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View Full Version : Just got a pair of 4312's that don't sound very good.



phreon
03-18-2013, 07:31 AM
Hi Folks,

I picked up a pair of 4312's at a local second hand store last week. The cabinets and covers looked good and the drivers looked great and they only wanted $20 so I bought them. I didn't realize until I got home and Googled them that they were pretty good speakers. When I got them hooked them up to my current system that has a pair of JBL Decade 26's (dumpster dive). The 4312's sound kind of dead in an A/B test. They seem to produce bass and OK mid but weak highs. The tone controls don't seem to do anything so I am guessing that the crossovers need a rebuild. They look like the first generation 4312 based on my Google searches. I can source the caps pretty easily but the pots look unusual. On the schematic I found in this forum for the early 4312 the pot is represented as a voltage divider with 2 variable resistors inside. Where do I source those? Are they custom JBL? Also should I save the old caps for authenticity? (I was thinking about putting the new caps in the old cardboard tubes but then the smaller .01mf is not encased in a tube and will clearly look new.)

I am also interested in hearing from anyone who rebuilt their crossovers as to what the difference in sound should be.
Thanks
P

grumpy
03-18-2013, 07:37 AM
nice set of speakers... I'd make sure all of the drivers and internal connections are made first.
No point in rebuilding crossovers if the drivers (mid and tweeter) need replacing.

Controls are L-pads, not potentiometers. Places like parts-express typically have these.

Lee in Montreal
03-18-2013, 07:42 AM
Rebuilding an old crossover is always a good idea. Coils usually don't shift but caps don't have a very long life. But be carefull with what you want. If you swap low or medium quality caps for much higher quality units, you may just have opened a can of worms. Let me explain. Any cap will have "parasistic" resistance on top of the nominal capacitance. When a crossover is designed, the tech usually takes in account that such or such cap will have a certain amount of resistance and includes it when designing the crossover. If you swap the old worn caps with, say, Solen caps, you will restore the capacitance values to their original specs, but as those caps have virtually no resistance, then the balance will be destroyed. At that point, you'll have to ad some small resistances to return to the original sound balance. The point of this story is basically not to overdo it. Replace a medium quality cap by a similar medium quality cap, or you'll be in for a long ride... ;-) Last time I rebuilt a three-way crossover, I took notes of old and new cap values, as well as internal resistance.

grumpy
03-18-2013, 08:00 AM
Perhaps you mean coils vs. caps when referring to effective series resistance
that you can measure with an ohmmeter (measuring a capacitor for something
like this would require more complicated equipment).

At any rate, capacitors can deteriorate over time, some types much more than others.
Unless they are blown, I wouldn't expect these (in this design) to be much of an issue.
The L-pads could need replacing, connectors do come loose, and the drivers could well
have been damaged leaving the woofers only to produce some sound.

Replacement drivers should not be hard to find on ebay, and updating the drivers
(and the crossover, see MkII schematic, to better protect the tweeter) would then
not be a bad idea.

phreon
03-18-2013, 09:14 AM
Thanks for the quick reply's. I will rebuild the crossovers and I have found that at least one tweeter is showing no resistance (open coil). The tweeter has 2 numbers inked on the back; one is 2482 and the other is "le25-2". Do you know which one is the correct model number? Does JBL still sell these?
Thanks
P

more10
03-18-2013, 11:21 AM
2482 is a big compression driver...

Lee in Montreal
03-18-2013, 12:20 PM
Perhaps you mean coils vs. caps when referring to effective series resistance

Nop. I meant residual resistance in capacitor. Or maybe I should have called it a loss factor. Looked into something I wrote three years ago in www.HifiLoudspeakers.info (http://www.HifiLoudspeakers.info) (a British vintage Kef forum).

http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1009

"Loss factor D in a reversible electrolytic is usually <0.1.
In a low loss elecrolytic D<0.05.
With Solens, D varies from 0.0001 up to 0.001"

Lee

macaroonie
03-18-2013, 12:50 PM
Thanks for the quick reply's. I will rebuild the crossovers and I have found that at least one tweeter is showing no resistance (open coil). The tweeter has 2 numbers inked on the back; one is 2482 and the other is "le25-2". Do you know which one is the correct model number? Does JBL still sell these?
Thanks
P


2482 is probably just a batch code which coincidentally matches a Compression driver designation.

The model number is LE 25-2. Easily found either by asking in the marketplace section in this forum or e Bay.
I have a pair but I'm in the wrong part of the world for you. JBL made heaps of these so they are readily found.
I believe re cone kits are NLA but there are equivalent LE tweets that will drop in. Chart for that is somewhere on this site.

Your L pads may just require cleaning , they corrode and loose the contact needed to make the tweet work.

Just had a look at E bay , dozens to choose from. Best to try here first , you are more likely to get a non abused set.

grumpy
03-18-2013, 01:20 PM
Hi Lee,

Fair enough... just not something your average joe is going to measure,
whereas coil resistance is (and which also can have an audible effect).

With the capacitors, one would normally replace bad or questionable parts with like
parts or same family of dielectric and construction... unless you were interested in
changing the design somewhat (possibly for the better, but still a change)... which
is what I think you were saying.

Thanks for the clarification and elaboration.

phreon
03-18-2013, 01:30 PM
Thanks everyone. The crossover is basically 3 caps and a an L-pad. I have ordered the caps and I will probably buy a LE25 from Ebay or I will check to see if this group has tweeters for sale.
Thanks to all.
P

(Weird coincidence, during lunch today I was checking craigslist for JBL tweeters and a guy was giving away a set of L-40's. I called him and I am picking them up later. I am a JBL magnet.)

phreon
03-20-2013, 03:41 PM
Thought I would update ...
I bought Dayton Precision Caps from Parts Express for the crossovers, they did not have 8mfd so I got two 4mfd and paralleled them, replaced the 3mfd and .01mfd as well. I cleaned the L-Pads. The crossovers are done.

I found one blown tweeter, the other drivers read between 6 and 8 ohms so I bought a pair of tweeters on Ebay. They should arrive this week so hopefully a weekend test will make beautiful (rock) music. This is the second time I have opened up JBL speakers and I am really impressed with the build quality. They really put extra effort in manufacturing a quality product. You do get what you pay for.

I will do a last post on this thread after the test.
Thanks to all for the help.

(I picked up the L-40's I mentioned, the woofers have been replaced with Pyle 10 inch, one tweeter is original, one tweeter is missing, the original crossovers are there as are the JBL labels. The cabinets were left in standing water and are swollen badly in the lower miter joints. Unless someone suggests otherwise I will strip the parts and toss the cabinets. Maybe a question for another thread.)

phreon
03-22-2013, 05:18 PM
OK, last post by me in thread.

Just received the new tweeter and after install fired them up. Recapping the crossovers definitely woke them up. They no longer sound dead. In fact they are amazingly bright, much more so then my Decade 26's. They are much stronger through the mids then the Decades, though I think the Decades produce more bass frequency but not as tight as the 4312's. They will take some time to get used to, I haven't had such clear mids in years so I need to spend time with them. Great fun.

Thanks all
P

berf
03-25-2013, 05:56 PM
You did well. Going by today's inflated ebay prices you're about $500 ahead of the game. PM me if you come across some free L19's.;)