New Project- 1935 RCA 74

[MikoKensington]

For over a decade I've been going to this one fleamarket. I've scored many things over the years. Guitars, amps, drums, speakers. But never a microphone.

There was an old coot named Glenn that had a little guitar store. He had junk to sell and the good stuff in cases hiding. He didn't want to sell anything the was worth a ****. He had this old RCA mic in his window ever since I started going there in 1992. I was ask to buy it but he never would.

So about 4 years ago two of my buddies go there and see the mic. One of them pressures him into selling so hard that he agrees. The other pays for it. $40.

So my buddy Cy who actually paid for it has been letting it hang out and collect more dust. The thing has no ribbon. And it's been horribly repainted. My grandpa used to do the same ****. Something's starting to show it's age. Slather it with some ugly gold paint.

Without further ado.

http://img.photobucket.com/albums/v473/MikoKensington/74rca1.jpg
http://img.photobucket.com/albums/v473/MikoKensington/74rca2.jpg
http://img.photobucket.com/albums/v473/MikoKensington/74rca3.jpg
http://img.photobucket.com/albums/v473/MikoKensington/74rca4.jpg
http://img.photobucket.com/albums/v473/MikoKensington/74rca5.jpg
http://img.photobucket.com/albums/v473/MikoKensington/74rca6.jpg

 

[rodabod]

Very interesting - it uses the same magnet setup as the Reslo ribbon microphones.

 

[MikoKensington]

DAY TWO

OK those pictures were taken at 4:30am. It's now 8:40pm technically it's the same day. But whatever.

I gave the painted parts a bath in Bix stripper. Then I gave them a water rinse. I then gave them a bath in laquer thinner to get the oily Bix residue off. Look kinda cool the way they are. I like the chrome and brass badge. I think I'm going to leave it that way.

I attempted to make a ribbon today. I cut it a little wide. But I quickly learned that the ribbon is probably half the problem. When I installed the ribbon and plugged it in. I got nothing. So I took it apart and found out how nasty this transformer is. The lams are gapped and looking just plain sorry. Where's CJ? You want a project?

http://img.photobucket.com/albums/v473/MikoKensington/rca1.jpg
http://img.photobucket.com/albums/v473/MikoKensington/rca2.jpg
http://img.photobucket.com/albums/v473/MikoKensington/rca3.jpg
http://img.photobucket.com/albums/v473/MikoKensington/rca4.jpg
http://img.photobucket.com/albums/v473/MikoKensington/rca5.jpg
http://img.photobucket.com/albums/v473/MikoKensington/rca6.jpg
http://img.photobucket.com/albums/v473/MikoKensington/rca7.jpg
http://img.photobucket.com/albums/v473/MikoKensington/rca8.jpg

Back soon.

 

[PRR]

> how nasty this transformer is. The lams are gapped and looking just plain sorry.

That should work. The lams overlap, so the gap is a non-issue. Might be a little less deep bass; won't be a no-signal.

Do both windings conduct electricity?

 

[rodabod]

Yeah, the transformer looks ok to me. As PRR says, have you tested it electrically?

You can check your ribbon assembly by using a multimeter set to M Ohms and attach one probe to the magnet assembly and the other to a ribbon clamp - there obviously should be no conduction.

For getting ribbon width correct, you can sometimes use the fitting which sits on top of the ribbon itself. The RCA mics may not have these but the Reslos do. It's basically a small piece of copper / brass which is exactly the same width of the ribbon - just sit it on top of your sheet of aluminium and you can then cut exactly the right width.

I've just got both of my Reslo SR1/L mics finished. I've not tried them yet, but I'll post the results on my thread.

 

[CJ]

Nice mic! I wish you would link to those pics, I had to do a cold boot after loading the thread, major hard drive trauma.

Those rounded L lams are limited in availibility. They were to keep the flux in the core as it rounded the corners I believe. Mag Metals still has a few. That might be non grain oriented steel since it is from way back. Grain oriented gave more perm I believe, which is why older cores tend to be bigger, like the old WeCo stuff.
They still make non oriented steel, so maybe you could talk Mag Met into staming you some samples. That would mean a set up fee.

You can do more damgae than good by trying to clean off the rust. Sanding the core can short out the lams, which are supposed to be electrically isolated.

 

[MikoKensington]

Well. I got sound. But it's awful. Sounds like changing the dial on an AM radio or something.

 

[rodabod]

That's strange.

Did you check for any conduction between the ribbon and the surrounding polepieces?

 

[MikoKensington]

Hey rodabod, how should the magnets be arranged. Someone got into this thing long ago and I don't think it was assembled right.

 

[Marik]

[quote author="MikoKensington"]Well. I got sound. But it's awful. Sounds like changing the dial on an AM radio or something.[/quote]

Miko,

Although the transformer is not the greatest, it should work fine.
A few suspects as for the sound you get.
1) Did you clean the clamp contacts? They get contaminated over the years and their resistance will be greatly increased. It will greatly affect the sound.

2) What material did you use for the foil? If it is a leaf found in a craft stores, its resistance may be too high for the transformer, and the bass will be loaded.

3) Does it sound kinda honky? Then you might overtightened the ribbon.

To measure the ribbon width, use calipers (preferably digital). Measure the gap width, then substruct about 0.02" for clearance (0.01" from each side). This value seems quite managable. If you can get tighter tolerances (i.e. to corrugate the foil very evenly, get the feel of ribbon installation, etc.) then try to get smaller gaps for clearance.

The magnet structure is actually different from Reslo, where it uses horseshoe magnets. In RCA they use bar magnets (cheaper) and there are more flux losses in the pole pieces. Overall, it has a huge magnet structure, which would result in better sensitivity, but much lower HF corner.

Next, 74B model uses alnico magnets, which are actual pole pieces, and have much less flux losses.

The right arrangement is if the S of all the magnets "looks" at the right (you can check it with compass and the red tip of the arrow will be attracted), then the top contact will be (+) of the signal. If it is S to the left, then your (+) is on the bottom of the ribbon.

 

[rodabod]

As Marik rightly said, you need to get the polarity of the magnets correct. If they are securely fixed to the rest of the assembly, then I would assume that they are in their original (correct) position.

If the magnets are loose, orient them correctly and then fix them with a very small amount of super glue.

Marik also pointed out that the clamp contacts must be clean. This is a very good point. I sometimes sand mine with wet-and-dry sandpaper if they are corroded. Otherwise, I will clean them with T-Cut and then remove any deposits using meths.

Meths in my opinion is very handy for re-ribboning. You can do a lot of things with it - eg. clean, hold ribbons in place.

You should take your time over fitting / fixing the ribbon. It is probably the most important part. From looking at the picture, yours might be slightly squint, but I'm not sure.

Looking good though!

 

[CJ]

If you ribbon guys are interested, I could slap together a DIY gauss meter with some of the old Hall chips we have here. It would just be a DC output voltage relative to the mag field. The sensor would go into a dual op amp, diff input, and then into the secon common mode amp for more gain. I could put a 4 turn trimmer on it so you could calibrate it off a magnet sample. Would run on single +24 supply. It might not be the greatest, but at least you could compare your magnets with a known ref.

cj