What is this? ( RTA 8TL )

mgebel70

Please excuse my ignorance; but I need help identifying this part. I believe it to be a capacitor, but I'm not familiar with this type. I just need to know what it's value is and what should I replace it with? It's out of a RTA 8TL crossover. I was having problems with sound from the tweeter. It sounded muffled. I did do all the proper steps to eliminate the receiver as the problem. It's definitely the one speaker. I pulled the crossover and removed all the caps. the two normal looking ones measured within specs; but this one read 0.048 nf on my meter. That reading is also what my meter typically reads when nothing is hooked up. I also measured resistance and it reads, out of range. There was no build up to this reading. And yes. I did discharge it, assuming it was a cap. Would this cap cause the muffled sound I was hearing?

westmassguy

That is a 16uf Mylar cap. If it's defective, your tweeter will sound distorted or not work at all

westmassguy

http://www.parts-express.com/solen-16uf-400v-polypropylene-capacitor--027-578

westmassguy

I don't know what you're budget is, but Sonicaps would be my preference. I believe they have a sale on now too. A good compromise, would be Sonicaps and Mills Resistors for the Hi-Pass Tweeter Section (12uf & 16uf), and Dayton or Solens for the Lo-Pass (34uf). your Ears will thank you.

mgebel70

Can I use any 16uf capacitor in it's place?

mgebel70

Thank you westmassguy.

westmassguy

More than welcome. There are better caps, such as Sonicaps, but these are conservatively priced, and better than what's in there now. I believe there's also a 12uf Electrolytic on that Crossover. You should consider replacing those as well, at least in the Hi-Pass Tweeter Section. There's also a 34uf Electrolytic in the Lo-Pass that deserves replacing as well.

mgebel70

You are correct. there is a 12uf and 34uf cap as well. I already have them pulled and laying on my work bench ( kitchen table ). I will definitely do as suggested and replace those as well. I appreciate all the help. I felt kind of dumb asking about the mylar cap; but I would feel even worse had I replaced it with something that didn't belong there and ruined my speakers. And by the way. For anyone else reading this and wondering. I did swap the tweeters around from the good speaker to the bad. I even went as far as removing the drivers and swapping them as well. Still made no difference. So the problem has to be with the crossover.

Thanks again westmassguy.

On3s&Z3r0s

One thing to think about when rebuilding the crossovers in RTA 8TLs with new caps is the size difference. I say this having recently rebuilt a pair myself and learned some lessons the hard way. I used 16uf and 12uf Sonicaps for the hi-pass and Solens for the lo-pass. All of these are a lot bigger than the original caps, and since the crossover mounts directly to the binding post cup you're going to have a real estate problem. My original placement of the caps overhung the board enough that I couldn't get the crossovers back in the hole cut in the cabinet so I had to re-do it. I actually ended up drilling a new hole in the crossover PCB to get everything to sit nicely, which may or may not have been necessary but made my life easier at the time.

So, this is probably obvious to any experienced speaker re-builder, but it definitely pays to think ahead to how the XO's will fit back in when replacing with caps that are times larger than stock. If you do end up going with the sonics and want to see my population of the board as an example, let me know and I can pull an XO and take a picture. Good luck!

mgebel70

Thank you On3s&Z3r0s: I never even considered that possibility. I'd of been pretty upset at myself had I soldered in all the new caps only to discover that it wouldn't go back in the speaker. That was definitely a good heads up. If for some reason I have problems with realty and can't resolve them; I'll contact you about those pictures. I still appreciate the offer. It's nice to know people are willing to go out of there way to help.

On3s&Z3r0s

I was wondering about that myself when I looked at the schematic and saw the two capacitors in series. I convinced myself that there must be something magic in the way the 12uF cap was paired with the resistor that kept them from going to a single cap there. You could fill a broken shot glass with what I know about crossovers, so I figured it would be best to keep everything the same.

As a counterpoint to that thinking, I will attach a picture of one of the "16uF caps" I replaced in the RTA 8TL crossover's I rebuilt. In case it isn't clear, it's a 12uF electrolytic cap paired with a 4.4uF mylar. Just goes to show that even back in the good ol' days Polk did some stuff that was a little dubious at times and didn't obsess over whether the details were 100% correct.

Given the fact that someone else has tried out the single 6.8uF cap instead and had good results, if I had it to do over again, I'd go that route myself.

mgebel70

That's some good info guys. Seeing how I haven't repaired my crossover yet ( waiting on parts ) I think I'll give the single capacitor a try. I ordered parts only for the one speaker. So I think I'll rebuild that one the way it was originally just so I can get it back in service; and then on the other I can try the single 6.8uf cap and see if I can hear a difference between the two.

mgebel70

I was also thinking. If I run a single capacitor instead of the two in series; Then the voltage rating of the single has to be same or greater as the two capacitors in series added together. Correct?

On3s&Z3r0s

mgebel70 wrote: »

I was also thinking. If I run a single capacitor instead of the two in series; Then the voltage rating of the single has to be same or greater as the two capacitors in series added together. Correct?

I couldn't speak to whether that statement is actually true, so hopefully one of the EE's would chime in. With capacitors in series I would think it's more of a "chain is only as strong as the weakest link" kind of situation for voltage ratings. That said, the electrolytic caps in my crossovers were rated for 50V and the mylar for 100V. All the film caps you're likely to replace with start at 200V and go up from there, so I think you're covered either way if you go with any of the usual Sonicap, Dayton, Solen, Clarity, etc.

westmassguy

On3s&Z3r0s wrote: »

I was wondering about that myself when I looked at the schematic and saw the two capacitors in series. I convinced myself that there must be something magic in the way the 12uF cap was paired with the resistor that kept them from going to a single cap there. You could fill a broken shot glass with what I know about crossovers, so I figured it would be best to keep everything the same.

As a counterpoint to that thinking, I will attach a picture of one of the "16uF caps" I replaced in the RTA 8TL crossover's I rebuilt. In case it isn't clear, it's a 12uF electrolytic cap paired with a 4.4uF mylar. Just goes to show that even back in the good ol' days Polk did some stuff that was a little dubious at times and didn't obsess over whether the details were 100% correct.

Given the fact that someone else has tried out the single 6.8uF cap instead and had good results, if I had it to do over again, I'd go that route myself.

The 2 ohm Resistor with the 12uf Shunt Cap is called a Contour Circuit. It's purpose is to modify, or in this case flatten the frequency response of the Tweeter. The Tweeter most likely has a gradual rise in output as the frequency drops. The Contour Circuit compensates for this. This is separate from the actual 2nd order crossover formed by the 16uf Capacitor and .4mh inductor.

Don't be overly concerned with voltage ratings for Metalized Polypropylene Caps.
Most start at 200 volts, and go as high as 600 volts.

When adding two or more Capacitors in Parallel to form a larger value Cap, it's always best practice to use Capacitors of the same type and manufacturer, and of equal value, or as close as possible. The 4.4uf and 12uf Cap shown, is a very sloppy way of doing it.

On3s&Z3r0s

Contour circuit... whaddya know. I figured it had to be something. I don't feel so bad for spending the time trying to fit all that stuff on the board now!

westmassguy

It's important, and it's there for a reason

mgebel70

Back again:

I just wanted to thank everybody for all the help. I finally received my parts this week. I installed all the caps, hooked up the speakers and powered everything up. Well.... The recapped speaker still sounded muffled. I pulled out the crossover ( which I am getting very proficient at now. Literally down to just a few minutes to remove or install. ) I removed all the caps and checked them; which I didn't do before the install. Turns out that one of the new caps was bad. Luckily for me; the bad cap happened to be the same value as one of the original caps that checked out ok. So I swapped the caps, put everything back together and gave it a listen one more time. All I can say is WOW. Sounds amazing. Now the one speaker that I didn't recap because I thought it sounded just fine... Well that one is next. I can absolutely without question, hear the difference between the two. I don't know if new caps typically would make such a drastic difference: But in my case they did. Again Thank You everyone for all the help. I can now sleep again. I can get quite obsessive about things. So much so that I have a hard time sleeping until I solve what ever it is that's puzzling me. These speakers were definitely keeping me awake at night.