Dead speakers in a cab?

[Sunbane]

Post summary:

How do I know for sure if the speakers in a cab are blown?
Can I measure them with a multimeter and see if they are busted or not?

Background:

I was called over to a friend to mess with a gutted old 2x12" marshall combo. The amp chassis was removed, but both speakers are still in it. It had previously been used as a cab by a bassist. The idea was to use it as a cab for a rack-mounted guitar amp.

The speakers were wired in parallell, but it lacked a jack to connect it to the rack unit. I printed out some wiring schematics from the 'net to be sure of what I needed to do, and then I installed a jack in it and plugged in the amp. But the thing will not make any sound at all. It doesn't even pop when I switch the amp on...it's as if the signal doesn't even get to the speakers, even though I'm pretty sure it does.

Question:

Could the speakers be blown? They look intact from the rear, but I haven't taken off the grille cloth to inspect them up front. Is there any way I can measure them to make sure if they are dead or not?

Unfortunately I didn't have another cab handy to test the amp with (nor a multimeter), but I also tested this cab with a keyboard that had speaker outputs - same result: Stone dead.

The speakers are two 12" Celestion 75W, rated at 8 ohms each. They were wired in parallell, which should make the cab 4 ohm. I tried reversing the wires from the jack, and also tried each speaker individually (making the load a straight 8 ohms). Still nothing.

The jack is a brand new mono tele jack for chassis use, and I made sure to solder the wires properly. The wires are also brand new - I took them from a freshly opened pack - soldering them to the jack in one end, and putting on new clips for attaching them to the speaker tabs in the other end.

The rack unit is a 2x60W stereo amp with integrated preamp, and the guy who owned it before (old friend of mine) said there is no problem running it on just one channel. It can take cabs between 4 and 8 ohms. It would give me a signal on the headphones output (but that doesn't go through the power amp). The keyboard was also known to work just days before, and it too gave me a signal through the headphones.

What do you guys think?

[jackson1]

To cut to the chase, this is just a dirty quick way to check a SINGLE speaker:

Take a 1.5v battery and connect it directly to one (1) speaker at a time, with the help of two wires (plus and minus). Don't hold it on too long, just connect the wire and WATCH the speaker cone. Release.

It should move slightly, either in or out depending on the polarity of the battery - doesn't matter right now. If it doesn't move at all, the coil wire in the speaker is broken.


You could also pull/press the cone with your fingers. Move it in and out. If it makes scraping sounds, it's probably dying or dead. It's the coil wire that hits the magnetic gap. Result of too much heat dissipated in the coil or a foreign object inside the magnetic gap.

Note: DC (battery) is not healthy for a speaker, but a 1.5v batt, for half a second will not cause any harm.

[Sunbane]

Thanks for your response Henrik!
I'm going back there on Wednesday, so then I will try to check the speakers with a battery like you suggested.

[jacksonaxes]

I use a 9-volt battery with no problems.

[jackson1]

I didn't dare write I use 9 volts too sometimes