Novice trying to repair a speaker: Speaker or crossover filter broken? How to check?

[Piri]

Dear hi-fi fans;

Short description:
Of one of my 3-way speakers the midrange unit does not seem to work.
I am asking for your help in trying to determine what is broken: the speaker, or the crossover, or...
I am not a technician, so I'm afraid of doing the wrong thing.
Especially because the inner workings of those speakers are not the usual construction.

More details:
I recently bought some old "new" speakers on an online bidding.
Brand name and model: Realistic Mach Two.
These are big 3-way speakers, and the midrange speaker of one of them does not produce any sound.
According to the seller, they had never been used, and it certainly looks that way:
Even though they are probably 20-25 years old, there's not the slightest scratch on them,
and the suspension of the woofers and midranges is in mint condition,
even if the suspensions are made of that kind of "mousse" that tends to harden and break after years of usage.

Now, there are 2 odd things about the inner construction of these speakers:

1. Both the midrange and tweeter are connected to a turning knob, which goes from "minimum" to "flat" to "maximum".
Mind you, these are PASSIVE speakers. These knobs sit in between the tweeter/midrange and the crossover.
Turning its know does not bring the midrange speaker to life though...

2. The crossover filter has an odd characteristic too:
I can see 2 bulb-like objects on each crossover, each containing a wire just like a classic light bulb.
They have the form of a classic fuse, but then one as thick as your thumb.
I had already read on the internet that they can really start to produce some light if the volume is turned up.
But as they are sitting on the crossover, I can hardly imagine that these are over current protections:
That would mean you need to almost dissassemble the speaker to replace them. So i guess they are not really fuses, although I am not sure.

So, I would like to find out what is broken: the midrange speaker, the crossover, or maybe the knob.
What can I do to at least diminish the number of possibilities?

1. I had thought of bypassing the knob, and connect the midrange directly to the crossover to eliminate the knob as a possible cause.
But the wires are soldered, and I do not know what kind of soldering I should use. Does anyone know what kind of soldering is used inside speakers?
I guess it's not the usual tin soldering...

2. Could I connect the midrange speaker directly to my amplifier to see if it works if separated from the rest of the speaker?
Or do I risk blowing the midrange or my amp?

3. I hope it is not the crossover...
I have no idea how I could test that, and those big "fuses" or "light bulbs" on them keep me puzzled.

If anyone knows these speakers or if you have read about them, please let me know.

Any ideas welcome.

Thank your very much in advance;
Carl

 

[tomlinscote]

if you can get the mid range speaker out of the box easily enough, then yes you can plug it straight into the amp and see if it works, there will be no damage to either either PROVIDED the speaker coil is not shorted out - you can test that by using a mulitmeter on the resistance setting it should read between 4 and 8 ohms (depending on the speaker) If the voice coil is shorted the meter will read next to zero, but if the coil is blown then the meter will read a lot or say open circuit depending on the meter.

As far as I am aware you can use standard solder in speakers I have and it sounds just fine.

If the crossover is shot you can buy new ones from say Maplins or try to get one from Realistic if they still exist may have to be from the US though as I think Tandy went years ago.....

Tommo

 

[expat_mike]

At first I wondered why bother with trying to repair old Radio Shack speakers.

However the Mach 2 certainly seem to have an appreciative user base for rock music.

http://www.audiokarma.org/forums/showthread.php?t=95768

Consequently follow the advice of Tomlinscote and see what you find out. I seem to remember another thread on this forum indicating a website where you can buy many types of speaker drive units, so finding a replacement should not be too difficult. And if its the crossover, go to maplins.

 

[andyjm]

Light bulbs use tungsten filaments - a material with a positive temperature coefficient of resistance. As the filament gets hotter, the resistance increases, specially designed 'bulbs' can be used as current limiting devices to protect delicate parts of a circuit. My guess is that your bulbs are crude over current protection for the drivers.

 

[Piri]

Hello folks;

Thank you very much for the advice so far!

I will try to put it to good use this weekend or the next, and I'll keep you informed.

I had already seen the posts on Audiokarma about those Realistic Mach 2's: Those old speakers do have a fan base indeed.
I had registered on Audiokarma, but currently they apparently do not allow new members to put posts due to capacity problems or something.

Actually, I like to play jazz and blues on these speakers because they have some kind of dirty 70's sound.
They have a BIG 38cm woofer, but contrary to what that figure probably makes you believe, the bass output is hardly dominant.
The advantage of such a big woofer is that, at low volume, you have some kind of natural loudness.
Like I said, I like their warm & dirty sound.

Thanks again, and I'll keep you guys posted.

Carl