175 driver seperation fix?

[Squarewave]

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]

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!!!
Lets us know what you find out.

[Squarewave]

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]

http://www.audioheritage.org/vbullet...gm+replacement

http://www.audioheritage.org/vbullet...gm+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]

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]

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]

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

[jcrobso]

Has it shifted or it is cracked? If the driver was dropped the magnet could have shifted.

[Squarewave]

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]

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]

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]

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]

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]

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]

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.