Have I bought a paperweight?

[chris661]

Hi all

Just got a (used, sold as faulty) guitar amp, but the reason I'm posting here is as follows - it's a drive unit problem.
I can't get any sound out of it, and the impedance measures up at a few million ohms (the meter won't measure it, it's that high). My semi-educated guess is that the voicecoil has become disconnected, so I'm looking at the resistance of the air or paper cone.
The cone still moves as freely as you'd expect from a PA speaker (high Vas...).
No change to impedance when I push the cone in or out. I did get some noise out of it once, briefly. Wasn't loud or anything, but sounded like there was a loose connection of some kind.

Thanks for any help
Chris
PS - I'm a Celestion G10-20, 8ohm impedance.

[adason]

That's an easy fix. Replace the speaker. Its burned.
Just make sure that the amp is ok first. Take the output form the amp and plug it into some other speaker.

[mondogenerator]

i agree, the speaker may just have been blown.

HOWEVER, it still may be worth checking that the braided wire from the terminals hasnt unattached itself from the terminals themselves--that is assuming you havent already!

If these wires look ok to you then chances are as the other poster said the VC is burned out.

CPC has a good range of celestion guitar speakers, from geen and blue back designs of the vox ac30 and other classic amps.

[event horizon]

Originally Posted by mondogenerator

HOWEVER, it still may be worth checking that the braided wire from the terminals hasnt unattached itself from the terminals themselves--that is assuming you havent already!

Most definately I know the owners of a repair shop for PA & some hifi kit (when hifi kit is taken in there). The chap that owns it lets me have the dead drivers for the magnets (handy to lower the Qts of drivers & shield them at the same time).

Cutting a long story short i was presented with a rather expensive Bowers & Wilkins (B&W) driver that was supposedly duff. Where the tinsel leads were attached to the cone there was a bad joint, resoldering it brought the driver back to life

[chris661]

Sorry, guys, but it's a definite gonner.
The wires were fine (when I posted this, the driver was in my lap), so I took off the dustcap, scraped away a little glue and measured resistance across the VC. open circuit, so it's dead.
The amp has a more interesting problem.
One of the resistors had been seriously burnt out. Changed it, all was fine until a 20v DC offset appeared from nowhere, after 20-30 mins of playing through it, at normal listening levels (at 3/10 on the amp).
Needless to say, I switched it off immediately, and began prodding around, and having a go with the hair dryer occasionally. Doing the latter gave a small DC increase on the output, but it still stayed less than a volt.
Put a sacrificial resistor across the output, turned it to 6 and played whilst watching the volt meter. Still nothing out of normal.
Going to mess with fuses and things for a while - back later on...

Thanks for reading
Chris
PS - it's a 150W PA driver for now.... (Em. alpha 10a)

[planet10]

Chris,

A better use than a paperweight for the dead driver, is to strip it down to the metal, screw it face "down" to a wall and use it as a magnetic holder for screwdrivers, bits, etc...

dave

[chris661]

Wow, that's a genuinely brilliant idea.
Now to persuade parents that I need a 10" driver attached to my wall.
The amp's around 20W (recorded 20V out, peak), so I'm hoping my back-up driver will survive the onslaught...

Edit - trying to get it reconned is pointless. It's over 30 years old, and a quick search on Google returns a more modern version of the G10.
Asking Dad, he says celestion used to have a different cataloguing system, suggesting the driver wasn't a dedicated guitar speaker (that's what the G stand for now...), it was a General purpose drive unit.
Still, even on ebay, it's not easy to find ones like it.

[DcibeL]

Back in the day, my friend had a cheap $10 10" subwoofer that he placed on his coffee table. It worked well as a candy dish, and a beercap collector. It's funny, now he lives with his girlfriend, and she made him ditch the speaker, and now we just toss the beer caps at her cats, they end up all over the floor. If only we had some sort of magnetic device to collect them!

Anyway about your speaker, 20V DC is a lot different than an AC or music signal. It will cook a speaker in no time. This is the reason for some protection circuits on amplifiers, not to protect the amp, but to protect your speaker if the amp dies.

[chris661]

The back-up driver stood it for around 7or 8 seconds - then I made it over to the mains switch and switched it off.
It turned out that one of the PSU rails was shorted to ground (on the circuit board) in some unusual way, and the earth from the chassis to the circuit board was non-existant, so the fuse was happy to carry on, as it saw no voltage on the chassis. For reasons unknown, after a length of time, one of the output transistors was switched fully on, put DC on the output, and tried to cook my speaker too.
Still, it's all fixed now and I have a nice little guitar combo, with just a clean amp. Perfect for my pedals......
Chris

[planet10]

So the dead speaker is just a symptom.

dave