Wednesday, April 19, 2017

Microphone Repair. Noisy Groove Tubes GT66, MXL R80 Blown Ribbon Replacement.


FIRST THE MXL R80

Here you can see the blown out ribbon, and what the PCB and transformer look like when removed from the R80.  I could tell something was wrong because the sound quality was weak and very distorted.  I couldn't really see the damage through the head grill, I had to take the grill off to inspect the ribbon.

The little rubber shock mount rubber grommet assembly has been removed as well to free up the ribbon motor for work.  You've got to have easy access to the ribbon motor by itself, without obstacles, to do this kind of ribbon work, in my experience.

I'm pretty sure I blew this one from careless handling.  Either a P pop without a proper vocal screen, or too close to a kick drum without a pop screen or 45 degree angle down placement.  Not sure which, but hopefully I've learned my lesson, and it won't happen again.  All it takes is one puff.  LOL.  This is the first ribbon mic I have destroyed in such a way.  Lucky for me this is also an upgrade opportunity, since I have some 1.8 micron foil on hand from Geistnote.

You can see that the center of the ribbon has been flattened of crimps, and the wire mesh pattern from the wire screens that used to be taped on with the yellow tape has been pressed into the ribbon!  Ouch.



Here is the new ribbon installed after being crimped in the Tube Wringer also from Geistnote.  I have seen other types of toothpaste roller tools out there, they are very easy to use once you learn the method.

Whatever foil was stock in the MXL seems a bit heavier than this 1.8u foil.  I also am not going to put the mesh screens back on, leaving the head grill wide open at this point, just need to handle it carefully.

When I was cutting my ribbon, I measured the gap, and cut the ribbon 1 mm smaller than the width of the gap.  This leaves a half mm gap to either side of the ribbon once installed.  It's a pretty tight fit, but I was happy that I was able to make it work without too much trouble.

This is my third re-ribbon after the Apex 205 I posted earlier.  It does get much easier with a little experience.  I got it perfect on the first try, this time.  The only thing I still hate is holding my breath, I might look into a surgical mask or something like that to block the breath.



In my opinion, this mic sounds expensive.  I don't see any need to upgrade the transformer in this particular MXL, just the mechanical and ribbon mods take it to a very happy place.

Right now this is my outside kick drum microphone, but I would love to get a few more for other instruments.  It's a little on the dark and midrange side of things, but takes EQ very easily, and has almost no artifacts of any kind in the sound, a very pure tone.  I think the body is fairly free of resonance or reflection points.

For maxium safety on kick drum (or bass amp, floor tom, vocal, etc) I am using a foam windscreen fitted over the top of the mic.  I also have angled the mic down 45 degrees toward the floor for double security.  Got that tip from a Royer video.  Keeps any wind blasts from hitting bang on flat to the face of the ribbon.  A secondary advantage is that the null of the fig-8 pattern is now minimizing cymbal bleed into this channel.  The bass drum sounds huge and there is little need for the subkick at this point.  Adding the subkick in gives a gut-wrenching subterranean punch to the drum.  Would be nice for some styles.



GROOVE TUBES GT66 NOISE REPAIR

My buddy Amos at Marshall Sound Studios handed me a couple cranky mics to attempt to soothe, see what I can do.  It's always nervous working for other people but it was a good challenge and test.

The mic was exhibiting a "seashore" type of windy, fluttery noise after being powered on for an hour or so.  It wasn't apparent on immediate power on, but once it came on it didn't leave.

Jim Jacobsen from JJ Audio has posted about these mics, apparently he's had a few on his bench.  He suggests the first thing to replace is the two small mica caps next to the pad and roll off switches.  He says to try that first, and if there is still noise, second call would be to replace the tube.

So I ordered the parts, a 33 pf silver mica cap, and a 3 pf silver mica cap, from Mouser Electronics.  The code on the 3 pF cap reads "3D" you should be a able to spot these, they are vaguely shaped like a smashed peanut or something of the sort.

I powered the mic on and ran it for 7 or 8 hours, and knock on wood, it seems to be fixed!

So now I know that small value caps are a good place to look for noise in condenser mics.  I had a similar problem with an AKG C451 e from the late 60s/early 70s.  There was a tiny polystyrene cap I replaced that fixed that mic.  The whooshy noise was pretty similar in that mic.

In other mics I've had moist capsules needing to be gently dried, bad tubes, dirty XLR connections, and things like that being other common sources of noise.



All in all, a very satisfying afternoon's work!  Pedals and guitars are fun, but mics give a special kind of joy I don't get anywhere else.

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