RTA 12C question

drumminman

Hi everyone,

I just joined yesterday and this is my first post.

Back in 1984 I bought a set of RTA 12C speakers, which I thought sounded great with an original Carver receiver (130 wpc).

I upgraded in 2002 and they've been just sitting since then. I'm finally getting around to setting up surround sound and I plan on using them as L-R mains, with a CS 10 center channel (on order). I'm planning on getting a small sub as well as using two small Energy e-xl's as my surrounds. The room is around 14 x 12 with 9' ceilings and open on two sides.

Since the crossovers are 26 years old I'm pretty sure the RTA's would benefit from replacing the caps and resistors. I found the schematics for the 12B, but I can't find 'em for the 12C. Are they the same? If not does anyone have them? The 6.5" woofers and the passive radiator are intact and look brand new.

I've tried different searches here and can find very little about these speakers.

Also, does anyone have experience combining the CS10 with the RTA 12 per above? Or is there a combination that would work better?

Lastly, I need the top hat for one as the original has pretty much disintegrated beyond repair. If anyone has one for sale, or knows of one please get in touch.

Looking forward to advice, opinions and comments from all you folks!

Thanks in advance,

Fred

ben62670

Welcome to Club Polk. If you can't get the diagram you can get the values of the caps and resistors fairly easily. Call Polk CS and see if they can help you get the diagram.
Merry Christmas
Ben

george daniel

then call Elliot @ Sonicap,,their CS is hard to beat.

geppy1

The crossovers between late Bs (1983) and Cs are the same with one exception. The only difference is in the late Cs that came with SL 2000s. They took the fuse off and put in a poly switch and a block resistor. I assume the resistor is to make up for the fuse??? Keith

DaveHo

There are also x-over components inside the cabinet which you will want to replace. The RTA 12's have one of the most complex x-overs in any Polk speaker. You're looking at a sizeable chunk of change to upgrade them.

-Dave

brgman

Fred-i just swapped my 12C components over to earlier cabinets i got from geppy1 and have grills and tophats if you need them.Mike

geoff727

drumminman wrote: »

I found the schematics for the 12B, but I can't find 'em for the 12C. Are they the same?

I am currently working on a pair of 12C's. The crossover is the same as the 12B (listed as "schematic rta 12b rev. pdf" here under the RTA series http://www.polkaudio.com/forums/showthread.php?t=38755), but with two small exceptions:

1. Between the polyswitch (RDE 050 A) and the first series cap for the tweeter (the 4.4 uf), there is a 2-ohm series resistor.

2. On the bottom of the diagram, where it shows a 27uf cap and a 7.5 ohm resistor across the terminals of the MW6501, those two components are backwards. Reverse the resistor and the cap, and it will reflect what's on the actual crossover board.

As always, verify the circuit by tracing it on your boards.

Good luck,
Geoff

drumminman

Great help guys.

Brgman, I need one top hat - everything else is in great shape. Let me know how much.

Geppy 1, I just popped the top hat and these have the fuse, so I'm assuming no polyswitch. They also have the SL1000 tweeter.

There are no numbers on the caps in the crossover on top of the cabinet, and the board has "RTA 12B: TDO Filter System" printed on one side. I just had a look at the schematics again and mine appears to be the orginal 12B crossover with the 3/4 amp fuse instead of the polyswitch.

Geoff, any tips on how to separate the board with the caps from the two large inductors? The nylon stand offs look like they'll have to be cut and replaced. Also, I read somewhere that if the electrolytics are replaced with polypro or better caps they do not have to be bypassed. And one more question: do the silver mica caps lose their value over time like the electrolytics and need replacing? What brand and values are you using?

Thanks Guys and Happy Holidays.

Fred

MNmike

If I remember correctly the board standoffs are barbed. So they are reused. You will have to desolder the inductors also. I found that there was some caps that needed to be desoldered also so you can read the values. Replace all but the bypass caps and any resistors with the values printed on them. (or close) You may want to also do the LFM in the cabinet also. Just wouldn't go with the expence of Sonics. Be sure to check the resister on the LFM. I've seen them half burned up before.

I'm currently looking at upgrades to orig RTA12 and RTA12b's. That is why I type this from work...I need the extra$$ with all them spendy caps.

I would use Sonics for the HFM and Daytons on the LFM (caps)

And Mills for resistors.

Get ready to spend $ and enjoy!

geoff727

drumminman wrote: »

Geoff, any tips on how to separate the board with the caps from the two large inductors? The nylon stand offs look like they'll have to be cut and replaced. Also, I read somewhere that if the electrolytics are replaced with polypro or better caps they do not have to be bypassed. And one more question: do the silver mica caps lose their value over time like the electrolytics and need replacing? What brand and values are you using?

MNmike is correct. There's a small flange you can squeeze with a pair of needle-nose to release those stand-off's.

Whether or not to bypass a larger cap with a small one (like a .1uf) is a matter of opinion (and application). Some like it, some don't. Personally, I shy away from opinions that offer an absolute "DO!" or "DON'T!". But that's just me. I investigate and try things for myself and form my own opinions. I won't say any more about this for fear of being shot down.

The rebuild I'm doing on the 12's has a mix of AXON capacitors for the larger values, and ClarityCap SA's for the smaller value tweeter series caps. If I were to add a bypass on the ClarityCap's, I would do it with something of high-quality, like a Mundorf Silver/Gold/Oil. But that's getting expensive, and I'm doing these for someone else and on their budget. There's a host of other changes to the speakers as well.

G~

MNmike

Yes, bypass caps are a opinion. Just as anything musical. But the good thing is one can add them latter with relative ease on the HF crossover.

As far as cap quality, you really want a higher quality cap for the HF where that frequency range stands out.

But again, that can be very expensive. And which brand to use is again a matter of opinion.

I'm with ya geoff727, I know what you mean. :frown:

Are you replacing the hookup wiring too?
What will you use for wire and how/what for terminations?

geoff727

MNmike wrote: »

Are you replacing the hookup wiring too?
What will you use for wire and how/what for terminations?

Yes, the person I'm refurbing these for (another forum member) was interested in new wiring. So, we are going with something relatively inexpensive to remain within the budget, probably Monster XP. I'm not sure yet if I'll use quick disconnects or solder them directly to the driver terminals.

G~

geppy1

drumminman , What are your serial numbers on the back?? Those are early 120C or late 12Bs 1984-85 so there is no Polyswitch.

drumminman

Geepy1 - the tweeters are SL1000's and the HFM are fused. I'm looking at the manual that came with them, and they're definitely RTA 12C's.

Hey Mike (Brgman), I'd like to buy one of your top hats. If you'd like to sell them as a pair only I could do that too. Let me know.

I hooked the speakers up couple of days ago. They haven't been played in at least 5 years, and they seem to have opened up a little after putting 10+ hours on them.

They are being powered by a Carver TFM 42, which is fed by the L-R main preouts from a Yamaha RX-V667 which is still burning in.

Still waiting on a CS 10 center channel, and looking at small subwoofers to complete the system. I'd like to hear sw suggestions if anyone has a smallish one (10-12") that they like.

Geoff727, MNmike - have you guys thought about doing point to point wiring on the crossover board? Is it too much of a pain?


Cheers,

Fred

brgman

Fred
I have the pair for you.I will pm you.
Those are great speakers.I have a pair and will never part with them.Clean em up and do what these guys tell you to get the most out of them and you'll be a happy man.
Mike

geppy1

That would be an early 12C from 1984-85. Serial probably in the 18,500 to around 21,000 range

geoff727

drumminman wrote: »

Geoff727, MNmike - have you guys thought about doing point to point wiring on the crossover board? Is it too much of a pain?

Point-to-point wiring is no more difficult or easy than soldering on a printed circuit board. If you wanted to do point-to-point on an RTA12, you could probably make a nice-looking crossover by using the appropriate size "prototyping board", laying out the components nicely and logically, and doing all the soldering underneath the board (like the picture here, which is one I did for an old Infinity speaker). The prototyping board could snap on top of the two big inductors, just like the existing board does. Done right (maybe with a Polk Audio logo attached to it), it could be a neat looking application. Or course, you lose some of the originality (and thus resale value, too). But, I suppose you could just keep the existing PCB and just point-to-point wire everything underneath it, too. There's a million creative ways to do this stuff.

The ones I'm working on will keep the existing PCB to correspond with the owner's wishes.

drumminman

Nice work geoff. What speaker is that for? You have some serious coin invested there.

At this point resale value is not much of a concern for me as I probably couldn't get that much anyway. I am interested in making them sound as good as possible though!

Fred

geoff727

drumminman wrote: »

What speaker is that for? You have some serious coin invested there.

Those were for a pair of Infinity RS-IIIa's, a project that is still on hold pending the completion of others. And you're right, those cost more to put together than what i bought the speakers for.:rolleyes:

drumminman wrote: »

At this point resale value is not much of a concern for me as I probably couldn't get that much anyway.

I always worry about this, although I don't know why i should. You are right, the absolute value of the speakers is often low enough that it's kind of dumb to worry about resale value.

drumminman

Hey Mike,

The forum won't let me do private email until I get 10 posts, though it let me send those first few. Go figure. . .

The price is fine; I'm also OK with a little dirt as long as they'll clean up OK. Send me another private email with your contact details and I'll get either a check or a money order in the mail to you.

Thanks,

Fred