You might take both midranges to a JBL authorized recone guy who knows and cares about classic JBL and find out if the earlier recone is genuine JBL factory stuff. If it isn't, have just that one redone with a genuine JBL kit.
Demagnetization from being overdriven is not an issue with underhung voice coils; these midranges have underhung voice coils.
same production run (from my understanding)Originally Posted by pyonc
==> i.e. same sourcing (suppliers), same raw material and same process of assembly for all the parts of the cone kits
The 2121 kits are still available from JBL, Gorden at Northwest Speaker Repair restored mine about a year ago. My money is betting that your speakers are ok, check all of your components and cables for a problem. Having these drivers restored can be costly. My 2 cents...
Well why not it's just one more pair...
4340's and 250ti's what an odd pair...
Thanks a lot, speakerdave. The one in the upper pic was done by our forum member, aka edgewound who confirmed he had done the reconing. I see 5/4/09 on the back of the cone. As for the other one, I'm not so sure of the authenticity of JBL original recone, but its dust cap is 100% aftermarket. JBL dealers want me to get both of them reconed for some reasonable reasons, but you suggest getting only one of them, namely the dubious one, reconed, which I also think is good and reasonable. As for demagnetization of these drivers, I checked their ohms with a digital multimeter, each reading at 6.4. How should I interprete this reading? Does it mean they need remagging?
This is wonderful advice. There is a ton of stuff interacting here, including the room. I may be missing something, but have you tried tweaking the level controls individually to get acoustic balance at your seating position? There is nothing sacrosanct here, the control settings don't have to be exactly the same side to side - they are there to compensate for "anomalies." Of course, instrumentation would help, but a good ear is the final instrument anyway (if it sounds good it IS good) - I'd experiment and see if tweaking the mids for best balance does the trick. Turn off the horn and UHF leaving only the woofer and mids playing and balance the mids. Then bring in the horns and balance similarly and finally the UHF. A 4-way is really hairy so takes patience. In any case, my experience with this old gear is that stuff tends to move - mostly degrading connections (as per JBL daddy above), both solder and mechanical (especially RCA) and you just compensate as needed and keep cleaning with deoxit and checking for cold solder joints ad infinitum! Speaking of cleaning, have you cleaned/replaced the pots (L-pads) in recent time? There is your first crossover villain. I actually keep my crossovers sitting on top of the speakers so I can service/tweak easily. Not kidding. Hey, there are 8,645 connections and they are ALL 30+ years old. My additional 2 cents . . .
Thanks for your kind explanation. I've got to check them first with the JBL dealers before I ship the drivers.
Thanks a lot for your insight. Actually I've never cleaned or replaced the pots (L-pads) since I bought 4343 used but in good contion from a forum member about this time last year, and I didn't check with the seller on this, either. Some member here also mentioned the possible crossover issues. Can you show me how to clean the L-pads?This is wonderful advice. There is a ton of stuff interacting here, including the room. I may be missing something, but have you tried tweaking the level controls individually to get acoustic balance at your seating position? There is nothing sacrosanct here, the control settings don't have to be exactly the same side to side - they are there to compensate for "anomalies." Of course, instrumentation would help, but a good ear is the final instrument anyway (if it sounds good it IS good) - I'd experiment and see if tweaking the mids for best balance does the trick. Turn off the horn and UHF leaving only the woofer and mids playing and balance the mids. Then bring in the horns and balance similarly and finally the UHF. A 4-way is really hairy so takes patience. In any case, my experience with this old gear is that stuff tends to move - mostly degrading connections (as per JBL daddy above), both solder and mechanical (especially RCA) and you just compensate as needed and keep cleaning with deoxit and checking for cold solder joints ad infinitum! Speaking of cleaning, have you cleaned/replaced the pots (L-pads) in recent time? There is your first crossover villain. I actually keep my crossovers sitting on top of the speakers so I can service/tweak easily. Not kidding. Hey, there are 8,645 connections and they are ALL 30+ years old. My additional 2 cents . . .
The DCR (DC Resistance) has nothing to do with magnet strength, the only way to measure it is with a gauss meter inside the gap as far as I know.
The 6.4 ohms reading is normal for an 8 ohms speaker, depending on material (aluminum, copper) and the type of driver it could even go lower, 5 or even 4 ohms. This is the purely resistive value of the winding, the impedance is based both on resistive and reactive components.
Thanks for exposing this audio beginner's ignorance on the relevance of the ohm to manet strength. Relieved to hear the reading is normal. I heard over time, the magnet strength of alnico units like 2121 is losing their power over time. 4343 with this 2121 unit first produced in 1976.
IIRC,an impedance sweep can show the effects of lowered magnet strength. Normal strength will show a huge increase in impedance at resonance, while a weak magnet will not increase as much.
Measuring the T/S parameters would be the first thing to do to find out if the two drivers in question are indeed mismatched somehow.
Mike Scott in SJ, CA
Drive 'em to the Xmax!
Nice to know, but it's only visible by comparison. Is there a way to really measure the magnet strength by computing TS parameters for a single driver?
Neat... did you test it?
Works quite well... have had one for a few years. Nice augmentation to a
mic/pre/laptop analysis system for audio FR measurements.
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