off ebay for pretty cheap. It came with no shockmount or case an looked like it had received a hit on the XLR port's shroud, but nothing unfixable.
I received it and inspected it, the hit on the shroud made it impossible to connect it so I simply filed away the bent metal until it was usable again (it now fits snugly and securely; and clicks in no problem).
However while testing it I noticed immediately that I was getting crazy levels of hum in the signal, that would lessen if I touched the cable's connector or certain metallic parts of the mic case.
I opened it and took out the XLR insert to check for faulty connections or solder joints but couldn't find anything. I can get the hum to go away by fiddling with the female connector's angle a bit.. so maybe I didn't file away enough? is there a break angle at which XLR cables don't work anymore?
I did test for continuity between the ground pin of the insert and the chassis' shroud and it's definitely grounded too.

Have any of you experienced something similar with a p420 before?
Thank you in advance for your answers.

 

[TwentyTrees]

If it's been given a ding like that, could a solder joint have been sheared inside perhaps?

 

[RoadrunnerOZ]

I’d open it up and look for any cable broken off or pcb cracks - it’s obviously had a heavy hit.

 

[cyrano]

Is it possible some of the filings got inside?

 

[RoadrunnerOZ]

Make sure the male pins in the socket of the P420 are all aligned with each other and that the whole 3 pin assembly is properly aligned with the chassis of the socket or is not sitting in too far.
I am assuming you had to file the inside wall of the XLR socket?
Can you post a photo looking straight on into the base of the mic?
Have you tested using a different mic cable.

 

[SoundsOfJill]

Make sure the male pins in the socket of the P420 are all aligned with each other and that the whole 3 pin assembly is properly aligned with the chassis of the socket or is not sitting in too far.
I am assuming you had to file the inside wall of the XLR socket?
Can you post a photo looking straight on into the base of the mic?
Have you tested using a different mic cable.

I tried several cables, with different brand connectors.
the pins and insert look like they're aligned properly.
I'll post a photo tomorrow

If it's been given a ding like that, could a solder joint have been sheared inside perhaps?

That's a possibility, I might dissasemble the PCBs from the chassis to check

I’d open it up and look for any cable broken off or pcb cracks - it’s obviously had a heavy hit.

I didn't see any PCB cracks but I'll double check

Is it possible some of the filings got inside?

I did make sure to protect everything from it but it's still a possibility. I'll try and vac away around the PCBs (being super careful to shield the headbasket and capsule beforehand)

 

[RoadrunnerOZ]

There’s not a possibility that your file scraped one of the pins of the socket XLR is there? The power for this mic is derived from pin 1 as the ground and the other two as + so any misconnection will result in failure. This mic requires minimum 44V phantom as well. 48V + 4V.
It’s also possible the pad, polar pattern or bass cut filter switches may have been damaged and/or their connection to the print board cracked.

 

[atticae.artifex]

I tried several cables, with different brand connectors.
the pins and insert look like they're aligned properly.
I'll post a photo tomorrow

Hi!

Are you still trying to fix the AKG P420?
Cuz I got one which had the same problem as yours, only without having been abused before ;-)

Anyway; it's a Design vs Manufacturing mishap that prolly a lot of these mics must have, cuz it's nothing more than a little too much Black Powder Coating (the outside color) on a spot that should've stayed bare metal (for obvious conductivity purposes)

If you open up the mic - u already know how to - please wear some clean, 'virgin' latex gloves, coz there's a lot of high-impedance parts in there that will get their carefully calibrated high-impedance ratings 'readjusted' when they get ur human skinn-grease on them...

Once opened-up, remove the capsule from the mesh and try not to touch- or breath- or even look at it funny for too long, cuz that thing is as delicate as a Condenser-mic Capsule !! ... cuz it is a condenser-mic capsule, obviously, but u get my point..

Okay.. ; so, by now, probably, u will have ur capsule safely aside so u can start filing some paint away from the area where the screws protrude into the upper-half of the mic (the mesh-half), that are there to keep the capsule - and the attached PCB - snug inside the mesh grille.

Why, u ask? Well, cuz those screws have a second function; they electrically connect the mesh grille with 'ground' on the PCB. They do that cuz that's how AKG designed the mic - idunno why tho.. I think maybe something-something Faraday-cage this and that, u know? - What they didn't do, however, is make sure the manufacturing facility knew what to paint and what not...

I hope this is clear enough, and I also hope you didn't already throw the mic into the trash, cuz it's a really nice mic... i think... If you have questions, lemme know.

Cheerz!!

 

[SoundsOfJill]

Hi!

Are you still trying to fix the AKG P420?
Cuz I got one which had the same problem as yours, only without having been abused before ;-)

Anyway; it's a Design vs Manufacturing mishap that prolly a lot of these mics must have, cuz it's nothing more than a little too much Black Powder Coating (the outside color) on a spot that should've stayed bare metal (for obvious conductivity purposes)

If you open up the mic - u already know how to - please wear some clean, 'virgin' latex gloves, coz there's a lot of high-impedance parts in there that will get their carefully calibrated high-impedance ratings 'readjusted' when they get ur human skinn-grease on them...

Once opened-up, remove the capsule from the mesh and try not to touch- or breath- or even look at it funny for too long, cuz that thing is as delicate as a Condenser-mic Capsule !! ... cuz it is a condenser-mic capsule, obviously, but u get my point..

Okay.. ; so, by now, probably, u will have ur capsule safely aside so u can start filing some paint away from the area where the screws protrude into the upper-half of the mic (the mesh-half), that are there to keep the capsule - and the attached PCB - snug inside the mesh grille.

Why, u ask? Well, cuz those screws have a second function; they electrically connect the mesh grille with 'ground' on the PCB. They do that cuz that's how AKG designed the mic - idunno why tho.. I think maybe something-something Faraday-cage this and that, u know? - What they didn't do, however, is make sure the manufacturing facility knew what to paint and what not...

I hope this is clear enough, and I also hope you didn't already throw the mic into the trash, cuz it's a really nice mic... i think... If you have questions, lemme know.

Cheerz!!

Right, I never concluded this thread. My bad.
So it turns out the way the p420's body shroud only gets good conductivity with the rest of the chassis if you absolutely CRANK the the hollow coupling screw at the bottom of it. I found about it while I was using it for a concert. The shrould could still rotate and move a little and that was causing it to break ground depending on the position it got in.
So yeah I now understand why it was screwed in with so much force from the factory.
On the upside it's a pretty decent mic. A bit bright but I've been using it a lot for singer-songwriter female vocals with plenty of success.

Hope it helps and thank you everyone for offering your help and advice.

 

[TwentyTrees]

Glad you got it sorted, and thanks for feeding back - I've a pair of these myself so I'll know to keep an eye on that now.

Speaking of which, if you find the P420 overly bright then it's very easy to tame the high end by just playing with one cap (leaves it sounding very U87ish):
https://groupdiy.com/threads/akg-perception-p220-to-neumann-u87-5-min-mod-p200-p100-p400-p420.67473/

 

[atticae.artifex]

Right, I never concluded this thread. My bad.
So it turns out the way the p420's body shroud only gets good conductivity with the rest of the chassis if you absolutely CRANK the the hollow coupling screw at the bottom of it. I found about it while I was using it for a concert. The shrould could still rotate and move a little and that was causing it to break ground depending on the position it got in.
So yeah I now understand why it was screwed in with so much force from the factory.
On the upside it's a pretty decent mic. A bit bright but I've been using it a lot for singer-songwriter female vocals with plenty of success.

Hope it helps and thank you everyone for offering your help and advice.

Thank you for replying!

Now at least I have a confirmation that my understanding of this mic's circuitry-issues wasn't just a phantom.. -power.. Get it.. ? Phantom Power... ??
Okay, nvm..

In my case the screws weren't that tight at all, though I can remember there was some type of colored sealing on the screw-heads, I think acting as a 'waranty void if broken' - substitute, which made the screws kinda hard to start unscrewing. Anyhow; i don't think I'd be able to turn them in so tight, that they would actually touch the mesh, but maybe the tightening itself could be be enough to scrape the underlying metal.. or something..

I prefer the way you found out about the cause of the problem, during a concert (!) - in comparison to how I simply tried to strangle the life out of the mic from Ground Hum-induced Rage (GHR), which accidentally made the mesh touch the screws for moment, silencing the noise.

Anyhow; bottom line is the mesh has to connect to ground on the PCB, whatever way you-, the reader-, might go at it.

Greetz, and good luck!

 

[pucho812]

Gnd noise aside there is a good mod for this to tame the brightness