Help with RTA12 super weak woofer output

eurekaiv

Hello... new (to me) RTA12 owner. Unfortunately, I've got issues already. :sad:

One of my speakers is very weak sounding. As best I can tell, the tweet is working fine but I get very little sound out of the woofers. It looks and feels as if they're moving some air but the volume is low and thin. I've tried swapping channels and the problem follows the speaker so its not the amp. I had them apart yesterday to Dynamat and JB Weld the magnets but that shouldn't have fried them. Plus, they seem to be moving fine and seemed to sound fine bare wired, though the latter is a little hard to say for sure since they don't exactly sound good as full ranges with one out of the cabinet. One of these woofers was replaced at some point with a later Polk model (IIRC it was dated '89) that looks identical and the magnet on that one seems a bit off center but like I said, neither of them are rubbing or stiff so I don't think they're blown or frozen. FWIW source is a Marantz PM17 Intgrated, Thorens TD160 or my PS3 for MP3s when I'm too busy to flip records every 15 minutes.

drumminman

Which RTA 12's do you have - the first generation, B's or C's? Were the speakers functioning normally before you took them apart?

Rule of thumb is the last mod you did probably caused the problem. I would start there and work back. When you had them apart what did you disconnect/unplug?

Are you getting sound of both woofers or only one. On the later models one of the woofers plays a different part of the frequency range than the other (can't remember exact numbers but it goes lower but not as high).



The later RTA's have two Xovers - one attached to the inside of the binding post cup and one on top. The upper Xover plugs into a socket on top of the speaker - is this connected? Did you reconnect the quick disconnects to the MW's and push all the way onto the tabs on the speaker basket when you reassembled everything?

Sounds to me like there's a weak connection some where if the worked before your mods.


Pics would probably help.

Oh yeah - welcome to CP!

eurekaiv

Thanks for the quick reply! :-)

The RTA's are first gen with the peerless tweets so no additional crossover.

I pulled the drivers from the enclosure completely. In this case, the replaced one had just bare wire twisted to the lugs so when I put it back in, I soldered it in place. Prior to soldering them back in I trimmed the speaker wire down far enough so that there was no oxidation apparent, and then tinned them. I'd be shocked if I had a bad connection there but I suppose stranger things have happened. I did check them over again visually last night and my joints looked fine. Where else could I have a weak connection? The leads at the binding posts are soldered to a lug that mounts to the posts and it appears to be epoxied to the terminal. I'll check them again but if something came loose there the tweeter would be exhibiting the problem as well.

I'm getting sound out of both woofers, but again, at a very low level and very thin sounding. Same thing when bare wired (though as mentioned they do seem to sound more "normal" that way) which makes sense as they're wired in parallel according to the schematic and what I saw when opening them up.

I don't see any connections on these that aren't soldered in place so I'm not sure about the board. Is there a place on the first gen RTA12s with a connector as opposed to a soldered connection? If so, I'd bet you're right. If not, I think my next step is to swap the crossovers around and see what happens. I don't look forward to having my soldering iron hovering above these things though.

Here's a couple pics I took prior to doing the work. The closeup of the woofer is from just before I went to remove them to add the JB weld. I snapped it just to make sure I had a reference for which wires were positive and negative. Maybe that's my problem, I'll double check. Would be a really dumb mistake since I checked it like three times while working but I've been prone to this sort of mistake in the past.

The crossover on the speaker that works (with labels for me so I can order replacement caps and resistors)

A woofer.

Next to my old Regas... BIG difference.

eurekaiv

Sorted... wired in one woofer backwards so it was playing out of phase. :rolleyes: Anytime black is positive I get confused. Not sure why anyone would choose that as a scheme but alas, my stupidity here. Thanks for takign the time to read and help me work through it out loud so to speak. Now to move on to upgrading my system around my new TT and speaker setup. :biggrin:

drumminman

Glad it was an easy fix.

As you probably know, those 30+ year old caps and resistors are long past their use by date. If you like the way your 12's sound now you'll be amazed at how much better they sound with brand new film caps and resistors.

eurekaiv

drumminman wrote: »

Glad it was an easy fix.

As you probably know, those 30+ year old caps and resistors are long past their use by date. If you like the way your 12's sound now you'll be amazed at how much better they sound with brand new film caps and resistors.

It's next on my list actually. Any rec's on caps? Was thinking Daytons since they're reasonable and these will get expensive fast thanks to size.

drumminman

Never used Dayton's, but some who have seem to like them.

Sonicaps are what I've used with both sets of Polks that I've done (RTA 12C's and 2.3 TL's), and I like them alot. For resistors I'd recommend something like Mills 12 watt. You can get both at Sonicraft.

Sonicraft is still advertising their 20% off sale here: http://www.soniccraft.com/specials.htm

eurekaiv

Cool, thanks for the tip. Do you know which value of resistor is correct for R3? The schematic I found posted to the forum here shows 1.69Ω but my crossovers have two 4.5Ω resistors in parallel (for a value of 2.25Ω) and actually measure exactly 1.9 on both.

drumminman

eurekaiv wrote: »

Cool, thanks for the tip. Do you know which value of resistor is correct for R3? The schematic I found posted to the forum here shows 1.69Ω but my crossovers have two 4.5Ω resistors in parallel (for a value of 2.25Ω) and actually measure exactly 1.9 on both.

If your Xovers look like they are from the factory, i.e. no mods or alterations by previous owners, I would go by the values that are there. Polk often used up old stock or made modifications on the fly during production.

Wonder why they would wire two resistors in parallel? Why not use the value derived at? Do paralleled resistors have increased power handling?

eurekaiv

Resistors may have been used in parallel in order to give a value that wasn't available off the shelf. This makes sense in the case of a crossover as you probably want a more specific value to tune the filter to the desired cutoff frequency. Plus, high power wire wounds aren't readily available in odd values with tight tolerances.

Two 5W resistors in parallel will also yield 10W of power handling capability.

drumminman

If you can find the correct value a Mills non inductive wire wound would be a good choice - inexpensive and sounds very good.

Tbone289

Another good cap--especially for the price--is the Claritycap PX at Madisound.

http://www.madisoundspeakerstore.com/px-cap-250vdc/

eurekaiv

Cool, seems about the same as the daytons. Thanks for the link.

Tbone289

eurekaiv wrote: »

Cool, seems about the same as the daytons. Thanks for the link.

You're welcome. I've used Dayton and these PX's, and I much prefer the PX.

jon s

The RTA12s have a very expensive crossover if you use higher quality parts. Each crossover has 14 caps IIRC. I used Sonic caps and it was roughtly $600 to replace everything.

drumminman

jon s wrote: »

The RTA12s have a very expensive crossover if you use higher quality parts. Each crossover has 14 caps IIRC. I used Sonic caps and it was roughtly $600 to replace everything.

+1 - same here.

However the OP has the original RTA 12's, which have only the top Xover I believe.

eurekaiv

Yep... only crossovers on the top. It's about $75 for Daytons and $150 for PXs at Madisound. I think I'm going to split it up and put the PXs in the highs and Daytons in the lows since the latter isn't part of the direct signal path and should have little influence there with any quality part. Then I can get the total to about $100 which I'm comfortable with.

drumminman

Sounds like a plan. Interested in your listening impressions post mods.

eurekaiv

I did my cap job last night and there does seem to be an improvement. I think the most obvious difference is an slight overall increase in the bottom end. Less noticeable but present is that the highs sound a little less grainy. Sometimes vocals had an almost raspy tone that seems to be improved. Of course, could be pure placebo.

Overall, it's a fiddly job getting the things off and back on but it mostly went off without a hitch. About 2/3rds of my standoffs broke when pulling the xovers?the plastic obviously did not age well?but I've found the correct replacement at Mouser and will order some today.

I am having one moderate problem however. The positive lead for one of the tweets is cutting in and out. It seems to be the wire coming off the voice coil making a bad connection at the lug. It must have loosened up a bit when I desoldered the lead initially. If I fiddle with it some it will stick on for awhile but eventually the vibration seems to knock it loose again. Has anyone had any luck fixing this? I tried to run a little solder into the base of the lug but it didn't make any difference one way or the other.