Torching FST ribbons on RM40's

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LosGatosSTI

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Torching FST ribbons on RM40's
« on: 19 Oct 2017, 04:14 pm »
I figure I should start a new topic for this as I buried my plea for help in a different thread....

I need your help guys.....

Bottom line is that I have now torched two pairs of ribbons on my old FST tweeters and torched two ribbons on brand new G2's I installed yesterday in my 2003 RM40's.  Thought I needed new tweeters because upon inspection of Brian's original installed tweeters in my RM40's, the metallic coating was flaking off the magnets on either side of ribbons and also the black fabric tape was not sticking to the magnets anymore. I thought this was shorting out the ribbons but I guess not.

I bought two Aurum Cantus G2's from parts express and replaced them yesterday into the RM40's.  Was listening happily and naively thinking I had solved the problem.  45 minutes went by like last time and then...Poof! two red fireballs at precisely the same time.  What I cannot figure out is why it happens at the exact same time on BOTH speakers.  This leads me to believe I have a problem with either the 14 year old Bryston 6BSST amp (still under warranty by the way) or something has failed in both speakers at exactly the same time? 

This time I had the Oppo 205 with RCA out to the CD input of Marantz Av7703 with XLR's out to the Bryston 6BSST.  The other two times it was with the Oppo 205 RCA out to Sunfire TG III with XLR out to bryston.    Source material was a "Beck" SACD being played with an Oppo 205 in all cases.  Oppo was connected by RCA's from the analog out stereo L and R outputs to the receiver CD L and R inputs.

I need your help guys.....

Could it be the Bryston due to the precise timing of failures?

Any help would be appreciated as I don't even know how to take the RM40's a part to check anything. 

Could someone please describe how to take the RM40's apart to look for damaged components?  I have a good FLUKE Multimeter, hand tools, and I am pretty handy and can solder basic stuff. 

What tools do I need and does anyone have instructions on how to change out or install new caps like TRT's?

Thanks and please feel free to call me also as it may be easier to figure out a solution

Thanks and feel free to call me.  If I don't answer please leave me a message where I can contact you to discuss my options.

(408) 221-9878

Scott

John Casler

Re: Torching FST ribbons on RM40's
« Reply #1 on: 19 Oct 2017, 07:47 pm »
If I had to take a WAG (wild ass guess), since I am not a speaker builder, I might think two possible issues:

1) Your amp is going into oscillation, and frying your tweeters
2) Both tweeter caps are fried.

Since they BOTH fail together, might point to the amp.


HAL

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Re: Torching FST ribbons on RM40's
« Reply #2 on: 19 Oct 2017, 08:35 pm »
Parts-Express use to sell new ribbons for the Aurum Cantus tweeters.  They also do speaker repairs if you want someone to install the new ribbon and test them.

Since the Bryston has been the same amp in both cases, sounds like ultrasonic oscillation.  You need an O'scope and know how to use it to see it.  Also see if you can check the crossover board to make sure something is not shorting out the series caps to the ribbons, or the L-Pads are going bad.

Do you see the ribbons moving a lot before the failure?

ZAKski288

Re: Torching FST ribbons on RM40's
« Reply #3 on: 20 Oct 2017, 02:42 am »
Hello Scott, the easiest way to get at the crossover, is to lay the speaker on its back. I normally take three firm pillows and position them on the floor. So when you tilt the speaker down they support the speaker. Remove the 4 screws on the base. Then remove the 8 screws on the passive. Gently pull the passive to remove it. Now you’re in. Remove the fiberglass to expose the crossover. The tweeter circuit is the blue and green wires. Good luck Zak

LosGatosSTI

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Re: Torching FST ribbons on RM40's
« Reply #4 on: 20 Oct 2017, 04:24 am »
Thanks you all very much for your replies and taking the time to help out.  I spoke with Mark from Evoke and also Ken today and they gave me some ideas on what to look for once I open up the speakers.  It may take me a couple of days to get the time but once opened up I will have to see what is going on with the crossover, caps, Lpads.   I will take some photos and report back on anything strange. 

Check back here in a few days,

Thanks,

Scott

Stimpy

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Re: Torching FST ribbons on RM40's
« Reply #5 on: 21 Oct 2017, 12:25 am »
Since the Bryston was in use in both systems, at the time of failure, it might be time to have it looked at.  It could be passing DC to the speakers, frying the tweeters.  You could check for DC-Offset yourself, if you have a meter?

LosGatosSTI

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Re: Torching FST ribbons on RM40's
« Reply #6 on: 21 Oct 2017, 07:48 pm »
I have a FLUKE 87 true RMS meter.  How do I check my Bryston amp for DC offset?  Step by step description would be appreciated as I do not want to damage anything further and I have never used a multimeter to analyze my amplifier or speaker components. 

The FLUKE 87 has a blue button on it which seems to activate a capacitance test function but have no idea how to use it.  Where do I need to plug in the test probes to the meter?  Is it possible to functionally test the crossovers with the meter and what and where do I probe?

I also have the left RM40 speaker opened up with access to the crossover.  What things should I check with the multimeter?  Please describe exactly how to do it step by step if possible. 

I created a video overview of the crossover in the left RM40  with the hope that someone with better training and experience with Brian's crossovers could take a look and help identify any issues with the components.  I also have photos but don't know if they are as helpful as the video will be. 

VIDEO is at this link on youtube:  http://www.youtube.com/watch?v=QEjgB3C-bBg

You may need to cut and paste the link into a web browser. 

Viewing the opened up the left RM40 and haven't found anything wrong visually.  The caps and the lpads all look fine with no visible defects or no evidence or failure or arcing.  One of the caps nearest the left side passive radiator cut out appears to have hot melt glue on it and has paper stuck to the glue but it does not appear to be exploded.  I will now look over the right RM40 but don't expect to find anything. 

I thought I found a smoking gun when I saw that one of the black wires going to the lower woofer was severely pinched between the passive radiator metal basket and the lower woofer magnet but I did not notice any wires coming thru the wire insulation.  I did not see any melted insulation or evidence of arcing on the basket or magnet. 

Photo below of pinched wire but it does not appear to have any wiring exposed or arcing so I do not think this is a problem. 



Crossover Pics:










I have other photos if needed.


« Last Edit: 21 Oct 2017, 11:25 pm by LosGatosSTI »

HAL

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Re: Torching FST ribbons on RM40's
« Reply #7 on: 21 Oct 2017, 08:56 pm »
The amps should be off before doing this, no signal to the amp inputs.  To test the Bryston for DC offset and noise, disconnect the speakers from the amp.

1) Set the Fluke 87 to DV voltage mode and connect the terminals to the one of the outputs of the amp.

2) Turn on the amp and watch the meter during the process.  It will stabilize to a reading after 10 seconds.  Record that number. 

3) Change the Fluke 87 to AC voltage mode and record that number as well. It may vary, so take the average.

4) Repeat for the other channel and record the measurements.

Post what you find.

Stimpy

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Re: Torching FST ribbons on RM40's
« Reply #8 on: 21 Oct 2017, 10:18 pm »
There's a good thread at Audio Karma, that discusses testing for DC-Offset.  It has some good tips on what the readings mean too.

http://www.audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/

"1. Speakers disconnected (or connect the meter to the 'B' speakers and set the front panel speaker control accordingly)
2. Input set to an unusued position (not Phono)
3. Volume control at minimum.
4. Balance in center
5. Tone controls either defeated or set to mid position
6. Set your meter to read DC, and set to a low scale (300mV scale is common) Connect directly to the Pos and Neg of the speaker terminals
7. Give the amp 10 minutes to settle. Report back...I'd like to see how healthy all these old amps are.

If you read:

0 - 15mV: Damn good!! If you read '0V', you may have a capacitor output, or your meter is set wrong

16mV - 50mV: An acceptable value, especially at the lower end of this range. 2nd harmonic distortion is probably twice to four times what manufacturer's spec calls for at higher frequencies. Probably not audible, as the distortion is mostly in the upper octaves. At the upper end of this range I begin to raise an eyebrow. :saywhat:

50 - 85mV: Something is certainly amiss, and while this is not enough to put your speakers or equipment in jeopardy, the amp is running nowhere near where it should. I'd venture to guess that most of the DC-coupled amps that are in use by forum members here fall into this range.

100mV to ?: A high enough voltage will cause the DC protection to kick in. This happens at a level determined by the designer, but is usually equivalent to about a diode drop (600mV)or so. Needless to say, if you are listening to an amp with 100mV or more of DC offset, you have no idea what the amp really is supposed to sound like. Indeed, some amps without a differential input are actually designed to have a bit of DC at the outputs, but this is triple-rare, and I don't think anyone here owns one. (in my book it's piss-poor design, but if you can sell it WTH..)"

Stimpy

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Re: Torching FST ribbons on RM40's
« Reply #9 on: 21 Oct 2017, 10:30 pm »
Also, if you get any types of noise (ticks, pops, faint thumps) when switching inputs, or skipping between tracks on a CD, this could be a sign that your gear could be passing DC.  If so, swap each piece of gear in and out, testing a source at a time, and each amp, preamp, receiver, etc., to try and isolate the offending source of the DC-Offset.  Once you find the culprit, replace it, and then head to a repair shop.  The offending gear probably needs rebuilding. 

LosGatosSTI

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Re: Torching FST ribbons on RM40's
« Reply #10 on: 21 Oct 2017, 11:18 pm »
Results using FLUKE 87 Multimeter below:

Ch 1  2.3 mV to 4.6 mV DC depending on how long I wait for voltages to settle.

Ch 2  2.3 mV to 3.4 mV DC

Ch 3 2.3 mV to 4.2 mV DC

Measured 0.9 mV AC on all channels.  This is the same voltage if I ground my FLUKE test leads against each other. 

I also measured the impedance of each channel also and they were all around 18 to 25 Ohms. 

Please keep in mind that the tweeter ribbons have all burned up at the exact same time and it is usually about 45 minutes to 60 minutes into the listening session.

I also took readings with my Sunfire TG III processor and Oppo playing the same Beck SACD source material and the voltages were similar to above.  Speakers were not connected of course. 

I have only been using the Bryston Ch 1 for the left speaker and Ch 3 for the right speaker over the past 14 years.   

LosGatosSTI

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Re: Torching FST ribbons on RM40's
« Reply #11 on: 24 Oct 2017, 08:23 pm »
Do you guys have any comments based on my amp readings above?  What else can I try?  I have hooked up a small pair of bookshelf speakers to the tweeter wires as suggested by Mark from Evoke speakers out of LA.  I will let you know what I find out from that experiment.

How did you get a good capacitance reading of the tweeter capacitor stack with it still installed in your wiring?  I thought that was not possible? 

When I use my FLUKE 87 meter and put the probes on the soldered branch of wire cap leads coming out and going into the capacitor stack on the lower part of the crossover I get a reading of 0.L microfarads.  FLUKE meter screen actually says 0.L whatever that means.  Basically 0.0 microfarads. 

When I switch to the resistance setting (by pressing the blue button momentarily) and probing the soldered branch of cap leads I get 4.6 ohms resistance across the stack of caps. 

Out of curiosity I used the Fluke meter and measured a small bookshelf speaker between the red and black hook ups on the back of the speaker with the capacitance setting using the Fluke 87 meter and it read 0.L.  When Fluke was switched back to resistance I measured about 5 ohms resistance.  I put a brand new 9 volt battery in the Fluke multimeter before doing any of my measurements to the amp or speakers. 

The Fluke also reads 0.L on the capacitance setting when I touch the black and red meter leads to each other.  I get a small capacitance reading (.37 nanofarads) when the meter leads are hanging in free air not touching which I believe is the capacitance of the test leads.  The REL button on the meter can make that test lead capacitance be taken out of the measurement.

See photos below:

 


Replacement AC G2 tweeters with melted ribbons pictured below:  Note: I used the original tweeter round faceplates and mounted them on the new G2 tweeters with store bought screws as the screws that are used on the G2 rectangular face plates are one size larger in diameter than the screws on the FST tweeters.  Also note the rectangular box that is on the back of the G2 tweeters that is not there on the tweeters that Brian used in 2003 when I bought my RM40's from him.






Please call me to discuss my options or ideas to try if you wish. 
Scott
(408) 221-9878


LosGatosSTI

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Re: Torching FST ribbons on RM40's
« Reply #12 on: 24 Oct 2017, 08:44 pm »
The bookshelf speakers I hooked up to the tweeter wires coming out from each RM40 tweeter seem to be playing only very high frequencies...no bass or midrange at all that I can detect.  This is good news I suspect?

jules

Re: Torching FST ribbons on RM40's
« Reply #13 on: 24 Oct 2017, 10:37 pm »
Was the tweeter you replaced on the RM40 a spiral ribbon or an Aurum Cantus?

There could be a difference between the efficiency of the ribbons. The G2 has a sensitivity of 96db and I suspect the spiral ribbon might be somewhat less. You could easily be putting twice or more times the power required through the G2. Aluminium ribbons like the Aurum Cantus are sensitive to being overpowered and you might need to throttle back the power coming through the crossover to the tweeter [= appropriate resistors] to get the balance correct.

What volume were you running the system when the tweeters flames out?

« Last Edit: 24 Oct 2017, 11:39 pm by jules »

ZAKski288

Re: Torching FST ribbons on RM40's
« Reply #14 on: 24 Oct 2017, 11:17 pm »
Hello Scott, there is a good chance that you burned up the 1.2uF capacitor (Auricap) in the tweeter crossover. The brown capacitor is most likely fine. You might want to unsolder them from the speaker post, to check them, to know for sure. Zak

HAL

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Re: Torching FST ribbons on RM40's
« Reply #15 on: 24 Oct 2017, 11:39 pm »
He will need to check both crossovers 1.2uF as both tweeters are bad. 

If you use your Ohm meter setting and measure across each of the caps with nothing connected to one end, they should read as open circuit. If you measure any resistance, then the caps are bad. 

LosGatosSTI

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Re: Torching FST ribbons on RM40's
« Reply #16 on: 25 Oct 2017, 12:15 am »
Was the tweeter you replaced on the RM40 a spiral ribbon or an Aurum Cantus? 

There could be a difference between the efficiency of the ribbons. The G2 has a sensitivity of 96db and I suspect the spiral ribbon might be somewhat less. You could easily be putting twice or more times the power required through the G2. Aluminium ribbons like the Aurum Cantus are sensitive to being overpowered and you might need to throttle back the power coming through the crossover to the tweeter [= appropriate resistors] to get the balance correct.

What volume were you running the system when the tweeters flames out?

Thank you for your reply i appreciate it.

Answers: My RM40's were purchased from Brian at VMPS with the FST tweeters and TRT cap upgrades already installed.  I bought the speakers complete in June 2003.  I replaced the original FST tweeters (14 years old) due to the issues in my first post detailed above.  I replaced them with recently purchased Aurum Cantus G2 tweeters.  They have 8 ohm impedance and have higher watt handling of 40 watts than the G2si I believe which is only 30 watts.  Same 96 db rating. 

Within 1 hour of replacement both tweeter ribbons on the new tweeters simultaneously lit on fire just like the other ribbons did on the original FST tweeters.  Volume was around 80 percent of max on Sunfire.  It was loud but not clipping.  Amps were NOT clipping at volume I was listening on my Bryston 6BSST amp.  I do not have spiral ribbons.

The lower left device in the photo (See "mark up 1" photo attached) did have some wrapping paper damaged or split but I don't know what that device is supposed to be?  Could the damaged wrapping paper be the tell tale sign of a failure of some type?  It looks like a transformer of some type because of the metal wire being wrapped around the black bar.  What are the other two items in the red circle on thee photo?   





Why do the bookshelf speakers that I hooked up to the tweeter wires still play if this particular component (Or others) was damaged?  Why would the bookshelf speakers only play high frequencies?

I'm just throwing these questions out there as I have never seen a damaged cap before so don't know what all these parts are in the photos I took.   

Thanks,

Scott

LosGatosSTI

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Re: Torching FST ribbons on RM40's
« Reply #17 on: 25 Oct 2017, 12:19 am »
Hello Scott, there is a good chance that you burned up the 1.2uF capacitor (Auricap) in the tweeter crossover. The brown capacitor is most likely fine. You might want to unsolder them from the speaker post, to check them, to know for sure. Zak

Which devices are you speaking about from this photo below?



 

What about all the other large Red 10 microfarad caps and other brown caps in this photo below? 





LosGatosSTI

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Re: Torching FST ribbons on RM40's
« Reply #18 on: 25 Oct 2017, 12:29 am »
Hal and Zakski....you must mean the "audience" device correct? 

So basically I would need to completely unsolder the 1.2 uF "audience" cap from each speaker and then use the Fluke to test it using the capacitance function? 

Do I just unsolder one end or BOTH ends? 

I have standard rosin core solder from radio shack...is that good enough to resolder them here or will it create any sound issues? 

Where do I get replacements caps if they are bad?

Does this logically explain the simultaneous torched ribbons from the exact same failed cap in each speaker? 

Thanks,

Scott

HAL

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Re: Torching FST ribbons on RM40's
« Reply #19 on: 25 Oct 2017, 12:46 am »
Scott,
There are two capacitors in the picture with the circle.  One is under the glue that looks like paper on the red cap.  Those two are in parallel, so the total value is the sum of the two values if you can read them.

The other device is an inductor that is basically a coil of wire on a ferrite core.  It will read a very low resistance.

You would need to trace wiring from there to the ribbon tweeter and isolate components by desoldering them to test.  Not an easy task.