Old ribbon mics - do magnets go bad?

[zebra50]

Hi!

I'm looking at an old RCA varacoustic ribbon mic for a customer, and the output is way down compared with most of my ribbon mics.

I've replaced the ribbon, swapped out the transformer for a sowter 8074 1:40, but I'm still way down on where I think I should be. Even with my quietest preamp, (API A2D) you couldn't cut a vocal track with this.

I noticed that the magnet doesn't pull at my screwdrivers as strongly as some microphones. So I wondered how badly these permanent magnets can /do fade with time, and if I can measure it. I was wondering about popping a couple of neodyns on the motor assembly to see if I can boost it that way!

Any thoughts or comments appreciated.

Stewart


PS - photo before the new ribbon went in.

 

[Rob Flinn]

I have been told that Coles have a big machine that they use to restrengthen the magnets in 4038's when you send them for reconditioning (keep your credit card & watch away from it).  My friend said that when he had his 4038's reconditioned that they had a much bigger output after they the magnets had been in said machine.  So I'm guessing that they do lose magnetism with age !

 

[Hank Dussen]

I don't know about ribbons mics but I know that old (magnetic) guitar pickups lose their magnetism and can be re-magnetised.

 

[rodabod]

Yeah, what Rob said. I think the Cole ones might be AlNiCo, but I'm not actually sure. The PGS1 datasheet would probably reveal that.

You could always use a Gauss meter if you were keen.

 

[ArR]

Hi Stewart,

Yes, time, heat and vibration will all do it eventually.

The new neodymium-iron-boron magnets are stupidly hard to demagnetise.
Sticking a couple of small ones on to the old magnet should (in theory) work fine.
i.e old magnet becomes a pole piece.
Gaussmeters are a bit pricey, but like you say, you can get a good feel for the pull on a screwdriver.
Or one of those 'magnaprobe' gimble mounted magnets are a good rough guide.
If you find NdFeB magnets are not strong enough (for the size they need to fit in)
then samarium-cobalt are even stronger (these de-mag at a lowly 120 degC so no recording in the oven).

PM me if you need NdFeB magnets, use hundreds of em at work 

Simon

 

[leswatts]

From the looks of it, that's an early alnico magnet. They can get weak, although I have some old '40's/50's
Shure dynamics and ribbons that still have ful output.

It can certainly be renewed. some turns of heavy buss wire around it and a huge  DC current source will do it. (with a keeper in the gap).

It would be good if you could find someone with a gaussmetter. I would expect something like 0.5 Tesla in the gap. Perhaps a little less.

I can't think of a way to boost it up with neos without seriously mucking its collector value. Need to find a way
that doesn't involve surgery.

Makes me think of the old magnetizer I had at Shure. Giant hunk of iron with big movable pole pieces. I think Ben Bauer made it. It was all old and rusty.

Les
L M Watts Technology

 

[zebra50]

Thanks guys, loads of great knowledge here as usual.

I was considering trying to re-gauss it, but was concerned that if I get the DC direction wrong I could end up trying to swap N & S , making things worse.

I'll try a couple of neodyns on the ribbon pole pieces and report back.

Simon - thanks for the kind offer. I should have a few kicking around but I'll certainly shout if I can't find a suitable size.

 

[Rossi]

If I were you, I'd call Coles if they can help you re-gaussing it. If you're lucky, you'll talk to a ribbon enthusiast and they'll do it for free. I think it would be a shame to alter the construction.

I recently bought alnico magnets for guitar pickups. As luck would have it, most of them were almost demagnetized (probably due to bad storage). I was able to re-magnetize them to about usual strength using a couple of very strong magnets used for name tags. Someone told me that shouldn't work, but it seems it does. Maybe the magnets "remember" their previous structure. Perhaps you could try that, too.

 

[Marik]

Hey Stewart,

Those Varocoustics have a good motor (the same as RCA77, but louzy transformer). As a few folks have suggested, that mic uses Alnico, which loses magnetic properties with time and impacts. I serviced quite a few of those and from memory the readings on my Gaussmeter were all over the place between 0.2T to 0.35T, but again, who knows what's that straight from the factory. The STC/Coles supposed to be some 0.6T over a larger 1/4" gap, but also have bigger magnet. The Varacoustics' gap is much smaller @ 0.066", so I'd expect some good 0.5T-0.6T there. You will need quite a bit of force to get your screwdriver out of that field.

In US there are a few companies to re-magnetize those Alnicos. It is my understanding they use very quick discharge of current. From what I remember the DIYers use a large bank of high voltage capacitors and discharge it with a rat trap (no kidding). If you don't have Gaussmeter you should still should be able easily figure out the N/S with a compass.

If you get stuck send it my way and I can mill down the edges and stick there 1/8" Neos--will work like a charm and will be a permanent solution.

Best, M

 

[zebra50]

Hi Mark,

... need quite a bit of force to get your screwdriver out of that field.

I can certainly pull out a screwdriver without much effort, so it is increasingly looking like the magnet is substantially weakened. If you look at the photo you can see that the mic has taken a few knocks over the years, which might have contributed.

I'm really tempted to have a go at remagnetising it. Methods on line seem to vary - some suggest high voltage caps, others suggest a car battery and 5 amps of current. The rat trap sounds like a cheap redneck high power switch - I like it!

If you get stuck send it my way and I can mill down the edges and stick there 1/8" Neos--will work like a charm and will be a permanent solution.

Thanks for the kind offer. It's not my microphone, but if I don't get anywhere I'll point the owner in your direction.

 

[ArR]

Stewart, I have found an ideal D.C. PSU at work.
A propper one (deliver 33A into a short all day and don't complain)  ;D
So if you don't find a solution, it's another option for you.

 

[zebra50]

Thanks ArR!

I managed to get an extra 10dB out of the mic by adding the neodyns, and the mic is now at least usable.

A propper one (deliver 33A into a short all day and don't complain)

Holy shit! That's a whopper!
Where is that supply? I'm guessing it would be easier to take a mic to the mountain, than to move the mountain etc.?

 

[ArR]

Holy shit! That's a whopper!
Where is that supply? I'm guessing it would be easier to take a mic to the mountain, than to move the mountain etc.?

Amazingly the mountain is a mere 1U rackmount, modern class D thing, pricetag however is mountainous 
Does not belong to me, my boss is not quite so cool as to let me borrow a £3k + psu, but is the type to find a bit of vintage restoration an interesting and fun use of company equipment during lunchtimes.

So mic to the mountain (Down Portsmouth way) is an option if your client really would prefer the mic unaltered from original.
Have gaussmeters etc so should be able to dial up original field strength in the gap if you can find out what that was.

Simon

 

[Rossi]

Go find the power source you must, young Jedi!  ;D

 

[zebra50]

Ha! Excellent stuff.

I've actually sent the mic back to the customer complete with a couple of magnets lightly stuck to the sides. I think he's going to get in touch with you directly. Do please post the result here so I get to hear the end of the story.

Stewart

 

[ArR]

We have made contact 
Will Keep you posted.
With pictures of course  ;D

 

[zebra50]

I like it when a plan comes together. 

The neodyns are attached with their own magnetism plus one drop of superglue. They should just snap off if you need to remove them.

I think the impulse may snap or stretch the ribbon!

 

[Marik]

We have made contact 
Will Keep you posted.
With pictures of course  ;D

Cool! If you have a Gaussmeter could you tell me what the field strength do you get after re-magnetizing... real curious.

The neodyns are attached with their own magnetism plus one drop of superglue. They should just snap off if you need to remove them.

Many times used similar solution myself  8). The problems, 1) because of increased front-back path the top end corner goes down, and 2) the pole pieces dissipate quite a bit of strength (that's why all the modern ribbons with Neo magnets don't have pole pieces). But hey, at least now it works  .

Best, M

 

[zebra50]

Cool! If you have a Gaussmeter could you tell me what the field strength do you get after re-magnetizing... real curious.

And of course before re-magnetising too! 

I really must look into getting a gauss meter.

 

[ArR]

The patient's vital signs will be closely monitored throughout the procedure 

Zebra 50 is that a Sowter Tx you put in ?