ACM-6802T: The Mod From You-Know-Where....

dgatwood

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Tonight, I started to disassemble my pair of 6802T mics in preparation for doing a capsule swap. Unfortunately, I found out about Chinese manufacturing quality and consistency firsthand tonight. Out of the six screws, I was able to remove three (two in one mic, one in the other) without drilling, and was able to get one more out with a torx driver after partially drilling. Of those four, I was able to get two (the two that came out most easily) to screw back into the mics without binding.

After drilling the heads, the beheaded screws came easily out of the threaded part with pliers, so apparently they were binding with the unthreaded part. That suggests to me that the manufacturer didn't drill the holes in the body big enough, but it is also possible that the holes in the screen were mistapped at a slant. Hard to say which. What's odd, though, is that some screws bind in holes where other screws don't. I'm utterly baffled.

So now I have two mics, neither of which can be reassembled without new screws, and they're a size that is pretty much impossible to purchase locally. My calipers shows them being about 1/16th of an inch in diameter, or about 1.5 mm. Not sure exactly.

Any advice short of drilling larger holes and tapping them for a size that's more readily available (e.g. the size of hard drive mounting screws)?

Oh, and I broke my 1/16th inch drill bit on the last screw because of the steep angle I had to drill at. I'll have to stop at Lowe's on the way to work tomorrow and buy a new bit. :mad:

Looks like it's going to be that kind of a week....
 

dgatwood

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And then, there was the little accident. The guts of one of them rolled off my kitchen counter while I was wiping the metal dust off the windscreen. Ended up with a wrinkle in one side o the capsule. Fortunately, I loosened the center screw and retightened it a couple of times and the wrinkle went away, so I can only assume the screw was too loose and the wire hitting the ground caused the metal tab to slide sideways a bit....

Like I said... clearly one of those weeks. :D

Oh, and my modding on a couple of SDCs the other day... I ended up buying some caps at Rat Shack because Fry's was out. Unfortunately, those caps were a different shape and after much effort, I still was unable to squeezed them into the space. I ended up going to a different Fry's about twenty minutes away to get more parts last night.

And then there was the whole accidental substitution of .001 uF parts instead of .01 uF. Didn't hear any real difference, but swapped them for the right part once I noticed the mistake (while modding the second mic).

Oh, and the second mic didn't work very well. Finally traced it back to a failed solder joint on one capacitor, causing my capsule voltage to be about 0.1V instead of about 0.5V (and various other voltages to be wrong throughout), resulting in a very weak output.

And then, there was the mod over the weekend in which I had almost exactly the same problem, but checked every solder joint and didn't find any problems. I traced the voltages, though, and found that all of a sudden, the voltage was basically zero after a resistor. Was the resistor bad? Nope. The capacitor on the other end of the resistor was shorted. Brand new film cap. *swears*

All in all, I've nearly bought out the .001 uF, .022 uF, and .2 uF film caps at three South Bay Fry's locations (Sunnyvale, San Jose, Campbell) plus the Sunnyvale Rat Shack. :eek:

Make that "one of those months". :D

Does anybody have any suggestions on the screw problem?
 

sweetbeats

Reel deep thoughts...
Not an ideal solution, but you could always use plated self-tapping screws...might be able to fit the exisiting holes...
 

dgatwood

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If I could figure out a way to get good electrical conductivity, I'd be tempted to fasten a couple of small neodymium magnets to the grille and make it removable without screws.

Something I'd like to do is to make all my mics have swappable capsules with plugs and some quick-release mechanism for the capsule (also TBD---possibly a neodymium magnet fastened to the underside of the plastic saddle with a couple of guide posts to keep it from sliding around).

Thoughts?
 

antichef

pornk rock
Would the magnet mess up the sound? If not, sounds like a good idea. I had to do some drilling on my ACM-310 - the nylon (I think that's what it's made of) hood-shaped thing that the capsule was mounted on was screwed in funny and not flush against the top. All I had to do was to drill a new hole in the nylon thing and it fits better. Last night I fixed a SM-57 that had noise (the ground wire from pin 1 had broken loose). I'm getting two ACM-583s in the current GB, and I've got two dual capsules waiting for them that I got from Ausrock.

I've been working on my 6802T also, but Marik did all the heavy lifting already - I'm merely pulling out the transformer to verify that it's the expected ratio. I pulled it out once, and have so far tried using my DAW output (sine wav) as a signal generator, but have run into all sorts of keystone-cops types of problems due to my ignorance/inexperience (alligator clip leads that are broken and so not conductive, but I was trying to use them anyway, for instance), and so far I've been unable to get meaningful test results. Last night to reassure myself I put the transformer back in (into a radically different circuit that Marik built) and it worked fine (whew! :D). I'll resume my "research" soon.

People have actually been *sending me their mics* to mod -- duh :D -- I did a pair of 603s for Guitarer, and there's another set on the way. I've got tons of parts, and it's easy, and there is a marked improvement, so I guess it's all good. Sooner or later Mrs. Chef is going to put the skids on it, though.

I've got three Fry's stores in the area, but don't by capacitors from them. Instead, there are two locally owned electronics stores that I frequent, each with an aisle or two of bins of capacitors (and resistors, and stuff), and one of them has a bunch of WIMAs and sells very cheap - I literally grab them loose out of the bins and hold them in my hand, walk up to the counter, we count and I pay. I get my hobbyist-grade tools both from those stores and from Fry's.

sorry to be less-than-helpful. I'd probably be making plans to drill out the holes. I'll take a closer look at the Peluso on my 6802T tonight and see if it looks like Marik went through anything similar
 
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BudgetMC

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Gadzooks. I'm planning on doing tube/capsule/transformer swaps on my ACM1200's (when I get them). But, you guys are doing a GREAT job of convincing me not to mess with anything else! I have no doubt at all that I simply lack the inner serenity necessary to trace voltages through circuits in search of faulty components.

Oh... the horror.

Maybe after my kids get bigger...:rolleyes:
 

dgatwood

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Thanks for the offer. I started digging around my closet just now and found that it's the same size and thread density as the screws Apple uses for the ultra-flat-head case screws inside the battery compartment on iBooks, and since I have a collection of random laptop bits, I had a few screws that would work. :D

The iBook screws are just a little over half the length of the originals, which turns out to be good. Part of the problem with the original screws is that the holes in the windscreen part appear to not be threaded all the way through and the screws were binding in the shallower-tapped holes.
 
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