JBL 4345 L-Pads replace

[Pavel]

Hi everybody!

I've got a pair of 4345 for several years and everything was OK. But now sound one of UHF driver became scratchy and or disappear at all. As I could understand, the reason is L-Pads.
Reading through the forum i've found two options:
1. Install new L-Pads
2. Replace them with fixed resistors
Mending or cleaning of L-Pads is no option - it's just temporary and not an effective cure.

Could you recommend a right seller in the Internet where I can buy a complete set of L-Pads for pair of 4345? I actually need to repalce just one R203 for UHF now, but I'd prefer to fix a problem totally.
From other side, I don't use L-Pads at all, so option to set fixed resistors would be very reasonable. What is right impedance of each resistors (to replace R201, R202 and R203) if L-Pads position is "0". And what is schematic of this change?

Thank for help, regards
Pavel

[martin2395]

Hi Pavel,

You should be able to find the 8 ohm L-pads for the UHF on Ebay without much problems.
The 16 ohm version for the 2421B and 2122H is very hard to find, parts express in the USA has them if you don't mind paying 30-40 dollars for the L-pads and another 40 for shipping + taxes.

The best way (sonically) is to replace them with fixed resistors, to do so you will have to measure the resistance on legs 1+2 and 2+3 on the L-pad first.
One resistor then goes in series on the positive "+" lead and the second goes between the + and the -

Please take a look at this thread, it explains clearly how an L-pad works:
http://www.diyaudio.com/forums/multi...able-lpad.html

[Pavel]

Martin, thanks!
Very valuable information and link!

The only thing I can't understand is how to measure correct impedance between legs of L-pad if it doesn't work properly...
I think that it should be kown values of standard impedance for that.

i don't mind to pay extra for spare parts, but I'm not so experienced to choose right one. Fot example, i find following shop, but I don't shure what to choose for R201-R203:
http://www.parts-express.com/cat/speaker-l-pads/306

[remusr]

Common problem. Due to lack of movement of contacts some corrosion will build up, worse if high humidity or smoke.
Access the LPads from the back or remove them. Make sure you document / photo wiring if remove them.
Spray electronic contact cleaner into the LPad rear openings and work the control back and forth 10-20 times.
Let dry for a couple hours.
Reassemble.
Play your music.
Spend savings on music or some nice beer. Vodka for you?
Repeat in 10 years.

[martin2395]

Common problem. Due to lack of movement of contacts some corrosion will build up, worse if high humidity or smoke.
Access the LPads from the back or remove them. Make sure you document / photo wiring if remove them.
Spray electronic contact cleaner into the LPad rear openings and work the control back and forth 10-20 times.
Let dry for a couple hours.
Reassemble.
Play your music.
Spend savings on music or some nice beer. Vodka for you?
Repeat in 10 years.

For me it worked for whole 4 weeks, after that they went scratchy again and I fully disassembled them and cleaned all parts of the l-pad separately.

[pyonc]

For me it worked for whole 4 weeks, after that they went scratchy again and I fully disassembled them and cleaned all parts of the l-pad separately.

Actually I've felt the strong urge to clean the L-pad for very long, but didn't do so because I didn't know how to.
How did you clean them inside? You have to remove the back panel of the 4343 first, but I see it' a tough job...

[hjames]

Actually I've felt the strong urge to clean the L-pad for very long, but didn't do so because I didn't know how to.
How did you clean them inside? You have to remove the back panel of the 4343 first, but I see it' a tough job...

I think the L-Pads are on a sub-assembly board that screws to the back of the baffle board (front panel).
They are often attached using screws that are hidden underneath the "foilcal" on the front panel.
So, you use a heat gun to loosen the glue holding that "foilcal" onto the baffleboard, and gently peel it off.
Then remove the countersunk screws and the lpad assembly will fall into the cabinet.

You will probably need to clip the wires running to the crossover or the drivers to free the lpad assembly
so you can get it out of the cabinet.

Some folks might consider it to be kind of a pain with such fiddly-bits to deal with.

[pyonc]

I think the L-Pads are on a sub-assembly board that screws to the back of the baffle board (front panel).
They are often attached using screws that are hidden underneath the "foilcal" on the front panel.
So, you use a heat gun to loosen the glue holding that "foilcal" onto the baffleboard, and gently peel it off.
Then remove the countersunk screws and the lpad assembly will fall into the cabinet.

You will probably need to clip the wires running to the crossover or the drivers to free the lpad assembly
so you can get it out of the cabinet.

Some folks might consider it to be kind of a pain with such fiddly-bits to deal with.

Thanks so much for your kind explanation. The main reason I want to do so is as some forum members here suggest, it'll really clean up any distortion (if any) from rusted contacts there. Actually I am starting to hear some audible (not that big) distortion from the HF drivers of both speakers (4343) these days while playing CDs, and I suspect that might have something to do with any rusted contacts of L-pads...

[martin2395]

Honestly, it's a s*itty job to do. I only cleaned mine because I was removing the 3143 networks and decided to keep the l-pads for the CC'ed 3155.

First you have to remove the foilcal, I used a hair dryer to soften up the glue and it came off nicely.
Don't try to remove it when it's cold, you will only damage the foilcal badly as it's made of metal.
Then you have to remove the bolts that keep the l-pad assembly in place, afterwards you have 2 options a) unsolder all the 9 wires on the Lpads b) remove the whole 3143 network including the speaker terminals.

The L-pads can be opened but the wire inside is wrapped around some kind of cotton/wool that swells up when it gets wet.

If you live in the USA you could aswell order new 8 and 16 ohm l-pads from Parts Express, they cost around 10-15 dollars a piece.

[pyonc]

The L-pads can be opened but the wire inside is wrapped around some kind of cotton/wool that swells up when it gets wet.

Thanks a lot, martin2395. Looks like it's still a tough job even after taking off the foilcals... Unless I'm absolutely sure, I had better stay away from it for the time being...
By the way, these days I start hearing some gentle distortion from the HF drivers while playing jazz CDs, something I didn't hear before. Definitely this is not scratchy, but sort of gentle high-tone distortion from vocals and piano. I doubt it has anything to do with the L-pads, but I'm not sure. Have you experienced this kind of distortion before?

[remusr]

Don't remove the foilcal, probably will crease it. Just open up the back, or remove the woofer & use a mirror, and spray into the LPads' holes with contact cleaner. May have to do it a couple times. Use a good quality contact cleaner, not WD-40.

[Pavel]

I've fixed it!
Special thanks for remuser, who recommed to use contact cleaner!

The recipi is following:
1. Spray "Contakt 60" in Lapds' holes to dissolve oxides
2. Make 15-20 rotation and wait for 10 minutes
3. Spary "Contakt WL" to flush dissolved oxides
4. Make 15-20 rotation and wait for 10 minutes
5. Spary "Contakt 61" to protect contact from oxides in future
6. Make 15-20 rotation and wait for 10 minutes

Enjoy!

[audiomagnate]

Sometimes they've been current/heat damaged an no amount of contact cleaner will help. AFAIK, PE only has the the tweeter pot.

[martin2395]

How on earth did you manage to get to the l-pads? Is the back of the 4345 removable?

[BMWCCA]

How on earth did you manage to get to the l-pads? Is the back of the 4345 removable?

The upper section of the back is removable.

And on my Kenrick clones, the L-pad assembly is removable without messing with the foilcals! Ain't modern technology great?