[undertowogt1]

I am currently trying to fix my fathers speaker and have hit a road block. The speaker are made by MONITOR AUDIO out of England, they are a 3 way speaker and seem really nice.I first want to say I am not extremely experience and repairing speakers / electronics (I am good enough to follow instructions in a kit and build a tube pre amp for example.) 

My father gave me his broken speakers a few years ago, I told him the cross over was obviously broken. I opened them up and discovered both Cross Overs were fried in the same spots, 4 Resistors were totally fried.

So I went to the electronics store, got some help defining the resistors, and fixed the cross overs. Fixing the cross over was easier than I thought.
My repaired cross over works perfectly. All 3 sections work, The Bass section / Mid section / and High End Section.

The issue is that some of the individual speakers will not work. Out of the 6 individual speakers, 3 are not making music.

Speaker Tower 1 - Woofer Works
                         - Mid Works
                        - Tweeter Broken

Speaker Tower 2 - Woofer Broken
                        - Mid Works
                        - Tweeter Broken

I trouble shot this many ways.
I bypassed the cross over totally, I bypassed the wire to the speaker and went straight to the speaker post, I used different amps, I used different wire.

The speaker them selves look in great condition, nothing looks burnt or fried on them. Is there something that can happen internally in a speaker to cause it to not work I am stumped

any thought are appreciated





.

thanks

[srb]

The speaker them selves look in great condition, nothing looks burnt or fried on them. Is there something that can happen internally in a speaker to cause it to not work

Yes, it's possible that the voice coil has a short or an open (break) even if it visually looks (and smells) fine on the outside. 

With the driver disconnected, put an ohmmeter across the terminals and you should read something in the ballpark of the impedance stamped on the speaker or on a label on the speaker (4 ohms, 6 ohms, 8 ohms, etc.).

I'm guessing you will probably find an open circuit, i.e. an infinite ohms reading.

Steve

[undertowogt1]

Yes, it's possible that the voice coil has a short or an open (break) even if it visually looks (and smells) fine on the outside. 

With the driver disconnected, put an ohmmeter across the terminals and you should read something in the ballpark of the impedance stamped on the speaker or on a label on the speaker (4 ohms, 6 ohms, 8 ohms, etc.).

I'm guessing you will probably find an open circuit, i.e. an infinite ohms reading.

Steve

Hi srb,

I just followed your suggestion. I got a reading of 6.9 on a WORKING WOOFER and OL on a BROKEN WOOFER. Would the break you are referring to inside the voice coil? All my BROKEN speaker give me an OL reading.

[JP78]

Hi undertowogt1,


OL means Open Loop, meaning there is no current passing across the speaker and it is most likely fried.

Best,

[undertowogt1]

Hi undertowogt1,


OL means Open Loop, meaning there is no current passing across the speaker and it is most likely fried.

Best,

So................... given my limited knowledge of speaker repair. The voice coil would have to be replaced? or the whole speaker?

[richidoo]

The fried resistors were probably parallel to the tweeters, there to attenuate the tweeters to match the other drivers. Resistors can usually take a lot more power than tweeter voice coils, so if the resistors failed then no chance for the tweeters. The fine voice coil wire gets hot enough to melt and open the circuit, but the parallel resistor is still in circuit and keeps heating red hot and burning the PCB until it eventually opens also.

Most tweeters are protected by capacitor from DC coming from the amp, so they tend to fail from too much power while playing music. But the woofers in a 3-way don't have caps to protect from DC, so they can be damaged by the amplifier putting out DC.

A midrange driver in a 3 way speaker usually also has a cap to protect from DC and a midrange driver has thicker gage wire in its voice coil so it may have tolerated the heat enough to survive, but it could also have overheated, melted the voice coil wire insulation and have shorted voice coil wires making lower DC resistance than it should. This low resistance can cause the amplifier to be damaged and cause it to put out DC. The DC would quickly damage the woofers which are not protected from DC by a cap.

So while the midrange drivers still make sound because VCs are not melted open, they might have shorted windings making very low resistance. So be sure to disconnect and check ALL the drivers with ohmmeter like Steve suggests. They should measure between 3-6 ohms. If it measures infinite ohms then the voice coils are burnt open. If it measures less than 3 ohms then might have shorted coils.  edit: 6.9 is fine 

You can also sniff the drivers for burnt smell. Open loop means open circuit, melted voice coil, no repair. Each broken driver would need to be replaced, but unlikely that original parts are available for them. You can look on Ebay and Craigslist, etc for similar Monitor speakers with shared parts. Search for info about the speakers to find the parts. If it is not a precious heirloom then you might just toss it up to lesson learned.

[undertowogt1]

This is all great info, thank you everyone for your help. I wanted to fix these and give them back to my dad with all the wood refinished and sounding great. I know he did like to crank the tunes up while tossing a few brews back.

 

[srb]

So................... given my limited knowledge of speaker repair. The voice coil would have to be replaced? or the whole speaker?

It's possible to replace voice coils, but it can be a difficult job that requires cutting, gluing and soldering not to mention the coil needs to have very precise alignment in the magnetic gap, and that's if you can even source the individual coils, spiders, tinsel leads, etc. that you might need.

There are professional speaker repair companies that can do it, but it's usually more of a last resort if a replacement driver can't be found or is prohibitively expensive.

I would first try to locate replacement drivers and see what they would cost.  Start with the model # and and serial # of the speaker (in case different drivers were used in different versions of that model), then collect any numbering and lettering on the drivers themselves.

Steve

[Russell Dawkins]

If I were you, and I wanted to re-make the speakers to as close to their original sound and appearance as reasonably possible (and you have the necessary wood finishing skils), I would completely abandon all thought of fixing the drivers—it would be a waste of time. Just find two matched tweeters with the same dimensions or remake the mounting plate for the tweeters to match the new ones and install them in the place of the others.
The woofer looks to be a KEF B-139 which are readily found secondhand. Here's an example:
http://www.ebay.com/itm/191907849414?rmvSB=true
I'd buy one and bolt it in.
Quite frankly, that speaker is never going to sound really great, so this is an exercise in nostalgia. B-139s were not a great bass driver, so it's not worth sourcing the original tweeters, even if you could find them. Just buy something with good reviews from somewhere like Madisound or Parts Express—you can get one that is guaranteed to sound orders of magnitude better than the original for around $20-25 each. Just bolt them in and use the original crossover as is.

At least the midrange driver is intact; that's the one that is most responsible for the sound of the entire speaker and, I'd guess, is probably the best sounding driver of the three!

[undertowogt1]

I don't mind the idea of replacing the woofer and tweeter, especially if they are not really good speakers.

[Doublej]

Got a model number on them? That will help gauge how good they were back in the day? I'd first look on ebay and craigslist for used exact replacement drivers then if that fails go to Madisound, Parts Express etc. for replacements.