R/E/P Community
R/E/P => Klaus Heyne's Mic Lab => Topic started by: jackreynolds on December 08, 2016, 07:39:41 AM
-
Hi All,
I have a quick question regarding an unwell AKG C414EB (silver body) I have on my bench at the moment.
When I engage the filter switch it makes a nasty crackling noise, which makes me think one or both of the HPF caps may need replacing, or possibly a dry solder joint on one of the HPF resistors.
Has anyone come across this sort of problem before? The circuit boards are tiny and the components are very tightly packed, so I don't really want to start dismantling anything before I know where to look.
Thanks in advance for any info.
Regards
Jack Reynolds
-
You have a dirty switch. Clean the contacts or replace it if that doesn't work.
-
Agreed. I use Caig Pro Gold. That usually cures reoccurrence,, after the cleaning.
-
Hopefully not hi-jacking the thread - which one out of these, Klaus?:
http://store.caig.com/s.nl/sc.2/category.292/.f
-
DeoxIT® Gold, #G100L-25C
(http://store.caig.com/s.nl/it.A/id.1568/.f?sc=2&category=292)
is what I use. I costed all of the Pro Gold products out, and found this format and quantity to be the most cost effective. But I believe the chemical formula for any of the Gold products is similar if not identical.
-
I always wash out any Caig products after use. The reason is it will attract more contaminants. Wash it out with a non-residue electronic contact cleaner or you will be back at it again.
-
And the most important advice:
Do not directly spray the contact cleaner into the microphone or you will destroy it.
Usually I unsolder any switch or control that I clean by stuff like that and put it back after.
As this is not a good solution with the AKG 414 ( The switches are not easy to solder) the best way would be to completely remove the capsule first, then protect everything except the switch with tissue.
Spray some amount of the cleaner into a glass and apply from there little drops, using a tiny screwdriver or something like that.
If the switch is still in the microphone there is no sensefull way to wash it after this procedure, it has to stay like this. The only thing that you can do is trying to rub out some of the cleaner with pieces of paper. To my remembrance this is possible as the contacts are accessible from the outside.
Before all that I would test if the switch is really the source of the problem. Does it appear as soon as you touch or slightly move the switch?
Or is it just the XLR connector?
-
Hello Kai,
Because of your concerns, the product I recommended avoids the issue. It is not a spray, but an small flask with a 2" needle applicator, not unlike a syringe, which helps to precision-target the oily fluid to the appropriate spots without spillage.
-
...the product I recommended avoids the issue. It is not a spray...
Important that you mentioned this fact.
-
Worth mentioning; every once in a blue moon, simply exercising the switch, knob, whatever, will wind up cleaning up the problem.
In this case, it might be worth carefully moving the switch back and forth a dozen or so times, with the mic unplugged.
-
Great. Thanks for the advice.
I have applied a tiny amount of deoxit gold (from a small plastic tube/vial)to just the contacts of the switch, so hopefully that will fix it.
I'm not sure how to clean away any residue, but perhaps an isopropyl alcohol swab?
I shall report back when I have had a chance to test it out,
Cheers
Jack
-
The alcohol will remove and neutralize the deoxidizer you just applied. Better to just mechanically wipe the excess off.
-
Just to update - my cleaning attempt does not appear to have fixed the problem.
I used some Kontakt60 and exercised the switch, followed by a tiny drop of Kontakt Gold - but the problem persists - a rumbling crackling sound.
I have therefore removed the capsule and isolated the switch enough to give it more of a wash with contact cleaner (having masked off the rest of the components)
Hopefully this will flush out whatever is stuck between the contacts, I have a feeling it some flakes of red paint which may have originally been securing screws in place as there is some evidence of this floating around elswhere inside the mic.
I will report back again shortly.
Jack
-
If the rumble remains with the capsule removed, it is a switch contact issue, most likely. If the rumble is gone without the capsule, the static / discharge sounds could stem from a contaminated capsule. If so, do not clean with mechanical means (brush, etc.)
-
Don't worry, I won't be touching the capsule!
The noise only appears when the 75Hz or 150Hz positions are selected on the filter switch. All the other settings ie the pad and pattern switches work perfectly, and the noise sets in a second or so after engaging the filter switch, which makes me think it could be a capacitor charging up and then leaking.
Having carefully removed the capsule and masked off the rest of the mic, I flushed the switch through with Kontakt 60, then wiped away any residue with sheets of paper as you suggested, then applied a tiny amount of Kontakt Gold to the gold contacts - which actually look very shiny and clean - and then again wiped away any residue.
Upon re-assembling and testing the mic, the same problem persists, so I'm not sure what the next move should be at this point.
Thanks for all your help so far. I am now familiar with dismantling a 414EB! I love the slide-in capsule mount and push fit electrodes for the capsule.
Cheers
Jack
-
The two low cuts are effected by a couple of tiny Tantalum capacitors (.1mfd.) They are always suspect in this mic, and I routinely replace them. Mechanically not easy, as they sit right in the center of the circuit board.
Schematic with position of caps here:
http://recordinghacks.com/microphones/AKG-Acoustics/C-414-EB
-
It looks like the tantalums may have been replaced with Wimas in the picture on recording hacks. They are two little green 35V tantalums in the mic on my bench.
http://cdn.recordinghacks.com/images//mic_extras/akg/C414EB-exploded.jpg
Would you recommend replacing like with like ie tantalum, or substituting a film cap?
On the schematic, i'm seeing two 0.1uF polarised caps, rather than 0.01uF, which appear to be part of the RC network for the filters.
As you say, it looks like quite a mission to get into the centre of the board there, but i imagine with some care and patience it won't be impossible.
Jack
-
Would you recommend replacing like with like ie tantalum, or substituting a film cap?
On the schematic, i'm seeing two 0.1uF polarised caps, rather than 0.01uF, which appear to be part of the RC network...
Either would work, if you use film caps there might be a slight sound change, because a significant amount of (LF) signal voltage is dropping at those, even in filter bypass.
Explanation: The characteristics of a condenser will become audible if there is a voltage present between both sides of it, so it acts as a frequency dependent resistor.
If this is the case depends on dimensioning and topology of the circuits.
Be sure to use the right value, best measure the ones you have taken out.
When calculating the x-over frequency the values given in the schematic (0.1uF) are correct.
Still it's possible that those of your specific microphone are different, as it might not be 100% coincident with the schematic.
-
there might be a slight sound change, because a significant amount of (LF) signal voltage is dropping at those, even in filter bypass
I don't believe so. There is a hard bypass before the caps, so I am not clear how the cap material would affect the sound in the flat setting?
On the schematic, i'm seeing two 0.1uF polarised caps, rather than 0.01uF
You are correct. It's 0.1µF. I have corrected the value in my original post.
KH
-
I don't believe so. There is a hard bypass before the caps, so I am not clear how the cap material would affect the sound in the flat setting
You are right, the trick is that the BC413 bias is switched off in bypass, so the whole filter is out of circuit.
This leads to the idea that even the BC413 might be faulty.
-
The two tantalums are marked +1 35V, but they look rather tricky to replace, so if there is a chance it could be one of the transistors, which do you think would be the most sensible to replace?
Or is there a way of testing the circuit to ascertain which parts are faulty?
It seems like replacing old tantalums is a good idea anyway, but I would rather not change the sound of the filter too much.
-
Wima MKS 02 2.5 mm mylar caps will sound smoother than the tantalums, plus you never have to worry about those parts ever again. They have a 2.5 mm lead spacing and should fit easily. Schoeps used those in their mics.
Also check the input coupling cap to the jfet gate, they sometimes used a ceramic disc cap, bad choice. Swap that out to a polystyene film or a Wima FKP-2 polypropylene film cap. That makes a huge difference in the resolution of that mic.
-
Let's stick to the thread's focus: troubleshooting of a specific problem.
The input coupling capacitor or considerations what components sound better or worse have no bearing on what the poster reported: noise, but only when the filter switch is engaged.
-
...the poster reported: noise, but only when the filter switch is engaged.
It's very clear now that the problem lies within the filter circuit. If I had to do it, I would change the caps first, then the transistor.