Metallic shimmer and distortion from peerless tweeter in polk monitor 7b

audio68geek

Hi there, i am new to this forum, and would very much appreciate some help on my repair/mod of the crossover, where i think the problem is located. Some info about me: I am a 50 year old audio-nut, love the hobby(sometimes income to!), and have been using a soldering iron since about. 78 and do all work myself if possible, including mechanical/metal work. Have worked in the norwegian phone company, Electric bureau(state company producing all telecomm products inc. Heavy duty millitary radios) and Ericsson service dep. In the 80s i was also into CB radio, and obtained a radio amateur certificate. I addition i have several computer/electronic courses, inc building my own solid state 10w amp from scratch. Nowadays collect and repair electronics that I can find for free or cheap, and sell online, or keep them for personal use. A year ago i came across A pair of Polk monitor 7b, that really surprised me, and i think They are really worth reparing/upgrading IMO. Before ordering parts for the xover it would be nice to know if this. Is a problem with the tweeter or xover. My bet is the xover, but best to be sure. I also noted some people recommended not changing the electrolytic ones with others made from different materials, some suggested keeping the 12uf mylar cap. Any useful advice/help on these topics would be very welcome!

mlistens03

My guess (I don’t have the most experience with the speakers, but I’ll give it a go) is that the tweeter diaphragm is off center and the voice coil inside of the tweeter is rubbing against the side of the coil gap. I don’t know if this is fixable or not, you might need to order new tweeters. I think the 7b replacement tweeter is the RDO-194, which you can order from Polk, but don’t go and order them now because I’m no expert on these specific speakers and a lot of guys here are such as @verb .

pitdogg2

The RD-0194 tweeter is a replacement for the SL-2000 tweeter. Most here replace all XO parts with the exception of the inductors. All caps are very long in the tooth. While the mylar caps may be fine, caps have come a long way and metallized polypropylene caps are leaps better than the mylar. Look for Sonicaps and mills resistors at www.soniccraft.com

There is really no direct replacement for the peerless tweeter except for a used peerless tweeter. While it is possible to use the RD-0194 the cut out is different as well as the screw holes not to mention that the XO's for peerless are different then the XO for SL-1000 or SL-2000.

I would recap the whole XO from the site listed

audio68geek

Thanks for all help so far, the scenario described by mlisten03 is exactly what i fear as worst case scenario, or loose windings in the voicecoil. My best hopes is that the tweeter is overloaded, the sound in the left ch is louder in anyway and has been since i rescued the 7bs from the el recycling 15 months ago. The tweeter problem started 1 month back. The left ch has in addition to being loud a metallic shimmer in especially female voices, there is also a tape-like hiss in the background, i dont think the tweeter is able to produce thi kind of noise/distortion by itself, in a couple days i will take apart the crossover, pherhaps buy a handheld, cheap(sadly) LCR-meter. This and the xover i take with me to a house in the countryside just across the Swedish border, where i have stored lots of components and speakers, Snell, revolver, kef and others collected over 20 years. From these i will try to find components that can be used to perform a valid test on my hypothesis. In any way, i wont give up, in that i think the 7bs are clearly worth the effort. If i have to find a new tweeter i will try to find it for free, you wont belive what kind of stuff i can find at recycling plants or given away to flea markets. Some examples: div. Electrocompaniet, div Thorens vinyl spinners, rega, denon vinyl,cd,amps, ls3/5a rogers, 3 yrs ago a guy gave me a Conrad j ls17 preamp, an ls11 phonostage(i think) a cj 250 w solid state amp, and an Oracle delphi mk iv with sme v and lyra dorian cart. All this i got for setting up and maintaining a very expensive system, consisting of: complete nagra system pre, pwr, pwr supply, ph stage, vib plates, burmester cd, loudspk, topline audioquest, nordost cables, and to top it a monarch grand prix direct drive, super high- tech. The peerless is made in denmark, and Copenhagen is a short boat-trip away, maybe i will look for them there. I also know that these where used in, among others in several models of the Tandberg studio monitor, for some reason they use two in each speaker. These are insanely priced on the net. But living in this oil-rich, completely spoiled country of Norway makes me optimistic about getting them cheap, or free. I will use a pair of dynaudio audience in replacement for the 7b, while diagnosed and repaired, and hook them up to a Mondial designs acurus dia100 mk2 i found among lots of old stuff. Bought it for less than $180, a fuse in right power section blown, otherwise in PC! Will make a Nice amp for 7b. As i progress i will post updates to how it turns out.

F1nut

there is also a tape-like hiss in the background, i dont think the tweeter is able to produce thi kind of noise/distortion by itself

The hissing indicates a problem upstream such as your pre amp or amp.

delkal

F1nut wrote: »

there is also a tape-like hiss in the background, i dont think the tweeter is able to produce thi kind of noise/distortion by itself

The hissing indicates a problem upstream such as your pre amp or amp.

This. Is the hissing in just one speaker? Swap the speaker cables and tell us what happens.

Faustin

Have you tried swapping the tweeters from cabinet to cabinet to see if any of the issues follow the tweeter?

audio68geek

Thanks for suggestions, have tried 3 different amp, even totally checked my rotel rb 870bx, adjusting bias, cleaned the whole circuit board of old fluss and all contacts, pots, cut all wiring not needed (remote spkrs, headphone amp) rearranged all spkr, pwr, intercn to receive as little RFI/EMI as possible. This because i have had trouble with RFI at this place before (cj relay-control chip selecting sources at will, vol up and down, going on/off as fit! Impossible to use in other words.) Having done much radio installations/work in 80s i first thought that was the problem, because the left speaker is situated exactly behind the fiber-opt

pitdogg2

Never heard of fiber optic causing problems.

audio68geek

Sorry touched something on the ipad. As i said the left only has this problem, which started about a month ago, and is becoming untolerable very soon, i will shutdown and disconnect the 7bs, replace them with dynaudio floorstanders i found at a garage sale with a denon dp47f(i think), sold the denon, dynaudio for free. Anyway this problem is AS follows: metallic shimmer most easy to hear in female strong ,high frequency voices, but you can hear all the time also. I have had a few people listening, they say they cant hear it, which was no surprise in that the tape-like hiss is VERY high frequency, very difficult to hear, and rises with increased vol. A theory is that the tweeter receives much to high freq, this will cause distortion lower in freq. In addition to being to loud, a theory(or hope at least) There is no question that the problem lies in the left spkr at least. In a couple of days i leave Oslo, i will dismantle it, even reluctant to do so, because it has worked well so far, and has a very tight seal. I have also checked and cleaned all connections to the spkr and changed to new fuses, to no result. When the issue is resolved i will do a thorough job on them, damp vibs, some of these fancy larrys rings would be nice, new xover comps, wiring, fuses, holder, new spkr terminals. I have some really rigid stands for them too, which is never used. As this progresses i will post the result with pics here.
Thanx for nok, Ray

F1nut

Another thing to check, your fuses may look like this.

audio68geek

Have been very busy lately, so it took a few days to go to a house in sweden, where all my equipment are stored. I dismantled the left crossover and took it with me. After a couple of days drinking beer with some friends, i started visual inspection first, then measured the two resistors 2.7 and 4.5 respectively. Their values was almost as from the factory, with very small deviations from original value, No sign of corrosion on solder conn either, but the 12uF mylar cap was badly corroded where the metal comes out. the color is green, more or less. The wires going from the print to woofer and tweeter are both badly corroded. The mylar cap seems to have green shatterings on the inside too. The coils, and everything else looks very good, for a speaker from 10-7-80 as is written on the back of the print. I am now very optimistic about getting it right. In a couple days i will de-solder the wires and the 12uF cap, do some meaurements, replace new wires and try to find a replacement for the cap, before ordering a expensive x-over parts. I also bought a new multi-meter and a handheld LCR-meter the other day, it will take some time to learn properly though.

getting back in a few days