L200 Studio Master Xover and Other Qs

[elektratig]

What a great site!

Hearing that I had built some speakers, my cousin asked me to look at his speakers, which it turned out were a pair of L200 Studio Masters. He said he'd bought them new in the mid-70s and they had "never sounded right." After dragging them out (he had one firing into the side of a couch), he was right -- one sounded dramatically different from the other, like the upper bass was largely MIA.

I had nothing with me, not even a multimeter, but eliminated a few obvious possibilities (tightened drivers, reversed amp channels, checked for phase). To make a long story short, I could find nothing obviously wrong (need that meter, but the drivers seem hooked up properly, surrounds OK). The only thing I can think of is that there's something going on in the crossover, which turned out to be a metal box attached to the back (inside) of the terminal plate on the back. It was getting late, and I decided not to press it after a couple of glasses of wine.

All of which lead to my questions:

1. Has anyone run into this before? The speakers really do sound quite different. Any suggestions?
2. How do I get into the crossover box? (There were a couple of bolts that looked like they need an Allen wrench, but the holes looked round, not hex.)
3. The boxes haven't been treated since the day he bought them. They look fine, just dry as the desert. What should I get for him to restore them?

Thanks in advance for your help.

Elektratig

[boputnam]

Originally posted by elektratig
1. Has anyone run into this before? The speakers really do sound quite different. Any suggestions?
2. How do I get into the crossover box? (There were a couple of bolts that looked like they need an Allen wrench, but the holes looked round, not hex.)
3. The boxes haven't been treated since the day he bought them. They look fine, just dry as the desert. What should I get for him to restore them?

1) You should remove the LF's - double-check their connections (solid color / non-black to Red). If consistent, then you should swap the LF's cabinet-for-cabinet, and (make sure you reconnect them as they were), and see if the symptom is cabinet (i.e., crossover) specific or LF specific. To remove the LF's lay the cabinets on their back, and work with them in that position.

2) I've not removed myself, but they may come off from the INSIDE. Once the LF's are removed this might be more clear.

3) pm Audiobeer, or hope that he's got ideas on this. He is the Cabinet Guru here...

Post your findings, and the type of wine ;)

[subwoof]

hey there - the crossover backs are held on with simple 1/8 rivets. Simply drill them out to get at the insides.

I have seen many of these crossovers go bad when one of the capacitors opens up - resulting in a major loss of HF or changed performance. loof for obvious burnt components ( like the resistors on the rotary switch ) if you suspect abuse by the non-wine crowd ( budwieser can brother-in-law types )

unless you have a capacitance meter, and a lot of patience, I would suggest an A/B swap with the other cabinet.

If you move the crossover, and the problem moves with it, the crossover *is* the problem. These can be fixed by some of the techies that prowl this forum.

If the problem doesn't move, then it's one of the components ( 136A or LE85 ). Again these can be repaired by a REAL jbl service center - not the "refoaming / good as new" jerks that advertise in college weeklys / laundrymat or on telephone poles...:)

sub

[elektratig]

I'm reviving this old thread, because I at last got back to my cousin's. Using test tones, we figured out that the problem was in the tweeter range -- shows you what a "golden ears" I am -- one channel was basically mia.

We swapped tweeters, and the problem moved, indicating that the problem lies with the tweeter driver, not the crossover.

Is there anything I can do to try to fix the tweeter. or do I simply have to try to find a replacement? There is nothing obviously wrong with the tweeter -- no obvious physical damage, and with a multimeter it measures the same as the other, about 6.5 Ohms.

Assuming I need to find a replacement tweeter, where do I go? Ebay? How much should I expect to pay? Alternatively, Subwoof mentioned a "REAL jbl service center" -- where do I find one? It is, according to the sticker, a Model LE 85.

We listened, in mono, to the "good" speaker playing the Glenn Gould 1955 Goldberg Variations -- these speakers are definitely worth saving!

Any help you can provide is much appreciated.

[boputnam]

:hmm:

Confusing data, fer sure... Getting identical impedance but no signal out of the LE85 compression driver.

You should use the "Search" button - but in the interim, you might start here: http://audioheritage.csdco.com/vbulletin/showthread.php?s=&threadid=326&highlight=le85

There is a wealth of information out here from very knowledgeable folk... :yes:

[subwoof]

Some possibilities:

(A) The diaphram is shattered and while the measured resistance is fine, there is little output.

(B) The magnet is weak and transporting this cabinet resulted in a magnet shift...While this usually also kills the diaphram, the magnet needs to take a vacation to california for recharging. This is 65.00 + parts + shipping so it's not cheap.

(C) There is "crap in the gap" and needs simple cleaning.

Since the driver measures correctly, "B" is the most likely...sorry...:(

The replacement diaphram is D16R2420 and is a LOT of $$$ ( over 200 + labor )

And to make it worse, if the magnet is weak, the other driver could be close behind.

PM me and I will foward the service center guy's number in syracuse.

sub