Strange issue on a solid state Fender amp

[saint gillis]

Hi, I've got a "Fender Ultimate Chorus", solid state amp, 2 speakers, 1 power amp for each speaker (schem attached)
It fell from a chair, speakers down to the ground, amp switched on.
One of the speakers stopped working. Then 8 months latter the 2nd speaker stopped working too.

I've checked the signal with a scope, the 2 power amps seem ok. Measured the speakers, both are open (no resistance between + -)

I'm a bit puzzled, do you think to shock simply damaged the speakers (it was quite a heavy shock) ? Or could it be something getting wobbly and turning the power amps to speaker killers for few seconds?
Can this design become a killer amp anyway?

Additional question, do you think it's ok to load these power amps with 4 ohms ? I've got a low quality 4 Ohms speaker which would be a good candidate to check for one year if the amp randomly kills the speakers.

 

[metalockpick]

Remove the speakers (they're bad anyway). Does the cone move up and down smoothly when you push on it?

Dropping can cause the magnets to shift and lock up the voice coil. The only fix when that happens is to replace the speaker,

Why only one failed from the initial fall is anyone's guess.

 

[saint gillis]

Remove the speakers (they're bad anyway). Does the cone move up and down smoothly when you push on it?

Yes they do

Dropping can cause the magnets to shift and lock up the voice coil. The only fix when that happens is to replace the speaker,

It doesn't seem to have happened this way

Why only one failed from the initial fall is anyone's guess.

Yes, that's strange

Do you think it's ok to load it with 4ohms ?

 

[KLDVOX]

I would stay with the original impedance.

 

[f67bird]

Have you checked the braided wires that go from the terminals to the cone?
They often go bad. Usually there are grommets in the cone where the flexible wires solder to the voice coil wires.
The grommets are often covered in black plastic type substance. You can gently scrape the black covering off of the grommets and check resistance at the cone where the voice coil wires solder to the braided wires from the terminals.
The wires usually fail at the very ends so it's best to check at the grommet itself and the terminal itself, not to mention the wires have string embedded in the wire so they can appear to be unbroken and still be bad.

 

[CJ]

Test the speakers, if they work it's the amp,

 

[saint gillis]

Test the speakers, if they work it's the amp,

no continuity in the coils, I don't see how they could work

 

[CJ]

there electrostatics,

 

[f67bird]

Am I missing something? The Fender Ultimate Chorus uses Fender Eminence Special Design 025923 12" 8-Ohm 75W Speakers which look like normal 8 ohm speakers with magnets, voice coils, and speaker cones to me.

 

[Disco Volante]

I believe we're witnessing an instance of the infamous CJ humour

I second the dropped litz-wire hypothesis. Connect some other drivers and see what happens.
You could spend a lot of time looking for a dodgy connection in the amps that might not even be there...
Good luck

 

[saint gillis]

Have you checked the braided wires that go from the terminals to the cone?
They often go bad. Usually there are grommets in the cone where the flexible wires solder to the voice coil wires.
The grommets are often covered in black plastic type substance. You can gently scrape the black covering off of the grommets and check resistance at the cone where the voice coil wires solder to the braided wires from the terminals.
The wires usually fail at the very ends so it's best to check at the grommet itself and the terminal itself, not to mention the wires have string embedded in the wire so they can appear to be unbroken and still be bad.

That was it, removed the dust caps, found a wire broken on both speakers, re-soldered them, re-glued the dust caps, it should be ok, thanks everybody

 

[KLDVOX]

well done, mate