Velocity/Ribbon microphone troubleshooting help needed:

[Vortex Joe - N3IBX]

Hello All,
            I just acquired an RCA 74B velocity/ribbon microphonium and can't get any output from it. This is the first mike of this type I've owned, and except for the obvious, don't know what to look for to troubleshoot it.

I downloaded a manual for it and checked all of it's connections and everything seems to be secure.

I did read that you cannot check it with a continuity tester for risk of damaging it. I don't know what that's about, but am glad I read it since I did plan on using a continuity checker on it like you would other types of microphones.

The problem is no output when connected up to my speech amp. Both the RCA 74-B and my speech amp have mating XLR connectors on them. Any help or advice will be greatly appreciated!

[Mike/W8BAC]

Hello Joe

    The reason for not using a continuity tester, or phantom power for that matter, is because of the ribbon itself. The ribbon is made of silver leaf maybe .003" thick. Putting the Simpson across it can let the smoke out. Phantom power will make it blow up real good.

    Find your magnifier visor and remove the screen. Use a strong light and have a look at the ribbon and the magnet on either side of it. Look for a torn ribbon or metallic hairs bridging the magnet and ribbon. Make sure the work area is clean and free of metal filings as the magnet is very powerful. Remember, just breathing on this thing can damage it. Don't pick the mic up and walk across the room without the wind screen installed.

    If you find the repair is beyond you, I know of 2 people that can help. When working they sound great. Good luck.

Mike

[W1GFH]

Hey Joe, does the ribbon look intact?

Google 'ribbon mic repair' for a dozen sources. This guy charges $125.....

http://home.earthlink.net/~jwebbmicrophones/

[k4kyv]

I would think it would be ok to use a quality DVM to check continuity.  I have a mid-grade Fluke and I believe the ohms scale operates with a milliamp or so.  That fragile ribbon does make an excellent fuse element, so I'd avoid anything more.

I have a pre-ww2 Electrovoice here, but it always picked up more hum from nearby power supplies than audio.  I think those things were designed for studio use where there were not usually any power transformers in the immediate vicinity.

[WD8BIL]

Joe ...... checkout this link to Dave's (W9AD) Page.

Might be helpful.

Dave's Mics

[Vortex Joe - N3IBX]

Buddly,Don,Mike,Everybody, I appreciate the leads and information. Oh, BTW, I emailed the seller of it and he explained it was used on a "white faced Johnson CB" without any difficulty. I don't think the speech amp in said CB would have had enough poop to drive the ribbon mike. Maybe I'm wrong. Regardless, I appreciate the info and would like to get it going.

Best Regards,
                   Joe N3IBX

BTW, Dave, W9AD has an incredible collection of RCA microphoniums!

[Mike/W8BAC]

Hi Joe,

The 74 might have worked on the Johnson but it wouldn't have worked very well. Good thing you rescued it. I picked up my 74 from the person that rescued my 20V-3 from WBAC in Cleveland TN. Unfortunately somebody buffed up the metal screen with steel wool. The fibers went right through the silk and collected on the large  magnet. No way of salvaging the ribbon. I brought it to Dave's (W9AD) place during the Gray's Lake weekend last fall. We spent several hours with wooden match sticks and tape cleaning the wool off and Dave made up and installed a ribbon.

I use several here. My favorite is the RCA 77D. The D model is the broadcast version hardened against RF. The 77DX is the studio version. I have an Altec 639B birdcage combination ribbon and dynamic and an Altec 670. The last one is an ElectroVoice V-1A. This one is interesting. Of all of them this one is high impedance with balanced line. I haven't had any rf problems with any of these, knock on wood

I have used Clarence Kane's service at ENAK in Putnam NJ. Clarence has been doing this since 1952 when he worked for RCA. Find him here.
http://enakmic.com/
He is the best and not out of line with pricing compared to the rest. Fast turnaround. You could drop it off. When it's fixed make sure you use a bag of some kind to cover it. I use Crown Royal bags.

One last thing, The spring loaded ball joint at the base of the 74 tends to ware out and the mic will flop over. Clarence can fix that as well. Good Luck and I look forward to hearing it on the air.

Mike

[Vortex Joe - N3IBX]

Hi Joe,

The 44 might have worked on the Johnson but it wouldn't have worked very well. Good thing you rescued it. I picked up my 44 from the person that rescued my 20V-3 from WBAC in Cleveland TN. Unfortunately somebody buffed up the metal screen with steel wool. The fibers went right through the silk and collected on the large  magnet. No way of salvaging the ribbon. I brought it to Dave's (W9AD) place during the Gray's Lake weekend last fall. We spent several hours with wooden match sticks and tape cleaning the wool off and Dave made up and installed a ribbon.

I use several here. My favorite is the RCA 77D. The D model is the broadcast version hardened against RF. The 77DX is the studio version. I have an Altec 639B birdcage combination ribbon and dynamic and an Altec 670. The last one is an ElectroVoice V-1A. This one is interesting. Of all of them this one is high impedance with balanced line. I haven't had any rf problems with any of these, knock on wood

I have used Clarence Kane's service at ENAK in Putnam NJ. Clarence has been doing this since 1952 when he worked for RCA. Find him here.
http://enakmic.com/
He is the best and not out of line with pricing compared to the rest. Fast turnaround. You could drop it off. When it's fixed make sure you use a bag of some kind to cover it. I use Crown Royal bags.

One last thing, The spring loaded ball joint at the base of the 44 tends to ware out and the mic will flop over. Clarence can fix that as well. Good Luck and I look forward to hearing it on the air.

Mike

Mike,
       I GREATLY appreciate the information and have emailed him for more particulars. I've always wanted to experiment with velocity ribbon mikes, and this is my first one. I've heard many things about them and they've all been good.

Interesting that you use a "Crown Royal" bag as a microphone cover. I do the same thing, and usually have it on one of my D-Ten-Fours, etc. The Royal blue color and size of the bag happens to fit in very nicely with the decor in my shack, and makes for a nice protective cover.

I guess I'll have to buy another bottle of Crown Royal so I can get a cover for my RCA 74B - hi!

Best Regards,
                   Joe N3IBX

[Mike/W8BAC]

Just a note Joe, I edited my post above. My mic is a 74 and not a 44. A small difference here but a big difference in the pocketbook.

Mike

[Bacon, WA3WDR]

Look and see if the ribbon is in place.  I have seen several ribbons blown out by a pressure blast, and the mike would not put out anything without the ribbon.  I saw one ribbon mike that must have been blasted, and it was intermittent because the ribbon was stretched and it was scraping against the magnet.  I realigned the ribbon, and it worked fine.

The ribbon is a conductive line between the pole pieces of a magnet.  Sound vibrates the ribbon, and motion of the ribbon in the magnetic field produces voltage, which is the audio output.  Usually the impedance of the ribbon is only a few ohms, so there is a transformer in the mike to step the impedance up, usually to about 200 ohms, so the mike looks like a 200 ohm impedance.   The mike preamp needs to be designed to work with a low voltage, low impedance source, but strangely the actual input impedance of the mike preamp should be high compared to 200 ohms.  Mike preamps are a strange science.

Most ribbon mikes are bidirectional.  The magnet pole pieces are open to the front and back.  This allows interviewers to share the mike with their guests, with the mike in between them.  This kind of a mike has very heavy "proximity" effect; get close to it, and it boosts bass heavily.  Voiceover professionals say that you have to "work" a ribbon mike because of this effect.  Many of these mikes have a near-far switch on the mike that cuts bass because of this effect.  With the right equalization, the mellow bottom quality of a ribbon mike can really work well with some voices.  Ohhh, and it can sound really mellow on a guitar amp... depending on the musical style and playing, etc.

The open back and the delicate ribbon can add up to trouble, though.  If you try to use such a mike on a bass drum, you will blow out the ribbon.  Likewise if you try to use it in the wind, you can get the same result.

I have also seen unidirectional ribbon mikes (RCA BK-5) that do not seem to have such a heavy proximity characteristic.  As I recall, they still have the distance-EQ switch, though.

I think the problem with a continuity tester and a ribbon mike is that the DC from an old-style, relatively high-current continuity tester can mess up the magnetics of a mike transformer.  If you magnetize the core, you screw up the performance.

For any given voice, there will be mikes that work well, and mikes that do not.  And a mike that sounds good with one voice can sound terrible with another, yet that voice can sound great with another mike.  So if the ribbon mike is not for you, then go find one that is.

[W1XYZ]

Rule of thumb: The preamp input Z should be at least 5X the ribbon mic output Z in order to avoid loading the transformer/ribbon and attenuating the low end.

The ribbon resistance should be less than an Ohm.  But don't test it by an ohmmeter or it will fly out of the magnet gap and be ruined.

Phantom power rarely damages ribbon mics.  If it does, it is usually because someone did the deed on the patchbay, where tip and ring are not simultaneous.

A sensitivity of less than 1 mV/Pa is normal for the old school ribbon mics.  Newer ones are a lot better, but the oldies are cool, even if you need 70 dB of gain.

RCA44 is very RF and hum sensitive.  77D and 77DX are better but RF can still get in there. 77D and DX are polydirectional and have a shutter that permits pattern changes from figure 8 (sort of) to omni, to hypercardioid.  The pattern change greatly changes the frequency response and the tone. (these are different but related)

All mic except omnis have some proximity effect. Bass up close is normal. Why spit all over the mic.  Back off!  Or, use the V1 or V2 positions, which switch in a shunt reactance to load the output, cutting the bass. I have the curves on the link below.

http://microphonium.blogspot.com

[Todd, KA1KAQ]

There's another thing to be careful of Joe, if you lay the mic down for any length of time. Someone may have mentioned this and if so, I missed it. Leaving the mic laying on its front or back can cause the ribbon to sag, degrading the performance. If you have to take it off its stand, lay it on its side. I had a box of 74Bs (sometimes referred to as the '44 Jr.') that I passed along over the years, some looked great, others had a lot of slack in the ribbon. None had the ribbon blown out, perhaps because they came from a radio station/TV studio originally.

Another issue I seem to recall is a general need for equalization, but that may be more a case of proximity and likely changes from place to place. I'm hoping to find an older 'pre-DX' RCA 77 someday, so I'll be curious to hear of your experiences. Once you work out the set up, you'll have one sweet sounding signal with that old mic. Especially fitting for 160, OM!

[WBear2GCR]

Scope across the ribbon itself, (the wires or contact points) without the xfmr primary connected ought to show output or not output.
No voltage being applied to the ribbon itself... it's a scope.

In a pinch, same deal (no xfmr connected) using a suitably sized series resistor to limit the current applied to the ribbon, and an ohmeter will tell you if you have a go - no go ribbon connection. Of course it will read the value of the resistor or else it will read open...

Visual inspection is good... but keep all metalic stuff FAR FAR away from that gap!!

Make sure of course that the magnet is still physically attached to the pole pieces (stranger things have happened).

You can test the xfrmr with some sort of signal placed on the primary (not the ribbon) and look at the secondary with a scope... it's a step up so, that input signal should be "mic level" but as long as the secondary is not loaded, doesn't need to be ultra low Z to drive the iron...

Nice find!

Do we get to see a pic?

        _-_-WBear2GCR

[Vortex Joe - N3IBX]

There's another thing to be careful of Joe, if you lay the mic down for any length of time. Someone may have mentioned this and if so, I missed it. Leaving the mic laying on its front or back can cause the ribbon to sag, degrading the performance. If you have to take it off its stand, lay it on its side. I had a box of 74Bs (sometimes referred to as the '44 Jr.') that I passed along over the years, some looked great, others had a lot of slack in the ribbon. None had the ribbon blown out, perhaps because they came from a radio station/TV studio originally.

Another issue I seem to recall is a general need for equalization, but that may be more a case of proximity and likely changes from place to place. I'm hoping to find an older 'pre-DX' RCA 77 someday, so I'll be curious to hear of your experiences. Once you work out the set up, you'll have one sweet sounding signal with that old mic. Especially fitting for 160, OM!

Todd - Sorry for the late reply but I haven't been able to check into the AMBBS as often as I'd like due to other "issues", and just found your comment.

I love my 74B, and was using it with a ART-13 through a HB hollow state preamp. I'd like to try it through my Behringer VX-2000 to a Gates BC1-T and compare it with the Behringer B-1 condensor microphonium I usually use with it. I hope you're able to get your mits on a DX-77. They have to be one of the kewlest mikes ever made, and sound good to boot! I guess I was just lucky to pickup the 74B I have on the net for $75, and with the help of Dave, W9AD who made me a new ribbon it now works. Definitely impressive to get behind and talk into, and it has seemingly good audio quality to boot.

Do you know anything about the Bruno "Velotrons"? I've always wanted to pick one of them up.

Hope to talk to you on 160M or 75M in the near future.

Very Best Regards,
                         Joe Cro N3IBX

[WBear2GCR]

Bruno Velotron?

Funny you should mention it. I have one here.

Checked the patent too.

It's an electret mic, built kinda like a little Electrostatic Loudspeaker element, except the electret is rather primative by todays standards, iirc. It was plastique film with rosin deposited on it, followed by a charge in a DC polarized electric field. The actual element that i have is ~2" wide and ~4 " long.

I opened it up, of course...

I managed to crack the yoke that held the mic body when I first got it, as it was cast out of "pot metal" as was the main body. When it was still working, it had significant output, like a hot xtal mic might... it has since died, lost the charge on the rosin, I presume...

I was thinking about "recharging it" and/or making a new film and coating it with some rosin dissolved in alcohol or similar solvent and sprayed via airbrush on the film... then trying to recharge the sucker...

But...

           _-_-WBear2GCR

[Vortex Joe - N3IBX]

Bruno Velotron?

Funny you should mention it. I have one here.

Checked the patent too.

It's an electret mic, built kinda like a little Electrostatic Loudspeaker element, except the electret is rather primative by todays standards, iirc. It was plastique film with rosin deposited on it, followed by a charge in a DC polarized electric field. The actual element that i have is ~2" wide and ~4 " long.

I opened it up, of course...

I managed to crack the yoke that held the mic body when I first got it, as it was cast out of "pot metal" as was the main body. When it was still working, it had significant output, like a hot xtal mic might... it has since died, lost the charge on the rosin, I presume...

I was thinking about "recharging it" and/or making a new film and coating it with some rosin dissolved in alcohol or similar solvent and sprayed via airbrush on the film... then trying to recharge the sucker...

But...

           _-_-WBear2GCR

Hello Bear,
             I've always been fascinated by the Bruno Velotron mikes since they were relatively inexpensive ($20) when they wer enew in the mid and late 1930's. In comparison, a new Astatic crapstal D-Ten-Four, or T-3 was in the neighborhood of $23 or so. I've always thought them to be pretty kewl looking.

One question about the Velotron: Were they not really velocity ribbon mikes? With an output such like you described, how could they be? The only thing hotter than a hot crapstal microphonium that I know of would be a carbonium microphonium, particularly a single button job. You could connect one via a coupling cap right into the grid of a #53 dual triode and have your mike preamp in one tube.

Best Regards,
                  Joe Cro N3IBX

[WBear2GCR]

No, not a ribbon at all!

Arguably the largest electret condenser element ever made??
As I recall the patent, it is rosin on plastic film, made into an electret via a HV charge!!

The element consisted of a stator, the electret on film, stator.
As I recall it put out substantial voltage when I put it on a scope.
Don't recall how much, but it was BIG swing for a mic!!

It's dead as a doornail now, but that was like over 10+ years back when I first go the thing, and it sat around not being taken very excellent care of... not bad care, but not excellent... probably too much moisture for the electret, or else it just gave up from old age??

Ya could make new ones, if you followed the patent and did a bit of experimentation!!

                 _-_-WBear2GCR