75T faint tweeter output/Bad Crossover

shodgson

I know i have seen threads on this from before, but figured since their last replies were from mid 2015, thought it would be worth a new thread.

Just yesterday I purchased a set of lightly used 75T floor standing speakers from someone who seemed trustworthy. I even listened to the speakers before I purchased them. But when I got home and hooked them up, I discovered one tweeter was very faint. I didn't notice this when i listened to them at his place because of how close the two speakers were to each other when I listened to them. The tweeter in the good speaker was loud enough I didn't notice the bad one.

While trying to diagnose the problem I pulled the bad tweeter out and noticed that the previous owner had already done this, the wire ends were all scraped up. He new there was a problem, and hid it so he could make the sale.

i put the good tweeter in the bad case, and it didn't work, i put the bad tweeter in the good case and it worked. it wasn't a tweeter that was bad. on to the Crossover.





Last night I continued to troubleshoot, and what I came up with is that the crossover is bad. (Hopefully this group can help me confirm this) When I jump the solder joints on the board for the tweeter, and bypass either of the capacitors for the tweeter, it works fine. When i touched either of the yellow joints to either of the red joints, the tweeter worked perfect.







I am wondering if there is a way to fix this. Can the capacitor be replaced? Or the entire crossover? Can I buy those parts from polk? The seller told me he was going to get the part from Polk warranty, but when he provided me the tracking number it is shipping from his town, not from Polk. He says he had a friend who happened to have three new crossovers for these speakers. we will see when it gets here.

I am very angry with myself for not listening to each speaker individually. I'm hoping my investment doesn't cost me too much more to fix. Hope the experts here can help.

IF this all goes well i am looking into getting 4 more T15's, a T30, and a PSW 108 to complete my 7.1 system.

Thanks, in advance from one stressed speaker buyer.

pitdogg2

appears to have cooked the resistor. common problem. Is the top bulged up on the black electrolytic capacitor?

Replace the 5wR75J resistor with a mills 12 watt 7.5 ohm resistor on both crossovers. Heck I replace both resistors with Mills on both boards.

The other larger resistor is a 15 watt 5.7 ohm resistor

Resistors can get hot I like to leave space between to board and the resistor on mine to help keep some heat off the board.

Can you solder? If so then you can also replace other items(caps) with film caps but they will be much bigger and you will need to get creative to get it all back on the board and back in the cabinet.


Welcome to Polk forum but keep a good hold on your wallet we'll help you spend some of it

shodgson

Thanks for the quick reply. So i know about circuit boards, but just to be clear, which one are you saying is cooked? (I have applied numbers in the below picture). Yes, the top of the one small black circular capacitor (#5) is bulged up.




So which two are you saying to replace with the larger capacitors? Is this a common thing to take a crossover that is designed by Polk specifically for these speakers, and rip it out and start "making it better"? For right now i just want to replace and get them working again. if it burns out again, id probably start improving rather than just replacing.

Yes, I can solder, but i might need to go buy a new solder sucker, if I remember right, i destroyed my last one.

Just for reference i am using a Harmon 1710S AVR, I plan to have 7.1 surround with these 75T's as fronts, the T15's for middle and back, and T30 or similar for center, and after a while ill probably get a PSW108, or maybe two since the receiver is able to do 7.2.

pitdogg2

Get the new crossover from the seller. He cranked the snot out of them and sent clipped signals which cooked the #3 resistor most likely which may of caused a problem with the #5 cap. My point was if it was not a warranty covered problem and you had replace 3-4 parts put better parts on. Mills resistors are cheap and vastly superior to the cement type. Going up to 12watts and same ohm value will add insurance so as not to happen again. ME I'd be on the seller for this problem if it's been offered. But remember to keep value under control or you will be seeing the same problem again.

shodgson

New Crossover showed up yesterday. And I'm happy to say, the speakers now sound fantastic. I would say the previous owners saying they owned them for 4 months, and didn't use them much, was a lie. Top picture is the new one. Bottom picture, the one that looks like it was in a fireplace, is the bad one.

tonyb

Glad they are working again. Previous owner probably drove the snot out of them at high volumes. Don't make the same mistake, everything has it's limits.

pitdogg2

I'd still switch out the (5wR75J) 5watt 7.5ohm resistor for a mills 12 watt 7.5 ohm for the added insurance. We have seen these cement 5watt resistors fail too many times.

Like Tony stated glad it worked out for the better.

tonyb

pitdogg2 wrote: »

I'd still switch out the (5wR75J) 5watt 7.5ohm resistor for a mills 12 watt 7.5 ohm for the added insurance. We have seen these cement 5watt resistors fail too many times.

Like Tony stated glad it worked out for the better.

Very true, and an excellent suggestion. Resistors are cheap, worth the added protection.

cowbot

So just to confirm, the resistor marked "5WR75J" is a 7.5 ohm, not a 0.75 ohm? I thought the R indicates the location of the decimal point (and 5W = 5 watt, J= 5% tolerance)

Thanks!

VR3

Looks like a .75 ohm to me

cowbot

Thanks for the confirmation. I wish Polk would publish cross over schematics for their old speakers.

SeleniumFalcon

F1nut

cowbot wrote: »

Thanks for the confirmation. I wish Polk would publish cross over schematics for their old speakers.

They did, top of the vintage speaker section. Not all, but a great many.