Nady RSM-3 on sale at MF...

[Smitty]

[KennyLusk]

[Michael_Joly]

Most likely I'll do both.
_________________
New MJE-K47H capsule head for SDC Mics -
Classic "Fet 47" Sound

[Michael_Joly]

Got my (now discontinued ) RSM-3 today - inspected, repaired and modified it.

First thing I noticed when I listened to it was the tell-tale sound of a ribbon too slack in the gap. Listening in headphones and rocking the mic back and forth (a test I always use with ribbons) produced a dragging "clang" sound. Took it apart. Sure enough, the ribbon was so slack it was bent into an "S" shape when vertical and bowed like a footbridge when horizontal. This problem made the mic have very low output and would have produced quite a bit of nonlinear distortion had I used it on a guitar cab.

Sagging RSM-3 Ribbon: http://origin.ih.constantcontact.com/fs076/1101219968445/img/33.gif?a=1101677217894] The photo shows the stock ribbon motor with the "footbridge" ribbon effect. After retensioning the ribbon I modified the headbasket and installed a Lundahl transformer. I cut out the resonant brass rings and installed a new stainless steel grille. Now this thing sounds more like the mic it was copied from.

Modified RSM-3 pic here:
http://www.oktavamodshop.com/popup_image.php?pID=90
_________________
New MJE-K47H capsule head for SDC Mics -
Classic "Fet 47" Sound

[Smitty]

got mine in the mail and my primary complaint was a corroded shockmount... grr. forunately there was silica gel in the mic box at least.

assuming i have two that might need a re-tensioning, how much would you charge for that, with and without the Lundahl upgrade?

please feel free to PM if you don't talk prices in-thread.
_________________
"I try to hate all my gear equally at all times to keep the balance of power in my favor." - Brad Sucks
meritbadgerecords.com
twitter.com/mattsmitty

[Michael_Joly]

They may not need re-tensionsing. Rock the mics slowly towards and away from you - rotate at least 45 degrees from vertical to allow gravity to pull on the ribbon. You won't hear any noise if the ribbon has some tension on it.

If the ribbon is off-center against one magnet face or if it has an "S" curve you'll hear it drag.

Its hard to do a visual inspection through the grille and mesh. The mic has to be disassembled to see if the ribbon has something close to proper tension.

A cheap and dirty way to check the resonant frequency of a ribbon mic is to make a recording of it while you bump the body repeatedly. Play the recording back and use the frequency analysis tool built into many software recorders. Depending on the preamp cutoff freq, A/D converter and software tool you should be able to see a large resonant frequency bump in the deep bass.

It would be most useful to have the ribbon resonant frqequency at some low pitched note - 32Hz say, rather than the single digit frequency that a super low tension ribbon would produce. The former is musical while the later emphasizes foot falls and other mechanical vibration.
_________________
New MJE-K47H capsule head for SDC Mics -
Classic "Fet 47" Sound

[chuckfurok]

mine makes all kinds of noise when I move it around. Guess I'll have to open it up and take a look at the ribbon. How do you retention the ribbon? Can a klutzy novice do this?

[Michael_Joly]

A novice? Yes. A klutzy one? Maybe

The core task is to remove the top half of one ribbon clamp, pry the ribbon loose from the face of the bottom clamp, stretch the ribbon and re-clamp.

Before that can happen you'd have to disassemble the mic. All these barrel type Chinese mics have the same basic construction - unscrew the bottom finish nut, slide off the barrel, then unscrew the frame from the headbasket. In the case of the RSM-3, the ribbon motor has to be unsoldered and removed from the top of the headbasket.

Once the motor is out (all fans / AC off - windows closed) you can place it on a clean surface that has had a magnet passed over it to pick up any minute metal particles. The RSM-3 has a fine mesh over the front and back of the ribbon cavity, this has to be removed to get a good view of the ribbon tension. As a side bar, there are two schools of thought regarding this mesh - 1.) one camp likes to have it in place to protect against random metal filings and also damp the ribbon, 2.) the other camp likes to have the ribbon operate as much as possible in free air and avoid parallel surface reflections. I like to think of this second approach as removing poles in the acoustic filter that introduce phase shift so that is my approach.

Once the ribbon is visible, you'll be able to loosen the clamp, tension the ribbon and reclamp. I like to use rubbing alcohol and the butt end of these special Vietnamese toothpics I get at my local market to handle the ribbon. How much tension? That's the value of empirical experience. Short of that, pull the ribbon just tight enough that it doesn't sag when reclamped. When the motor is picked up watch how the air current flexes the ribbon. A ribbon that is too loose will have a lot of play and will seem floppy and too under-damped visually. Not good.

If you've gotten this far, you'll enjoy seeing how the ribbon is offset in the magnetic gap - just like the basic claim of the Royer patent, how 'bout that.

Of course there are more exact, measurable and repeatable methods to set the ribbon tension / resonant frequency by measuring the system impedance but this involves test equipment more readily found in the basement laboratories of participants at "The Lab" and probably not among most TOMB readers.

A caveat - it is possible that the ribbon will not lift easily off the bottom clamp because it has become fused to it. Prying it up can break it at that point. Then you get to learn how to source aluminum sheets, cut ribbons, corrugate them and install them But, in many cases the ribbon will lift right off the clamp and allow itself to be retensioned.
_________________
New MJE-K47H capsule head for SDC Mics -
Classic "Fet 47" Sound

[DryCounty]

Wow -- mad props to Michael for all his help throughout this thread. Your mod looks amazing by the way, nice grill on the headbasket! When might this mod become available?

I just got mine today as well, and will have time this weekend to experiment. Really looking forward to trying this mic out and checking everything.

Thanks again Michael!
_________________
"I would imagine that the inside of a bottle of cleaning fluid is fucking clean." -Hedberg
-----
Tulsa Drone
Dry County Records • Richmond, Virginia
twitter: drycounty

[RodC]

I hat to retension mine. Mine was so bad you could see it with all the mesh inplace. It was laying aginst one side of the mesh, about 80% of the ribbon was in contact!

Pretty easy fix, I wish I had seen this thread before I started it, I wasnt too confident I did it right before I saw this thread!

Thanks Michael_Joly!
_________________
'Well, I've been to one world fair, a picnic, and a rodeo, and that's the stupidest thing I ever heard come over a set of earphones'

http://www.beyondsanityproductions.com
http://www.myspace.com/beyondsanity

[RodC]

BTW, mine was fused about 50% of the way to the bottome clamp, I scraped it of gently with a sharpend jewlers screwdriver.
_________________
'Well, I've been to one world fair, a picnic, and a rodeo, and that's the stupidest thing I ever heard come over a set of earphones'

http://www.beyondsanityproductions.com
http://www.myspace.com/beyondsanity

[Michael_Joly]

Thanks for the props. RodC - that's amazing that 80% of the ribbon was touching the mesh! I've never seen one that loose.

I've been wondering about the causes of the slack tension. I can think of three - 1.) poor surface contact between the clamps allows the ribbon to slide out. 2.) improper installation at the factory and 3.) ribbon goes slack as a result of transit.

I haven't seen #1 in the Chinese ribbon mics. The clamp blocks are wide and deep so any surface imperfections are a small percentage of the total available clamping area. And the fact that the ribbon is usually mildly stuck to one of the clamps means it hasn't moved. I have seen this problem in the Oktava ML-52 which has very narrow ribbons and small contact area clamps held down by screws that sometime bottom out before the clamp faces are tight together.

2.) I suppose improper factory tensioning could be a problem, but its just about as easy to do this job right as it is to do it wrong. Its hard for me to imagine a worker who does these for hours on end, in a flow state, would tighten the clamps on a ribbon that was so slack it bowed completely out of the gap. Small variations in tension are par for the course in these mics though and that accounts for the different bass tunings.

And 3.) transit problems. I think I'll have to test this myself. I also couldn't resist the $99 RSM-4 so I'll take it apart, check tension / measure resonance then whack it around to simulate the combined effect of boat / air / truck travel. Then recheck. Now with both an RSM-3 and RSM-4 in the shop at the same time I want to see if the ribbon motor is the same in both.
_________________
New MJE-K47H capsule head for SDC Mics -
Classic "Fet 47" Sound

[RodC]

I don't think it could be #1. The top of the ribbon was flush with the top of the clamp and there was some excess at the bottom. so I sorta figured thats where I needed to pull it.

I wonder if the corrigation should have sharper creases, I'm sure this affects the sound?? They really dont appear to have sharp corners like others I have seen. If they are more rounded I bet they would tend to loose their tension pretty fast.

I think its storage/transit stretching them out. It came in a huge foam box, but that isnt going to absorb everthing. Plus they cant really know what type of vibration this thing is going to see in boats/airplanes/trucks/conver belts during transit.

How in the heck would you adjust the tension if you found the resonance off a bit? I had a heck of a time just keeping it streight as I tightened the clamps. And as you tighten the clamps the tension changes a bit???

Im willing to bet there are lots of ribbon mics out there with the same issues, just no one has the guts to open one up and look at it. I aint rich but at $130 many ppl aint going to be afraid to open er up! lol

BTW I wonder what a good ole piece of gum wraper hit with a hammer a few times to flatten er out would sound like in one of these? lol
_________________
'Well, I've been to one world fair, a picnic, and a rodeo, and that's the stupidest thing I ever heard come over a set of earphones'

http://www.beyondsanityproductions.com
http://www.myspace.com/beyondsanity

[RodC]

[Michael_Joly]

Looked at my RSM-4 and compared it to the RSM-3 today.

The ribbon motors are similar in design concept, implementation is a little different. The RSM-4 has shorter magnets and ribbon than the RSM-3. The ribbon in the RSM-4 is symetrical in the magnet gap while the RSM-3 uses the Royer offset ribbon technique. As has been noted before, the RSM-4 use a 6 micron ribbon while the RMS-3 is 2 micron.

The RSM-4 uses a rather unique, but potentially flimsy system of ribbon clamping. The clamps are actually made out of fiberglass PC board with a plated contact surface. I don't have a super low resistance range VM to measure the contact resistance of the two clamping systems so I can't comment there.

Transformer in the RSM-4 is significantly smaller than the one in the RSM-3 - don't know if this affects bass SPL handling. Lundhal has a miniature transformer that will fit in these mics though. I really do prefer the sound of the Lundahl transformers - more "hi-fi" with less ringing distortion. Frequency response is extended at both ends but don't expect condenser mic HF levels.

The particular RSM-4 I got also has a severely sagging ribbon. I was planning to take it out anyway and install one made from the 1.8 micron aluminum sheeting I've got.

The RSM-3 with shock mount and wooden case was a good deal at the Stupid Deal blow out. The RSM-4 at $99 is probably not as good a deal as the Apex 205 at the same price which has a 2 micron ribbon, more robust ribbon clamps and a multifaceted headbasket.
_________________
New MJE-K47H capsule head for SDC Mics -
Classic "Fet 47" Sound