Help with RTA 12B Crossovers

tholl1go

Hello,

First time poster here.

I recently bought a pair of RTA 12Bs in great physical condition with all drivers functional. I tested the capacitors when I brought them home and found that they were all open (OL).

I ordered and installed replacements, but these all tested open too. Then I went back and tested the old capacitors that I had removed and found that they are within specs.

So, the problem is clearly something else in the crossovers. Does anyone have any suggestions as to how I could troubleshoot and possibly repair them myself? I have successfully replaced capacitors in other speakers before, but that is the extent of my crossover knowledge.

Any suggestions would be most appreciated. If I have to I can take them to my local repair shop, but really trying to avoid those high prices (and I love fixing things myself).

Gardenstater

Welcome and nice score.

Something doesn't compute here. When you say you tested the "capacitors" and found they were all open are you trying to say that the DCR of the pos to neg on the binding posts was open? I take it you have a meter capable of measuring capacitance because you later say the old ones are in spec.. I'm also assuming you know that capacitors don't pass DC current.

Go back over what you wrote because it's a tad confusing to say the least.

F1nut

What are you testing them with and what are you testing for?

tholl1go

Thank you for the welcome.

I am using a multimeter to test capacitance by attaching the leads to the capacitor wires. Basically, when the capacitors are not attached (soldered) to the crossover board, my multimeter shows that they are in spec. But when they are attached to the crossover board, my multimeter shows OL when I conduct the same test (multimeter leads to capacitor wires).

Gardenstater

Well I think you've proven to yourself that you usually cannot measure a capacitor when it is soldered into a circuit on a PCB, depending upon the circuit of course. The more complicated the less likely to get an accurate reading.

F1nut

tholl1go wrote: »

Thank you for the welcome.

I am using a multimeter to test capacitance by attaching the leads to the capacitor wires. Basically, when the capacitors are not attached (soldered) to the crossover board, my multimeter shows that they are in spec. But when they are attached to the crossover board, my multimeter shows OL when I conduct the same test (multimeter leads to capacitor wires).

That dog don't hunt. Test them out of the circuit.

Gardenstater

Supposedly you can test a capacitor to see whether it is bad or not, in circuit, but only if you have an ESR meter. https://www.yamanelectronics.com/test-capacitor-without-desoldering/

But if there are other components in series or parallel with it, you can't measure the capacitance while it is in a circuit.

tholl1go

I didn't realize that was usually the case. I have recapped four other pairs of speakers and the test worked fine for them in circuit. Very strange.

What really made me think there is another issue is the sound that these speakers produce. I've read the rave reviews about them, but to me they don't have anything on my (much cheaper back in the day) Advent 5002s or Avid 103s as far as clarity, detail, bass response, etc. I was figuring that a recap is what was needed like with my others, but, meh. It's all subjective, though.

Maybe I don't have enough power to drive them. Would have to wait until I can afford more. Currently using a vintage Sony STR-7065 (very conservatively rated at 65 watts @ 8 ohms, 85 @ 4 ohms).

Thank you very much for your help.

F1nut

That receiver isn't up for much. That said, check that drivers aren't locked up and check for air leaks.

CH46E

Make sure you have the right speaker on the right and the left on the left. There should be a L and R on the model number on the white sticker in the rear.

They really are a fine speaker. A recap is definitely worth while.

When I got mine, someone had jammed the quick disconnect behind the crossover in the wrong position. Make sure they are plugged in right. This will really screw up the sound if they are not right.

Do they have the correct Peerless tweeter in them?

CH46E

If you are standing in front of your speaker there should be two black and one white wire. Make sure the white wire is facing left if you are still in front of each speaker. Goes for the left and the right speaker. White wire to the left.

Gardenstater

tholl1go wrote: »

I didn't realize that was usually the case. I have recapped four other pairs of speakers and the test worked fine for them in circuit. Very strange.

That's an RTA speaker and it has a complicated crossover, with capacitors in parallel with each other. Not sure why you are getting a 0 capacitance reading though. You should just be getting a much greater reading than the actual. Even if you disconnected the 3 prong plug, pulled the fuse, and disconnected the drivers, you still wouldn't get accurate readings without desoldering. The ZERO reading is a mystery though, to me anyway.

Gardenstater

Also, if you are worried that they are anything but awesome speakers, reading the review here: RTA-12B, Stereo Review, Jan 1982, p. 32, should reassure you. Of course everyone's hearing is different but just sayin':

https://www.americanradiohistory.com/Archive-All-Audio/Archive-HiFI-Stereo/80s/HiFi-Stereo-Review-1982-01.pdf

tholl1go

I reconnected the 3-prong plug in the proper position and now I'm very happy (thanks, CH46E). I did another A-B test with my others speakers and there is no comparison . . . A different league indeed.

They have the SL-2000 tweeters. Based upon everything I read in discussion boards, I ordered the RD0194 at the same time that I ordered the capacitors, but still waiting on the tweeters to arrive.

Definitely keepers!

CH46E

If they are indeed RTA12B and not 12C or 12A then the RDO194 is not the correct tweeter. You would need a pair of Peerless 5514 or 7514.

tholl1go

From what I have read in these forums and elsewhere, late 12Bs used SL-1000s, the crossover of the 12B is the same as the 12C (which used SL-2000s), and the RD-0914 is the direct replacement for the SL-1000 and the SL-2000. What am I missing?

CH46E

Someone with a bit more knowledge than I will have to explain it.

I've never known a 12b to have anything other than the Peerless. I'm pretty sure the 12b and 12c are NOT identical crossovers. I could be wrong. I dont retain information like unused to.

pitdogg2

Some RTA12's did come with sl1000's I've seen them. That whole 12 series can be very confusing.

Gardenstater

I've been searching through Stereo Review and High Fidelity and Audio back issues and I haven't been able to find a 12B with anything but a Peerless (so far). I did find a 12C with a SL-1000 in the Oct 1984 issue of Audio. Aside from that one all the other 12C I have found have had the SL-2000.

Oct 1984 Audio 12C (with SL-1000)




Oct 1987 Audio 12C (with SL-2000)

tholl1go

Here are some of the discussions I read before ordering the RD-0194s:

https://forum.polkaudio.com/discussion/152847/rta-12-which-model-are-these/p1

https://forum.polkaudio.com/discussion/131663/rta-12b-help-binding-posts-tweeters

https://forum.polkaudio.com/discussion/156043/rta-12b-rta-12c-differences

https://forum.polkaudio.com/discussion/96857/rta12-differences

https://forum.polkaudio.com/discussion/57052/rta-12b-different-caps

https://forum.polkaudio.com/discussion/106104/sl-1000-to-peerless-swap

Gardenstater

Are you sure you don't have a 12C? What's the serial #, MW #, and dates on the XO and MW's? If you are feeling like you'd like to experiment (Peerless vs RD0-194) , you might try a pair of Peerless. There's a pair of Peerless 5514's avail right now on eBay for $60 shipped but the seller is giving offers of $50 (I got one).

tholl1go

If the serial numbers on the binding posts are correct, I think I might actually have (or what started out as) a 12B and a 12C, both with SL-2000s installed. Both crossover boards are green and labeled RTA 12B.

Left:
Binding Post Serial # L12B 18133
XO Serial # 22 19884 (don't see a date)
MW6600X dated 1/12/83
MW6600X dated 1/12/83

Right:
Binding Post Serial # R12C 21247
XO Serial # 22 19494 (don't see a date)
MW6500 dated 5/13/83
MW6501 dated 2/28/85

Gardenstater

OMG the plot thickens!

I'm assuming the crossovers are electrically the same? Do they have polyswitch or fuse?

Somebody been messin' around here for sure!

Gardenstater

According to geppy1 in one of those threads you posted:

"There are late RTA 12Bs that have a higher serial number (on back) than early RTA 12Cs and the speakers are identical.
Early 1984 is where this occured (18,000-19,000 ish) Early 1984 is a mess"

So apparently there was some screwy stuff going on even at the factory in the transitional period between the 12B and 12C. I'd like to think an audio dealer wouldn't sell someone a 12B Left and a 12C Right but who knows lol! I'm sure Polk wouldn't have put two different MWs in a cabinet though!

Our only schematic in the archives shows MW 6501 in the 12B.

Also, my understanding is (please correct me someone, if wrong) that the SL-1000 came out before the SL-2000 and was even shipped out to people as a warranty replacement for the Peerless. I'm skeptical that either of those speakers was originally equipped with an SL-2000.

tholl1go

Both crossovers are fused.

Now my new question is, are any of these three different MWs not suited for what I have? I'm reading old discussions and have read (including from F1nut) that at least two of them have different specs

Gardenstater

Off hand I don't know the specs for MW 6600X but the MW 6500 and 6501 are pretty darn close as per this thread (except for DCR). I think you could easily check the DCR (DC Resistance) of all 4 of your drivers.

https://forum.polkaudio.com/discussion/133992/thiele-small-parameters-for-mw6500-drivers

Both crossovers are fused OK, but are they totally the same electrically (ie. same value components), and do they match the schematic in the archive?

tholl1go

Yes, the crossovers have the same values.

The 6600Xs are measuring 7.4, the 6501 7.9, and the 6500 6.7.

These discussions make me think that the 6500 is the odd duck and might need to be replaced with a 6501 or 6600x for best results:

https://forum.polkaudio.com/discussion/154649/mw-6500-vs-mw-6501

https://forum.polkaudio.com/discussion/189752/mw6600x-specs

tholl1go

And the crossovers do match the schematic.

Gardenstater

Yeah that was my feeling as well. Too bad we don't seem to have the Thiele Small Parameters for the MW 6600X anywhere on the forum.

@KennethSwauger do we have these specs avail. anywhere at all? Thanks.

There I tried lol.

Gardenstater

I think maybe your speakers were assembled by Polk Salad Annie herself (heh heh.......sorry I crack myself up sometimes).

tholl1go

LOL. They do seem to be Frankenpolks, but for $100 before modifications, I can't complain.