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Restoring a classis Oscillator for Loud Speaker / bench work

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  • Restoring a classis Oscillator for Loud Speaker / bench work

    Having a spare moment in the shop, waiting for parts to arrive, I extracted a classic Oscillator from my pile of stuff in one corner of the shop. A Bruel & Kjaer 1022 Beat Frequency Oscillator. I'm about to set up another small test bench over at the Guitar Dept/Rental Depot down the street, and start going thru the large inventory of speaker cabinets, where I know there has to be a large pile set aside with one or more blown speakers.

    In my shop at the main rehearsal studio complex, I have a smaller successor to the B & K 1022 (the B & K 1023), which, like it's predecessor, can drive speakers directly at low (but adequate) wattage (7W for the 1023, 2.5W for the 1022). This has always been adequate to use to drive the speakers in any of the combo amps I have up on the bench, when checking for blown, buzzing, and rattles without going thru the combo's amp to do that. For the larger bass cabinets....8-10's & such, I'll have one of my shop power amps to supplement for higher wattage.

    It had been several years since I had this 1022 BFO powered up, and did know it needed a little work. I moved it to the bench, plugged in my IEC Male to Nema 5-15 Female adapter, it having a Nema 5-15P Male Pins on the recessed power connector, and powered it up. The Meter lamps and Frequency Scale lamps were all dead, so the only hint it woke up was the low current indication on the power analyzer feeding it, and, in switching it on and off, seeing the meter move from the power transient. Initially, I wasn't getting any output. I rotated the Automatic Scanning switch (used for local/external frequency scanning), and that woke it up. So, switch contacts on that need attention.

    I moved a known working 12" woofer to the bench, and connected it to it's Matching Transformer output terminals, set it for 6 ohms.....the one normally used for driving speakers. I printed out the Frequency Calibration procedure from the operators' manual, and set the frequency dial to 60Hz, as this instrument uses Line Frequency as a scale reference using a Beat Frequency procedure to calibrate it. Set the Oscillator level to mid-scale, and pressing the momentary Power Frequency Beat button in, I first used the Fine Frequency control to try and find the beat frequency. No luck, so set that Fine Freq control back to mid-position, and used the recessed Coarse screwdriver control and adjusted it, finding the beat frequency, where the output level meter is now swinging considerably about the mid-point. Went back to the Fine Control, and finally achieved a steady reading between either direction where it would be swinging again. Checked the tuning by adjusting the Frequency to 120Hz, and all was fine.

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    What I've always loved about the Bruel & Kjaer generators is their frequency knob lets you sweep 10 octaves with one turn of the wrist. While the frequency range on this 1022 is 20Hz-20kHz, you can tune it lower than 20Hz. Setting the frequency dial to 20Hz, with the Frequency Increment set to 0Hz, you can then carefully detune to the left. If you go down 20Hz, you'll stop the oscillator, and if a speaker is connected via the output matching xfmr, it will object loudly. But, going that low, I can set the output to around 2V or so, and can take it down to a couple hz (detuning by 18Hz.

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    At these low frequencies, if there is any cone rubbing problems, you can both hear it and often see it......sometimes spot the cone separating from the surround or on the spider suspension below the cone. There's plenty of force being applied to the speaker. Plus, these oscillators have modulation provisions....warble tone. I tend to use warble tone from them when checking speakers as it's easier to listen to, and you can set both the rate and bandwidth of the modulation. Even use it for limited low frequency sweep, using 1Hz rate and 100Hz bandwidth, setting the Frequency dial to 120Hz....it will sweep from 20HZ to 220Hz, or even greater, setting the dial to a higher frequency. You wouldn't want to let it go below 0Hz though.

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    When I began restoring this, I placed the face of the instrument down on three short 2 x 4's, so I could remove the four special long metal shoulder washers that hold the housing to four long threaded shafts attached to the frame of the chassis, and lifted off the housing. Then, moved back to the bench, facing the rear of the unit. I didn't get any images inside this, but it's very similar to that of the 1024 Sine-Random Generator I restored a couple years ago.

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    The Meter and Frequency Scale lamps look like 3AG-style fuses (1/4" dia x 1-1/4" long), 6.3V/250mA. I had a small stash of them, and also checked to see just how common they were by googling 'fuse-type lamps', finding prices ranging from $1.10 thru $1.65 or more from a variety of sources. The meter lamps (two of them) were mounted in open-frame clips, only one installed, burned out. I didn't have my service manual here, so I wasn't sure if they're in series or parallel. Put two new lamps in, checked with the power applied, and now had the meter illuminated. The Frequency Scale lamps are harder to get at. I had to remove the smaller concentric Fine Tune knob, then remove the larger Frequency knob, then turn the instrument around to face the back, and using a long metal shaft, I was able to tap on the four plastic pegs from the large black scale housing, driving them forward in their tensioned holding clips, turned the instrument around to face the front, and finished carefully prying the large housing off the front panel. That left the frequency pointer still mounted to the tuning shaft, and access to the pair of meter lamps, one on each side of the shaft, underneath that large green disc behind the knobs. I replaced the two burned out lamps with new ones, then carefully returned the scale housing, seated it flush with the panel, and restored the two large tuning dials. Now the Frequency Dial lights up.

    The Output Level control was behaving raspy, as well as the meter when muting the generator wasn't going all the way down to zero, suggested I had some caps to change. I didn't find any date codes inside, but would guess this was built in the early 70's. I was able to replace the main power supply caps easily, though the output filters of the supply weren't really accessible without having to remove a lot of wires from the harness, so there, I did resort to clipping the old caps off right at the body, leaving long leads to which I was able to wrap my new caps around and quickly solder them into place with a small heat sink clip at the PCB end of the left-over wires, so I didn't damage the PCB solder joints.

    The Level Control was the hardest part to restore. Wire Wound pot, and hard to extract. I had to remove a shielded wire and the source/ground wires to it, having to reach in with the soldering iron and grip the wires. Easier removing then putting back together, for sure. I opened the pot's cover, carefully wire-brushed the wiper surface at the two ends where it was raspy, then applied Caig contact lubricant, exercised the control, and put it back into place, successfully re-soldering the wires. Now the level control is well behaved without any of the discontinuities I was getting.

    It also has a step attenuator, accessible in the ATT position of the Impedance Matching switch, which provides 10dB steps from 12V down to 120uV full scale, output thru the B & K shielded socket (I have one of their BNC adapters installed in that connector).

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    I see these instruments from time to time on ebay for not much money. There's an earlier version that's all tubes, with the meter assembly on the outside of the panel, rather than recessed like this is. The smaller B & K 1023 that sits atop a stack of instruments (top Grn box below the headphones in the last picture) on my bench usually fetches several hundred dollars. It took years to find one affordable. So, after the basic maintenance, I have another solid workhorse that I can park over at the rental depot and start digging into all the Marshall, Orange, Fender, Ampeg and loads of others to cobble cabinets back together. We're not set up for doing re-coning here. We had considered that when I came on board back in 2009, but, the inventorying of so many different cone kits, and the limited shelf life of adhesives........it was just cheaper to either send the salvageable speakers out to be re-coned, or replace them. Still costly, but, should be another interesting adventure.
    Last edited by nevetslab; 11-22-2019, 05:02 PM.
    Logic is an organized way of going wrong with confidence

  • #2
    Loudspeaker Test Station & most common bass cabinet failures

    Friday I set up a small Loudspeaker Test Station at our Equipment Rental Depot (CenterStaging, LLC Burbank, CA) to start going thru all of the failed guitar and bass cabinets first, to see what I could cobble together from the pile, before moving on to the preventative maintenance end of checking thru the rest of the rental cabinets inventory.

    I added the spare Bruel & Kjaer 1027 Sine-Random Generator to the 1022 Beat Frequency Oscillator I just restored. Added an old Yamaha P2100 power amp that I also just repaired, it having one channel not working. I got one of our 4ft banquet tables from inventory, set that up at the end of one of the tall shelving units in the warehouse, parked the two oscillators at one end of the table, with the Power Amp standing on one end underneath the bench. I had to add some bungie cords to strap the tall 1022 BFO to the tall shelving frame so it doesn't get knocked down by accident, or shaken loose by any earthquake that sometimes occurs.

    I added my laptop for logging in my service notes, along with a Fluke 8060A DMM, my small Rion NA-61 Precision Impulse SLM and a collection of tools.

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    The most common failure mode I found was radial crease cracks on the lightweight cones of the 410 cabinets from everyone....Ampeg SVT 410HLF, SWR Goliath 410, Eden 410 XLT, TC Electronic RS-410-8, Aguilar GS410 so far......all of these have accordian-style surrounds where the cones terminate. Long Throw Cones...essentially. What seems to happen when pushed hard in the first octave with bass, is the cones can develop their own flexure joints out near the surround, which now can jump out of of the magnetic gap, eventually ripping the cones over time at these radial fractures. On the front of the cabinet, you can barely see the crack marks unless you look close. On the back side of the cones, they have developed cracks, on the way to eventually tearing open.

    Using the B & K 1027, which conveniently tunes down as low as 2Hz, AND the Yamaha P2100 will pass that nicely to the cabinet, tuning the oscillator down into the 5Hz range, you can easily see the cones now flexing at the cracks, though you don't see this behavior at 40Hz and above. You do begin to see it a bit at 32Hz (around a Low Open B string on a 5-string bass). I was able to cobble a couple cabinets back together with woofers that hadn't yet failed in this area, swapping out the same Eminence 10" woofers of a particular cabinet. I'll get some additional photos what clearly show this. You can somewhat see the radial cracks just out near the edge of the cones before terminating into the surrounds, under 5Hz LF drive, with the cones extended out forward.

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    A lot of the cabinet repairs were just a matter of missing or loose speaker mounting screws, loose cabinet panels, missing spacers & screws from the metal grilles. Two of the center domes were crushed....in particular the upper left speaker above in that SWR Goliath 410 cabinet. Using Gaffer's tape to try and get it to adhere to the dome, and give a quick upwards pull only pulls paper pulp off the dome. I've had resort to using Dental Picks...poking thru the dome with the right-angle picks, and carefully pulling up from the insides. It didn't do a perfect job, but, considering it was fully crushed from slamming into the back side of the metal grille, it's a whole lot better than before.

    And, of course, I've encounted a few kazoo's from burnt voice coils, as well as open voice coils.
    Logic is an organized way of going wrong with confidence

    Comment


    • #3
      Great test setup & terrific reporting nevets! Also a great way to use what would otherwise be a doorstop, the Yamaha SS power amp with one channel kaput.

      On those fuse looking lamps, they're called "festoon" lamps. They were popular in 70's hi fi receivers (Marantz for instance, maybe Pioneer too) where they ran on 8 volts, at least the ones I've seen. For some time they were very scarce but within the last couple years LED replacements have been offered. I've seen 'em in the Parts Express catalog, and in multiple colors no less, so you can be festive as you please with your festoons. I expect 8V rated parts would work just fine on 6.3V, maybe not so dazzling bright, but I wouldn't complain.

      Back in the 80's a local studio made a habit of installing festoon lamps in their outboard racks, mounted in lie-down fuseholders, and running them off cheap Radio Shack 6.3V filament transformers. They called 'em "dummy lights" why I don't know. Having some extra low level illumination in the studio sounds like a brilliant idea to me.

      I've seen similar damage on 10 inch speakers. Most recently, a Celestion 125W rated unit in a Trace Elliot 1x10 cab. Now I told its owner, it is pure folly to hook up your 350W Genz Benz and expect that rig to give you any satisfying bass wallop. Feh, you know how musicians can be, "I got away with it so far ... so good. Just put in another 10" that you think will do the job, and light weight too, I'll pay the neodymium premium." That poor little Celestion was ripped halfway around, deep in the accordion-pleat creases. So... Mister Bass Man got one of the new Jensen neo 10's and he's happy with that. Only the future can reveal how long it will last despite any claims of hi power ruggedness. We'll see.

      Thanks also for mentioning the dental pick trick for - somewhat - fixing crushed dust domes.

      And - it's a treat to see all that kool old B&K test gear put to work.

      All good stuff! Looking forward to further reports.
      Enjoy. Every. Sandwich.

      Comment


      • #4
        I have a large stock of the glass bulbs that look like plain fuses. They snap right into fuse-clip-like holders behind the panel dial. Is that your festoon lights? I mean like hundreds. They are in 6v, 8v, 12v. I also still have some of the small glass bulbs with wire leads that mounted inside panel meters like VUs. They look sorta like reed relays. If anyone is looking for some, let me know.
        Education is what you're left with after you have forgotten what you have learned.

        Comment


        • #5
          Originally posted by Enzo View Post
          I have a large stock of the glass bulbs that look like plain fuses. They snap right into fuse-clip-like holders behind the panel dial. Is that your festoon lights? I mean like hundreds. They are in 6v, 8v, 12v. I also still have some of the small glass bulbs with wire leads that mounted inside panel meters like VUs. They look sorta like reed relays. If anyone is looking for some, let me know.
          The B & K bulbs are mostly 6.3V/250mA, same size as a 3AG 1/4" dia x 1-1/4" long fuse.
          Logic is an organized way of going wrong with confidence

          Comment


          • #6
            Originally posted by Leo_Gnardo View Post
            Great test setup & terrific reporting nevets! Also a great way to use what would otherwise be a doorstop, the Yamaha SS power amp with one channel kaput.

            On those fuse looking lamps, they're called "festoon" lamps. They were popular in 70's hi fi receivers (Marantz for instance, maybe Pioneer too) where they ran on 8 volts, at least the ones I've seen. For some time they were very scarce but within the last couple years LED replacements have been offered. I've seen 'em in the Parts Express catalog, and in multiple colors no less, so you can be festive as you please with your festoons. I expect 8V rated parts would work just fine on 6.3V, maybe not so dazzling bright, but I wouldn't complain.

            Back in the 80's a local studio made a habit of installing festoon lamps in their outboard racks, mounted in lie-down fuseholders, and running them off cheap Radio Shack 6.3V filament transformers. They called 'em "dummy lights" why I don't know. Having some extra low level illumination in the studio sounds like a brilliant idea to me.

            I've seen similar damage on 10 inch speakers. Most recently, a Celestion 125W rated unit in a Trace Elliot 1x10 cab. Now I told its owner, it is pure folly to hook up your 350W Genz Benz and expect that rig to give you any satisfying bass wallop. Feh, you know how musicians can be, "I got away with it so far ... so good. Just put in another 10" that you think will do the job, and light weight too, I'll pay the neodymium premium." That poor little Celestion was ripped halfway around, deep in the accordion-pleat creases. So... Mister Bass Man got one of the new Jensen neo 10's and he's happy with that. Only the future can reveal how long it will last despite any claims of hi power ruggedness. We'll see.

            Thanks also for mentioning the dental pick trick for - somewhat - fixing crushed dust domes.

            And - it's a treat to see all that kool old B&K test gear put to work.

            All good stuff! Looking forward to further reports.
            The Right Ch which wasn't working, now IS working. It was a gift from a colleague years back, and have been meaning to get to it.

            After looking at the back side of the lightweight 10" cones, I couldn't help wondering if cutting out blown 10" speaker cones, and selectively cutting wide patches, I was wondering if with the proper adhesive, would these stiffen the cones in that region that is currently now an additional 'surround' flexure? It would no doubt change the character of the speaker somewhat, adding weight to the cone....but, when you have (4) 10" speakers that are going non-linear on you, wonder if it would work. I suspect there has to be a stockpile of speakers yet to be found in all the 810 cab's, 410 cab's that I haven't yet seen/heard.
            Last edited by nevetslab; 11-26-2019, 05:42 AM.
            Logic is an organized way of going wrong with confidence

            Comment


            • #7
              The patches would be worth a try, at least an interesting experiment.
              Another thing that sometimes works on the dustcaps is a powerful shop vac, just the plain hose end.
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

              Comment


              • #8
                Well, Nevet, if you ever need any of those bulbs, drop me a line.
                Education is what you're left with after you have forgotten what you have learned.

                Comment


                • #9
                  No kidding- I just ordered a bunch of those fuse type lamps for stock minutes before your post. I'll keep this in mind for next time.
                  "Yeah, well, you know, that's just, like, your opinion, man."

                  Comment


                  • #10
                    Originally posted by Enzo View Post
                    Well, Nevet, if you ever need any of those bulbs, drop me a line.
                    Will do.....thanks!
                    Logic is an organized way of going wrong with confidence

                    Comment


                    • #11
                      Originally posted by g1 View Post
                      The patches would be worth a try, at least an interesting experiment.
                      Another thing that sometimes works on the dustcaps is a powerful shop vac, just the plain hose end.
                      I hadn't thought (or remembered) the shop vac approach. Though, as I was in 'foreign territory' and not in my shop where my shop vac lives, that approach still awaits me. I'm not a fan of poking holes in the dust cover, even though that has worked. I'll have to go back and coat over the tiny holes.

                      I'm pretty sure I have some failed 10" woofers in the shop, which would be good for cone material from which to cut patches from. Bonding them to the back side of the cone would be the plan, as that's where the real cracks appear. What type of adhesive would I want to use here? I'd assume one of the adhesives commonly used in re-cone kits.....while their shelf life is a concern. At least, I'm swimming in 'bad' 10" woofers to try this experiment with. I'll have some more close-up images of the defects, along with some images of the cones driven forward.....probably could do that with a lab supply. I haven't taken one of those over to the test station yet, though that seems like it would be a useful tool. Seeing the defective woofers lunge forward at a few hz under sufficient drive signal to 'over-extend' their excursion is something to see, relative to a cone that is still undamaged.
                      Logic is an organized way of going wrong with confidence

                      Comment


                      • #12
                        Back from a couple weeks' straight effort at our Inventory warehouse where my Loudspeaker Test Station is set up. While the instrumentation use to reveal excess cone excursion beyond what the surrounds allow, drive level strong enough to continue forcing the voice coil/cone further forward and finding the cones now flexing on the jagged radial creases (which will eventually tear the cone apart), that has been effective. I haven't yet added a DC power supply to the setup, which will also force the woofer cones far enough to see that as well. And, in many cases, using the Sine-Random noise to add noise transients to yield abrupt barking if the voice coil jumps forward out of the magnetic gap has helped identify some of the speakers in 4x10 and 8x10 cabinets, what ALWAYS works to qualify a cabinet to be suitable for rental use is the Fender Jazz Bass we have for checkout use...fresh roundwound strings, so with a powerful amp, like a GK1001RB, it and the bass are enough to bust ANY of the cabinets I've gone thru to reveal speakers that just CAN'T produce solid, crisp, full bass. I had some GK 410RHB cabinets that I had to swap out seemingly good speakers, before they'd stand up to the Fender Bass played aggressively. Same with the Ampeg SVT 810 cabinet, it being a sealed non-ported box, using light cone Eminence 32 ohm woofers. A second SVT 810 cabinet, which sounded ok on the instrumentation, just fell flat on it's face, being unable to do the same without farting and fluttering on the bottom notes on the E-String....even up to the 7th fret which is always a much thicker sound than the same note on the A-string.

                        I've seen a lot of lightweight cones with the radial creases.....very common on the Aguilar GS410's, DB810's, using the same woofers. See it as well on those used in the GK410's & 810 cabinets. Also had to add 2" #7 dry wall screws to tighten up GK410RBH cabinets, which were buzzing and resonating, misdirecting you to think it was bad woofers not being able to go the distance under high SPL.

                        I did run into one GK cabinet that puzzled me...GK NEO412 bass cabinet. Two of the three in our inventory were quite new, and produced much stronger bottom end with that Fender Jazz bass, while a third NEO412, with an older grille mounting method, it's sensitivity was about 20dB down at 40Hz, 13dB down at 80Hz, 11dB down at 125Hz relative to the newer NEO412's. But, it still sounded solid playing bass thru it, same GK 1001RB amp. I haven't removed the woofers from both cabinets to see if they're the same model. My guess is that perhaps the magnets on that older cabinet have weakened substantially, affecting the sensitivity. If you had the older and newer cabinet connected to the same amp, you'd never hear the older one! I haven't yet contacted GK on that.

                        I also haven't yet had the time to try and restore any of the marginal light-cone speakers with the radial creases to see if they can be restored for further use. Perhaps there's a thick coating of some type that could be applied to the back side of the cone that could restore that present tendency to flex like a surround from the damage? All uncharted territory for me presently. What I have determined is that Yamaha P2100 power amp doesn't have sufficient power to push these speakers to the amount of power they're used to being subjected to with the typical high power amps in use these days.

                        As most of the cabinets are ported (Ampeg SVT 810 being one exception), there's no steep HPF on any of the bass amps that I'm aware of in our rental inventory to roll off the low end below the box tuning. As speakers will do their best to respond to the energy fed them, once they're below the box tuning, they'll continue to try and reproduce the signal until they just crap out. 5-string bass, producing ~32 Hz fundamental on open B string can be a killer.

                        I've a lot to learn in this latest venture, though after some 35 cabinets serviced, I have cobbled together a number of failures back into serviceable cabinets, and will also no doubt inherit a few workable woofers to populate a couple cabinets here in the shop, while they'd crap out on stage at high SPL levels.

                        I'll have more photos and details coming.
                        Logic is an organized way of going wrong with confidence

                        Comment


                        • #13
                          Cone Surround & Spider Testing with DC Power Supply

                          Here, I've put an Aguilar JDK-12 8 ohm speaker on the bench, face up, with my Kikisui PAC 16-10 0-16VDC @ 0-10A Power Supply to check this speaker's clicking sounds heard when driven hard. Set the DCV for Max, and then using the Current Adjustment, turning it up reveals where the clicking sound was coming from. The glue bond around the surround/cone/voice coil area separates, making that clicking sound. In the following images, you can see the difference from idle to applying DC current, where the voice coil moves forward, and then the cone assembly separates from the spider.

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                          In the next two images, using a different light-cone 10" woofer, which had some minor repairs made on the cone/center dome joint, it has radial creases visible in the cone, which under applied DCV, you can now see the excursion goes beyond where the surround limits, now flexing the cone at that crease...a common problem I've been seeing on the Aguilar GS410's and 810 cabinets using similar Eminence speakers

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                          With this fixed voltage/current application, you can't leave the supply voltage applied long, as smoke will begin rising from the voice coil!
                          Last edited by nevetslab; 12-05-2019, 09:08 PM. Reason: Added additional image
                          Logic is an organized way of going wrong with confidence

                          Comment


                          • #14
                            Semi-related story: Your tests remind me of a shop I worked at in the late 80's. Attached to our shop was a car bay where they installed car stereos. They'd bring us all the old crappy factory speakers to play with when they replaced them. We used to clip a suicide cord to them (AC plug on one end and alligator clips on the other- for those not in the know) and plug them directly into the wall. We'd then bet on which speakers would last the longest before burning up or exploding. Not something I'd recommend doing, but good fun if you're careful.
                            "Yeah, well, you know, that's just, like, your opinion, man."

                            Comment


                            • #15
                              Originally posted by nevetslab View Post
                              Here, I've put an Aguilar JDK-12 8 ohm speaker on the bench, face up, with my Kikisui PAC 16-10 0-16VDC @ 0-10A Power Supply to check this speaker's clicking sounds heard when driven hard. Set the DCV for Max, and then using the Current Adjustment, turning it up reveals where the clicking sound was coming from. The glue bond around the surround/cone/voice coil area separates, making that clicking sound. In the following images, you can see the difference from idle to applying DC current, where the voice coil moves forward, and then the cone assembly separates from the spider.

                              [ATTACH=CONFIG]56211[/ATTACH] [ATTACH=CONFIG]56212[/ATTACH] [ATTACH=CONFIG]56213[/ATTACH] [ATTACH=CONFIG]56214[/ATTACH] [ATTACH=CONFIG]56215[/ATTACH] [ATTACH=CONFIG]56216[/ATTACH]

                              In the next two images, using a different light-cone 10" woofer, which had some minor repairs made on the cone/center dome joint, it has radial creases visible in the cone, which under applied DCV, you can now see the excursion goes beyond where the surround limits, now flexing the cone at that crease...a common problem I've been seeing on the Aguilar GS410's and 810 cabinets using similar Eminence speakers

                              [ATTACH=CONFIG]56218[/ATTACH] [ATTACH=CONFIG]56219[/ATTACH]

                              With this fixed voltage/current application, you can't leave the supply voltage applied long, as smoke will begin rising from the voice coil!
                              Your posts always are so well articulated sometimes it reminds me of Mr Wizard's World.
                              nosaj
                              Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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