KA1J Posted February 19, 2018 Share Posted February 19, 2018 Hi, I have a Chorus on the bottom and a Forte II on top fed parallel at 4 ohms. A a pair of these make up my sound system. I replaced the tweeters and mids with Crites replacement 2.5 years ago and rebuilt the crossovers with Crites update kits at the same time. Amp is a McIntosh and delivering 75W/channel. I've never used the amp wide open, maybe 75% full volume at maximum. At first all was excellent and the sound superb. I've been away from the equipment for awhile and hooked it to my television and ended up buying SVS Subs to accompany the Crites. It soon became apparent there was something wrong with the sound and I discovered both of the Chorus tweeters were not working in the least. I removed the tweeters and while still connected, listened to cymbals and zero came through the tweeter pressed against my ear. I checked and no connections had come loose between the crossovers and tweeters. I removed the tweeters and measured the ohms at the terminals and they came out .6 ohms and 1 ohm. Reconnecting the wires did not change these readings. It's my recollection that the tweeter should be somewhere around 5-6 ohms resistance. I took an AAA 1.5v battery and was able to generate a tiny click when quickly making contact with the terminals, Doing the same with a car stereo speaker with combined tweet/mid/woof (measures 3.6 ohms) gave an assertive click. My guess is either both tweeters have failed or both crossover rebuilds have failed or some combination of both. There can't be more than 200 hours of use on these over the last two & a half years. Any ideas what I might try at this point? I don't want to rebuild the crossovers again, I'd have to get new ones. Maybe there's something that can be done with the tweeters before replacing them again? Thoughts? Thanks. 1 Quote Link to comment Share on other sites More sharing options...
Alexander Posted February 19, 2018 Share Posted February 19, 2018 Not to answer your question but to have those speakers in parallel must be putting a heck of a load on your amp with less than 3ohms Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted February 19, 2018 Share Posted February 19, 2018 11 minutes ago, KA1J said: Hi, I have a Chorus on the bottom and a Forte II on top fed parallel at 4 ohms. A a pair of these make up my sound system. I replaced the tweeters and mids with Crites replacement 2.5 years ago and rebuilt the crossovers with Crites update kits at the same time. Amp is a McIntosh and delivering 75W/channel. I've never used the amp wide open, maybe 75% full volume at maximum. At first all was excellent and the sound superb. I've been away from the equipment for awhile and hooked it to my television and ended up buying SVS Subs to accompany the Crites. It soon became apparent there was something wrong with the sound and I discovered both of the Chorus tweeters were not working in the least. I removed the tweeters and while still connected, listened to cymbals and zero came through the tweeter pressed against my ear. I checked and no connections had come loose between the crossovers and tweeters. I removed the tweeters and measured the ohms at the terminals and they came out .6 ohms and 1 ohm. Reconnecting the wires did not change these readings. It's my recollection that the tweeter should be somewhere around 5-6 ohms resistance. I took an AAA 1.5v battery and was able to generate a tiny click when quickly making contact with the terminals, Doing the same with a car stereo speaker with combined tweet/mid/woof (measures 3.6 ohms) gave an assertive click. My guess is either both tweeters have failed or both crossover rebuilds have failed or some combination of both. There can't be more than 200 hours of use on these over the last two & a half years. Any ideas what I might try at this point? I don't want to rebuild the crossovers again, I'd have to get new ones. Maybe there's something that can be done with the tweeters before replacing them again? Thoughts? Thanks. If it were only one, I'd diagnose it differently than 2. With both tweeters out I wouldn't look at the crossovers as it's unlikely that the both went out PLUS you've confirmed an issue with the tweeters using a meter. I'd say they were over driven and need replacement. You might visit with Bob about tweeter protection OR get an amp that can handle the load a little better. 1 Quote Link to comment Share on other sites More sharing options...
KA1J Posted February 19, 2018 Author Share Posted February 19, 2018 Thanks for the replies, The amp is a McIntosh MC275 VI set to stereo (75W/ch) and has the option of driving at 4, 8 or 16 ohms. I have it in the 4 ohm position. I've never put the amp up to maximum, I'm not into pain... With this particular equipment I can't imagine how I would have overdriven the tweeters. Unfortunately, I threw out the original tweeters after upgrading so I don't have those to now compare with. 1 Quote Link to comment Share on other sites More sharing options...
Max2 Posted February 19, 2018 Share Posted February 19, 2018 They're likely both toast, but just unhook a speaker and hook it up directly to the tweeter with an AVR or something, of course at a very low level. That's a lot going on for that 275 to feed. Given a Damping factor of 22 and it really being a two channel amp, no telling how low the ohm dips are with your content, which are likely increasing the THD numbers well above where they are spec'd by the factory. Quote Link to comment Share on other sites More sharing options...
Alexander Posted February 19, 2018 Share Posted February 19, 2018 8 minutes ago, KA1J said: BTWThanks for the replies, The amp is a McIntosh MC275 VI set to stereo (75W/ch) and has the option of driving at 4, 8 or 16 ohms. I have it in the 4 ohm position. I've never put the amp up to maximum, I'm not into pain... With this particular equipment I can't imagine how I would have overdriven the tweeters. Unfortunately, I threw out the original tweeters after upgrading so I don't have those to now compare with. Let be make sure I understand what you have, is it that you have a pair of forte II's & a pair of Chorus I's? If so your amp sees 2.67ohms (based off of advertised impedance values). BTW are you a ham? KA1J looks like you are, I am KJ4NKN General class Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted February 19, 2018 Share Posted February 19, 2018 32 minutes ago, KA1J said: Unfortunately, I threw out the original tweeters after upgrading Cardinal sin! 1 Quote Link to comment Share on other sites More sharing options...
KA1J Posted February 19, 2018 Author Share Posted February 19, 2018 Max, I have no AVR, the TV has only an optical or HDMI out. I have the optical going to the C2500 and have HDMI connected to the computer and through a soundbar. The SVS subs are self powered and they connect to a pair of outputs on the C2500. I have no way to measure THD, all that good test equipment is long gone, unfortunately. Not enough room along life's pathway and I had to thin much I wasn't using. I could put the amp at a very low output and connect to it, I suppose at this point it won't matter. Alexander, Yes, been a ham since 79, & nice to meet you, my main interest is DX, contests and CW. I've been under the impression each of these speakers were 8 ohms & with two wired in parallel, the net impedance would be 4 ohms. If they are not 8 ohms, then being in parallel is a bad mistake on my part. To come up with 2.67 ohms you must be looking at different information than 8 ohms. CECA, yes. I thought they were out in the garage but nobody goes there except for me and they were not where I remember them to be so I must have thrown them out. I restore old motorcycles and always keep old parts, it's a habit... Surprised these are gone, they must have been pitched during a cleanup last year 1 Quote Link to comment Share on other sites More sharing options...
Alexander Posted February 19, 2018 Share Posted February 19, 2018 I could never get CW down, just one of those things I guess I started in 92 (KD4UQZ) but had family to raise and could not spend any time on it. Got back in in 09. I like DX'ing the most but have been on a few nets before I moved to NC. Haven't had much of a chance to hang any wire yet. The HOA is a party pooper for antennas but I have 4+ acres of woods so I want to through up a long wire and a 75/80m loop when the time is available. And the forte II's (and I's) are rated at 4 ohms but actually go down into the mid 3's IIRC. Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted February 19, 2018 Share Posted February 19, 2018 49 minutes ago, KA1J said: I've been under the impression each of these speakers were 8 ohms Yes, nominal but throughout the frequency range they can dip quite a bit. Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted February 19, 2018 Share Posted February 19, 2018 For example, this is a Forte I. Look at the dip around 130Hz, it's below 4 Ohms. Edit, the link says Forte but Graph says Quartet but you get the idea. It isn't a constant 8 Ohm load on the amplifier. Quote Link to comment Share on other sites More sharing options...
KA1J Posted February 22, 2018 Author Share Posted February 22, 2018 A follow-up. I'm not sure what killed them but the tweeters are indeed fried. I removed them and looked carefully and both of the coils were fried. Not the whole coil was fried, just the lower edge, the edge closest to the titanium dome/board,. I could have re-wound them with magnet wire, I think I have some of that same diameter but I just re-ordered a replacement pair as this way there will be no issues. Bob sent them immediately & they should be here tomorrow. What concerns me though is why they fried. The 75W per channel from the MC275 doesn't exceed their specs and both of them suffered the same damage. If it were a transient of some kind from the power line it would have surely damaged the C2500 or the MC275. If nothing else, I had the Forte connected in parallel with the Chorus and those tweeters are also titanium replacements and they are both working perfectly. One tweeter on the Chorus I failing I can picture, but for two of them to fail and the Forte pair equally attached & unscathed doesn't make sense to me. Odd. 1 Quote Link to comment Share on other sites More sharing options...
absolve2525 Posted February 22, 2018 Share Posted February 22, 2018 Is there ferrofluid in these tweeters? It can get thick and muck up the coil if it dries over time. Perhaps it needs to be replaced? Not sure, just an idea. Quote Link to comment Share on other sites More sharing options...
KA1J Posted February 22, 2018 Author Share Posted February 22, 2018 No, they were Ti replacements for the originals, maybe close to 3 years old. The Forte are different tweeters than the Chorus but they too are Ti and the same age. The forte are fine, the Chorus both are toast. 1 Quote Link to comment Share on other sites More sharing options...
Alexander Posted February 22, 2018 Share Posted February 22, 2018 1 hour ago, KA1J said: No, they were Ti replacements for the originals, maybe close to 3 years old. The Forte are different tweeters than the Chorus but they too are Ti and the same age. The forte are fine, the Chorus both are toast. The k75k in the forte I/II and k79k in the Chorus I/II are the exact same tweeter with the exception of the k79k has ferrofluid (or did when new) and the k75k did not. Don't know why the k79k's were hurt and not the k75k's but anytime you have the impedance lower than an amp was designed for the chances of tweeter killing clipping is increased. Just a possibility. Quote Link to comment Share on other sites More sharing options...
KA1J Posted February 22, 2018 Author Share Posted February 22, 2018 It was obviously something definitive to affect both of them. Not knowing the exact cause of the Ti tweeter failures, I'm now concerned about the coils also being damaged in the woofers. I can't get to them without removing the cone. At this point I'd like to rebuild them just to be sure they're OK but can't find a kit for the K-48-E, only the K-48-K. I plan to keep them but if I ever did sell them I'd want to only sell something that is known to be 100% intact. 1 Quote Link to comment Share on other sites More sharing options...
Alexander Posted February 22, 2018 Share Posted February 22, 2018 Not to claim to be an expert by any means but usually only the tweeter gets hurt unless something in the crossover goes and then could take something else out with it. Maybe we can get some of the big boys in here and help out. Quote Link to comment Share on other sites More sharing options...
pzannucci Posted February 22, 2018 Share Posted February 22, 2018 Too much load on the Mac or an alignment problem on the tweeter diaphragms (cleaned voice coil gap). Very unlikely to be caps. You can always swap the tweeters between the Fortes and chorus's just for grins. If they work you definitely have the answer. The crossovers should be the same (outside of 40 ohm resistor) if Bob just does a parts swap. Quote Link to comment Share on other sites More sharing options...
KA1J Posted February 22, 2018 Author Share Posted February 22, 2018 What I did try was hooking up the tweeter from the Chorus, to the tweeter connections in the Forte; Nothing came from the tweeter. What I should have done was try the forte tweeter in the Chorus. Bad investigation technique on my part, I know better... Not wanting to repeat myself, I'm still trying to wrap my head around why these two exact same pieces failed. One component failing I get, but two tweeters burning out & each connected to separate channels in a not over-powered stereo system (75W/channel) seems unlikely. And I haven't played anything mono in a long time. The most taxing piece I can think of I've played was "Lucky Man" by ELP and I had the amp at 75%, hardly seems excessive & even so, the Forte tweeters equally connected are fine as frogs hair. It is possible I installed both of the tweeters incorrectly and they lasted for a few years and then both failed around the same time. The tweeters themselves had no defects in construction so the answer lies elsewhere. The new replacements came in just now, off to get them installed. 1 Quote Link to comment Share on other sites More sharing options...
Alexander Posted February 22, 2018 Share Posted February 22, 2018 The 40 ohm swamp resistor in the forte II's is what might have saved it's tweeters? Looking at the Chorus schematic it does not have one as pzannucci pointed out. Quote Link to comment Share on other sites More sharing options...
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