Advice on Restoring L100A

[HMayes]

Hi Folks,
My dad purchased a pair of L100A when he was younger, and now has given them to me. They are in alright shape. They still sound pretty good, although I know they could sound a lot better. I don't know a lot about speakers beyond the basics, but I am pretty handy. I'd like to restore these as much as possible.

Both tweeters have dents in the cones. All of the foam on the speakers are badly deteriorated (does that affect sound quality?). The woofers and mid-range speakers don't have any physical signs of damage or wear that I can see, but I don't really know what I'm looking for. I haven't opened the cabinets up yet, but I'm sure there are some corroded parts in there, especially the two dials on the speaker face. What should I look for when I open the cabinet? I know there are two devices (crossovers?) in there that send the appropriate frequencies to the different speakers, beyond that I don't know what's even in there.

I just bought a new Onkyo TX-8555, hope that is a decent match for these speakers.

Anyhow, I'm just looking to restore these for my own use in my house. Any advice would be greatly appreciated. Thanks!

-Mayes

[Eaulive]

Hard to know if they sound right when you have no reference.
However most of the parts are available for restoration but they're not cheap and the labour has to be done by a knowledgeable technician. No offense intended.

Are both speakers sounding the same? put them side by side and do an A/B test (from the same source, not a left right test from a stereo source)
It won't assure you they're OK, but you will spot if one is not, at least. Then if they're the same my advice would be to enjoy them :)

I have four of them as you can see in my avatar.

I'm not in the US so I can't tell you where to go to have them checked but somebody will probably chime in.

--------EDIT---------

Wait a second, you say "All of the foam on the speakers are badly deteriorated"... As far as I know, the only foam on this cabinet is the ring around the tweeter and the effect on the sound is very sublte, mine are gone as well and I don't remember hearing them with the foam anyways.

Do you have woofers with foam surrounds?

[robertbartsch]

These are classics that should be kept and cherished for future generations.

Unfortunately, JBL recently decided to discontinue the manufacture of replacement parts for all non-professional legacy systems such as the L-100. I assume there are some recone kits in inventory that might last for a period but eventually that will run out. OEM recone kits can be purchased only by authorized service reps.

If I had dented tweeter domes, I would buy used replacement tweeters of the same model that are often seen on Fleabay.

Crossovers rebuilds are common and involve replacement of old and leaking capacitors.

[HMayes]

Thanks for the advice folks. Actually, after looking at them just now, the foam is not as bad as I thought. There is a narrow, maybe 3/4" wide strip of foam around the subwoofers. It is in excellent shape. The foam around the tweeters has no spring to it though and kind of flakes away when you touch it. So I guess that foam is pretty bad.

Also, upon closer examination I notice that the speakers are clearly marked, "L100", but they look EXACTLY like the L100A depicted on this page: http://www.audioheritage.org/html/profiles/jbl/l100.htm

In this picture you can see the narrow black ring of foam around the woofer, and the large foam around the tweeter. My speakers have the tweeter diagonally positioned beneath the mid-range cone, which leads me to believe it's an L100A not an L100. However my speakers do say "L100" on them.

Also there is some crackling when adjusting the Presence and Brilliance dials. Should I just leave those? Is it worth replacing them? Should I treat them with some kind of anti-corrosion spray or something?

I've heard someone say I should replace the filter. What exactly is that and is it really worth replacing? Also, if I could get my hands on a wiring diagram, I think I could probably change out the crossovers if it's not too complicated. Is this really something an expert has to do or is it something I could attempt?

Also, one of the white plastic tabs that holds the grill in place is broken. Is there a place I can find replacements?

Thanks for all the advice so far folks.

-Mayes

ETA: I see that the crossovers available on ebay include the presence and brilliance dials. It looks like those two dials adjust the cutoff points for which frequencies go to which speaker drivers, correct? Also, it looks like I can pick up some mint tweeters with foam for about $150. I am considering changing the tweeters out and the crossovers. What do you folks think? Anything else that should be done to help restore these speakers?

[jcrobso]

"It looks like those two dials adjust the cutoff points for which frequencies go to which speaker drivers, correct?"
No, not correct, they are just level controls, just try moving them from end to end a couple of times, usualy once you find the correct level that you like they are rarely touched.

[Don C]

The dark foam around the outside of the woofers is in this case a sealing gasket, intended to seal the woofer in case it was mounted behind a baffle. On older woofers it was cork, on some others it was thick cardboard. When the woofer is mounted in front of the baffle as in this case, that gasket is only decorative, it won't hurt anything but appearance if it deteriorates.

[4343]

<SNIP>

Also there is some crackling when adjusting the Presence and Brilliance dials. Should I just leave those? Is it worth replacing them? Should I treat them with some kind of anti-corrosion spray or something?

I've heard someone say I should replace the filter. What exactly is that and is it really worth replacing? Also, if I could get my hands on a wiring diagram, I think I could probably change out the crossovers if it's not too complicated. Is this really something an expert has to do or is it something I could attempt?
<SNIP>
-Mayes
<SNIP>

Cleaning the L-Pads (Dials, Level Controls) is sometimes difficult without removing them from the cabinet. I use De-Oxit in a spray can that comes with a tube that lets you direct the spray where it will do the most good, i.e. inside the metal can on the back of the control. Removing them from the cabinet can be difficult as well, a lot of JBL models have a metal foil type label that is glued on over the nuts that hold them in. Removing the "foilcal" as it's called can be a disaster for the foil, unless you are VERY careful! I have used heat to soften the glue whilst sliding a very thin, wide blade under it, being careful to not bend the foil AT ALL. As noted already, rotating the knobs end to end several times will sometimes clean enough of whatever is causing the scratchiness off to set them where they sound best. Normally, they are not re-adjusted once the speakers are positioned in your room and the proper settings have been determined.

There is actually three filters in a L100: High Pass for the midrange (to filter the bass out), Low Pass for the midrange (to filter the treble out) [I guess I was wrong about that, see below...] and High Pass for the tweeter (to filter the midrange out). Together these are referred to as the Crossover, Filter, Network, or some combination of the three terms. The filters are very simple, and several improved versions can be found on the web with a little research. I would recommend getting the existing Filter Network working as it should before venturing down the Rebuild/Replace/Upgrade path, it gives you a base to judge any improvements from...

[macaroonie]

Just about all you need is here ( and more ) This fella seems to know what he is about , its a hefty read but you will learn loads :)

http://www.troelsgravesen.dk/JBL_L100.htm

[Fred Sanford]

There is actually three filters in a L100: High Pass for the midrange (to filter the bass out), Low Pass for the midrange (to filter the treble out) and High Pass for the tweeter (to filter the midrange out). Together these are referred to as the Crossover, Filter, Network, or some combination of the three terms. The filters are very simple, and several improved versions can be found on the web with a little research. I would recommend getting the existing Filter Network working as it should before venturing down the Rebuild/Replace/Upgrade path, it gives you a base to judge any improvements from...

I don't think they're even that complicated- at least on the later L100s, the midranges don't have a low-pass.

je

[4343]

I don't think they're even that complicated- at least on the later L100s, the midranges don't have a low-pass.

je

Ah, the Less Is More approach then, eh? So just a pair of High Pass filters then... A semi-acoustic crossover!

[VSN]

Ramblings from a converted L100 fan,


I must say, the L100 was a great speaker back in the early 70's, yet after many years of loving those babies, I have learned to move on!

There were four versions of the L100:

The very first model was the L100 Century made from 1971 to 1974. This model is very unique in many ways from the vast majority of L100’s found for sale today. Some call this model ‘rare’, yet in actuality it had the longest production run of all the models and many thousands were made. The main reason you do not see this version as much is they were far outnumbered by the enormous volume produced as the later models gained popularity. The most notable difference of this early model is the drivers are in a straight vertical row. Other differences exist in the crossover network, phasing of the transducers, and the cabinet bass port. Lower value capacitors and the use of an inductor in the LX12-10 network produced crossover points of 2.5K Hz & 7.5K Hz. All the drivers were in phase with each other, unlike the later models where the woofer and midrange are 180 out of phase. The bass port was only a small hole in the cabinet without an internal tube, and the tweeter was the LE20 round, not the LE25 square.

The second model released was the L100A Century which was made from 1974 to 1976. The most notable difference was the midrange driver became offset from the bass and tweeter axis. The circuitry changed to higher value capacitors and eliminated the inductor shifting the crossover points to 1.5K Hz and 6K Hz and became the N100 network which remained the same in all future models. The woofer phasing was changed to be 180 out from the midrange to cancel out the duplicated tones where the 123A-1 and LE5-2 overlapped. The bass port diameter was enlarged along with adding the internal tube to enhance the low frequency response, and the LE25 square tweeter replaced the LE20 round tweeter.

The third model was the orange label L100A Late which was made from 1976 to 1977. This model was virtually identical to the black label L100A Century excluding the foilcal label dropped the word Century and a polarity change occurred to the woofer. Up until this point all previous models had used the 123A-1 positive firing woofer which was now replaced with the 123A-3 negative firing woofer. As the new -3 woofer fired in the opposite direction as the previous model, it was required to be wired backwards inside the cabinet to maintain the correct polarity as the network circuitry remained unchanged. This means the positive input wire now went to the negative woofer terminal to maintain the out of phase relationship with the midrange. JBL did not want to confuse the service centers by suddenly changing the internal wiring of the well established L100.

There is really a fourth L100A “Late-Late” model yet JBL does not identify this as a separate model even though the internal wiring changed to the studio monitor color code, the face of the cabinet got a slight groove around the face perimeter plus the presence and brilliance controls went from a +/- 3 scale to a 0 to 10 scale. Somewhere in those last few years, the rear input connectors changed from the metal spring loaded style to the plastic twist-lock style which were used on many other late 70’s models such as the L112 & L150, plus they were relocated higher up toward the middle of the cabinet rather than offset near the woofer end.

I have upgraded many L100 cabinets with the 4312A crossover, 035Ti tweeter and 104H-2 midrange which IMHO kicks butt on the screechy L100. Then again, I have sold all my L100's long ago in favor or larger systems, and I have never regretted it for a moment!

My suggestion would be to discover what you are missing by moving up the food chain, try the L112 if you want to stay small, the L100T3 is bigger yet even better. Both of these could be had for what the L100 fans will gladly pay for your Dad’s pair…..

Then there are the 240TI which really sound good for under a grand! Or find a pair of L5 on the cheep, the options are endless, yet ALL will sound better than the L100!

Most important, just have fun and enjoy.

[Swerd]

I built entirely new crossovers for my L100As, resulting in much improved sound. http://www.audioheritage.org/vbulletin/showthread.php?t=13105

Those crackling old variable L-pads that change the volume levels of the mid and tweeter can be cleaned, but they will continue to oxidize and crackle. With the new crossover, they are not needed at all.