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Squarewave
01-12-2010, 11:54 AM
Hi all,
Like a good newbe, I tried looking around the site for info on this, but struck out.
I have an early 175 masher ser#1250 that I sifted out of my elephant graveyard. I did test it about 25 years ago when I stripped it out of a C-34 enclosure. The rear end of the driver was loose but still attached and working. Nothing has changed, but now I would like to repair this.
Wax seals are gone. I have never cracked one of these open. I did not want to touch it before talking with more experienced people. My guess would be that this driver took a hit and the two screws holding the driver together were striped out. Any guidence would be most helpful
Thank you
Dan

jcrobso
01-12-2010, 12:44 PM
Carefully loosen the screws, they are short. there are two short wires that go to the diaphragm. You can lift the cap and slide it to one side. Look at the screws, are they stripped or are the holes stripped? DO NOT touch the diaphragm it's self. There is a strong magnetic filed in the area where the diaphragm is so be careful!!! :blink:
Lets us know what you find out.

Squarewave
01-12-2010, 02:00 PM
I loosened and removed the screws, they were both "tight" ( had to be broken loose) and threads looked good. They also feel tight when I run them back in, how ever I did not force them down at all. When I try to seperate the two peices, I "think" I can feel a VERY strong magnetic pull between them. It's either that, or something is hung up somewhere inside.

The gap between the two parts is 1/8" or so on one side and tight 180 degrees around the other side. When I try to push or pull, that gap can travel 180 degrees to the other side or anywhere in between. Wherever it is, the gap remains roughly the same size.

Is it possibile someone installed this 180 degrees off, and it is hanging up?

I am also afraid that when I am pulling hard Ill get it apart, and then it will rapidly pull back together, hit on edge and damage the diaphram. How hard should I have to pull on these peices to get them seperated?

Thank you
Dan

jcrobso
01-12-2010, 02:17 PM
http://www.audioheritage.org/vbulletin/showthread.php?t=11558&highlight=driver+diaphragm+replacement

http://www.audioheritage.org/vbulletin/showthread.php?t=20572&highlight=driver+diaphragm+replacement
Scroll down, shows pictures of a drive disassemble.

There can be two locating pins. The cove is made from aluminum so no magnetic pull. Try slipping a small screwdriver into the crack and slowly work it around the cover.

Squarewave
01-12-2010, 02:37 PM
OK
I now realize that my "moving gap" is between the front of the diaphram mounting plate and the front driver housing. My rear cover is stuck, may be orig. paint, or a little corrosion holding it on, but its quite tight. I'm going to carefully score around this seam with a razor knife and see if I can get it to budge. I will report back.
DC

subwoof
01-12-2010, 02:44 PM
if you can move the top plate that easily, you have a very weak magnet and it needs to be recharged, period. a simple tap with the HANDLE of a hammer will give enough whack to break the seal loose.

Post a picture or 2 of this "gap"

Squarewave
01-12-2010, 03:18 PM
I decided to cleve the rear cover off with light tapping on a utitility knife blade..worked great! There is nothing easy about pulling the plate back from the front housing. I'm fairly strong, and it takes all I have to move it. Element looks teriffic from the rear

Is it field pull only that holds the element mounting plate to the front housing?
If so, it seems somthing is holding it apart (I would guess).

If not, what else holds the plate to the housing and what might stop it from seating?
I can't get this to seat properly.
thanks,
DC
http://www.audioheritage.org/photopost/data//523/175_gap_1.jpg

jcrobso
01-13-2010, 08:23 AM
Has it shifted or it is cracked? If the driver was dropped the magnet could have shifted.:blink:

Squarewave
01-13-2010, 09:04 AM
25 years ago I pulled this driver out of a C-34 looking just like this,
It's likely the whole enclosure was dropped before I bought it.


Just popped the horn off, and It looks as if the magnet has shifted.:(
Is this the end for 175 ser#1250, or is it repairable?
many thanks
DC

Steve Schell
01-13-2010, 12:27 PM
I have seen one instance where a 175 driver had a pot metal die cast throat insert that had grown into a different shape and pushed up the top plate. Junky pot metal sometimes does this; the piece will be physically larger with cracks on its surface. Later on JBL solved the problem with a plastic throat insert that does not deteriorate over time. Of course the plastic ones sometimes break when shocked, so solving one problem lead to another. In any event, the driver will have to be taken apart, parts replaced as necessary and then the magnet recharged.

Squarewave
01-13-2010, 12:57 PM
Steve,
Thanks for the insight. That could be the problem here, especially with the age of this driver. Do you think I could remove the element mounting plate myself? Still not sure how it is attached.

Way back in my sound co days, I used to have my JBL stuff serviced @ LA speaker service in Hollywood (JBL authorized). Don't even know if they are still around, or are still good. Any idea?
Thanks Again,
Dan

SMKSoundPro
01-13-2010, 03:14 PM
Dumb question to ask, but here goes nothing...

Were the horn mounting bolts too long?

I have dis-assembled a 2470 driver by inserting allthread into the (3) 1/4" bolt holes to push the magnet pot apart.

(I then gave it to the admiral where he redrilled the top plate so that it could used with 2425 diaphragms.)

scotty.

Squarewave
01-13-2010, 04:05 PM
Scotty,
Bolts seem ok, but I like your I idea for pressing it out.
I think jcrobso and Steve have nailed it. Either the magnet shifted, or the pot metal throat insert has crystalized and exploded, or a little of both. I'm leaning towards the latter as this driver still works pretty well.

It's looks like the repair won't be too cost prohibitive, but want to get it apart and see before I send it out... if possibile.
Did you remove the diaphragm mounting plate before you did this pressing? Still wondering whats holding that plate on the mag pot,I don't see any screws, or clips.

HP told me this in is just second down the LHF list for age/ser#, so I'm going to try saving it.

Thanks!
Dan

SMKSoundPro
01-13-2010, 04:28 PM
No removing the plate. Just insert three long threaded 1/4"x20 bolts or allthread and it will push the driver apart into your hands. Careful not to pinch your fingers!

(I am not responsible if you ruin your driver by following this post.)

subwoof
01-13-2010, 04:46 PM
I have a black 175 with a magnet that couldn't hold a paperclip. Makes a great teaching tool. the throat needs to sit just right into the recess or it will pop up like yours.

Squarewave
01-13-2010, 07:28 PM
I think the magnet is ok but if I'm going this far, I might as well recharge the mag as well. That should put this 175 back in shape for the next 50 years or so.
Can anyone recomend a service shop on the west coast?

herki the cat
01-14-2010, 10:11 PM
[quote=Squarewave;275685]]=cover off! Is it field pull only that holds the element mounting plate to the front housing? it seems somthing is holding it apart If not, what else holds the plate to the housing and what might stop it from seating?
I can't get this to seat properly.
http://www.audioheritage.org/photopost/data//523/175_gap_1.jpg [Quote/]

Well, Squarewave, In your picture, "! The diaphragm assembly of this driver is still mounted on the motor top plate, and it should have been removed immediately before disassembly of this driver !" This plate has shifted against the motor center pole piece remaining tightly locked there by the magnet force that holds this driver together.

Without a doubt, the voice coil has been crushed against the motor center pole piece during attempted disassembly of the motor structure for no other purpose than to inspect the magnet. Unfortunately JBL has permanently discontinued the original aluminum diaphram of the LE-175.

"In order to form a concentric precision "0.036 inch" voice coil gap", The seating of this plate is provided by a concentric ridge machined under the top plate to nest the plate precisely against the inner diameter of the motor iron cup. This concentric ridge device is now displaced, preventing latteral movement to re-seat this plate correctly.

By the way:
edgewound (http://www.audioheritage.org/vbulletin/member.php?u=2021) http://www.audioheritage.org/vbulletin/images/statusicon/post_old.gif
06-13-2005, 02:20 PM/#6 (http://www.audioheritage.org/vbulletin/showpost.php?p=205951&postcount=6) Senior Member: Edgewound, & #5 Senior Member: Earl K both have excellent credentials featuring Care in Cleaning Compression Driver Gaps & Phasing Plugs, & Care of delicate Replacement Aluminum Diaphrams, & Repairs.

http://www.audioheritage.org/vbulletin/image.php?u=2021&dateline=1138655526 (http://www.audioheritage.org/vbulletin/member.php?u=2021)

---------------------------------------------
Cheers Herki the cat

Squarewave
01-14-2010, 11:40 PM
"Without a doubt, the voice coil has been crushed against the motor center pole piece during attempted disassembly of the motor structure for no other purpose than to inspect the magnet."

Hey there herki the cat,

Thank you for your input on this.

1: While I did do a little careful probing and prying, I have not tried to remove the plate yet. That's why Im here, to learn before I attempt it.

2: the driver has looked just like this since I've owned it (30 yrs).
I never used it it for my small town sound co because of this gap. I just put it up on the shelf in my elephant graveyard.

3:I fired it up today w/o horn and it appears to still be working almost as well as other good 175's here, though I can't explain why.

I will be removing the diaphragm as the next step. On the chance that I still have a good element here, can you give me the proper steps for removing it?

Also, thanks for the referal. I spoke with edgewound today.. he sounds like the perfect JBL tech!

DC

Squarewave
01-15-2010, 01:57 AM
With everyone's help I have managed to solve and fix the problem. I removed the element,(thanks herki the cat), then the top plate.

Steve had it right. Indeed, the pot metal throat insert had cracked all the way around near the plate. Both peices were still well attached at either end, but of course, the crack was not on an even line going around the insert. It appears that someone had been in here before me, and did not get the crack alligned to fit tight before they re-set the top plate. I was able to get it lined up. The top plate fits to the mag pot perfectly now, and the driver sounds GREAT! 175 serial#1250 is back in service.

I would like to thank you all very much for helping me solve a mystery I've had on the shelf for the last 30 years!:applaud:

Dan

PS I'll get some pics up on this soon for the next guy.

herki the cat
01-15-2010, 02:54 AM
[quote=Squarewave;275794]I have managed to fix the problem. I removed the element,(thanks herki the cat), then the top plate.Steve had it right. Indeed, the pot metal throat insert had cracked ......the top plate fits to the mag pot perfectly now, and the driver sounds GREAT! 175 serial#1250 is back in service.
Dan [Quote/]

Squarewave, congratulation! & Thanks to Steve ! You were indeed fortunate the top plate was still partially nested in the motor pot without hurting the voice coil. There is no more stock of the tangential edged aluminum diaphram which is common to LE-175, LE-85, 2410, 275, & the pro' 2420. Temporarly however, JBL is still supporting the 375, & 2440 which is a cloned WECO 594-A quite excellent to 10,000Hz with a new JBL 375 diaphragm.

Dan, You are a courageous fellow & an insperation!

Cheers Herki the cat

Squarewave
01-15-2010, 01:01 PM
herki the cat,
Thank you! I'm lucky it went this well.
Here's pix of the 175. Sad news about replacement diaphragms, are there any good aftermarket replacements for the 175? or are we forever SOL?
http://www.audioheritage.org/photopost/data//500/Mag_Pot.jpg
http://www.audioheritage.org/photopost/data//500/Top_Plate.jpg
http://www.audioheritage.org/photopost/data//500/175_success_.jpg

jcrobso
01-15-2010, 03:25 PM
http://www.parts-express.com/pe/pshowdetl.cfm?&Partnumber=294-720

toddalin
01-17-2010, 10:28 AM
[quote=Squarewave;275794] There is no more stock of the tangential edged aluminum diaphram which is common to LE-175, LE-85, 2410, 275, & the pro' 2420.

Maybe and maybe not:

http://www.speakerrepair.com/mm5/merchant.mvc?Screen=PROD&Store_Code=genem&Product_Code=10-251&Category_Code=l_jbl_dias

"Genuine JBL replacement diaphragm for 2410 driver."

http://www.speakerrepair.com/ebaypics/dia_jbl_d16r2410_f.jpg
http://www.speakerrepair.com/ebaypics/dia_jbl_d16R2410_b.jpg