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I recently had an amp blow during a gig so took it to my friend who's an electronic engineer to have a look.

 

Amp is a Peavey 1500 which was running a pair of Cerwin Vega subs in bridge mode, when the amp went there was a burning smell, when I looked down the red DDT light was on and when I shut the amp down it wouldn't switch back on again.

 

I only ever turn the amp up to about 2/3 power and am extremeley cautious with my peak values on the mixer but I guess what I was asking the amp to do with the subs was too much for it.

 

Anyway, before we opened up the case I thought i'd try plugging the amp in and switching it on just to make sure it hadn't magically fixed itself!

 

I plugged the amp in (the power at the amp was off) and as soon as I switched the power on at the socket on the wall there was an almightly BANG and I could see a big ball of blue explosion through the front grill - this was with the power on the amp switched OFF!!

 

After both our hearts had calmed down a bit we removed the cover and there was no sign of any burn marks anywhere?

 

Is there a common component that goes on amps when you drive them too hard, my mate seems to suspect the Toroidal Transformer but isn't quite sure how he'd go about testing it?

 

Also why the explosion with the power off?

 

Oh and another thing, neither of the two fuses in the amp were blown and the breaker didn't go either....

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If the switch on the amp was off then the fault must be before the switch, maybe a damaged cable or IEC socket on the back.

 

Are you sure it was OFF?

 

If he cannot check the toroidal then is he really likely to be able to fault find on the electronics?

 

Jim

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Switch was definately off although on the Peavey it doesn't say off and on it says Power and On, switch was turned to power.

 

I don't think he could really be bothered to have a proper look at the amp at the time, he seemed more interested in fixing my guitar amp so we could 'jam' :rolleyes:

 

I've 'accidently' left it in his shed as I know what he's like for fiddling with broken electronic things so hopefully he'll have a better look at some point.

 

Anyway, the cable I used to test the amp was different to the cable I used on the night it blew but i'll get him to check the IEC connectors at the amp.

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why wasnt the amp clipping when i done a sound check,and the 3 hours beforehand

Maybe because when you did the sound check it was in an empty room where it seemed loud enough, we all tend to push the volume up as the night progresses.

It is you that posted that you were pushing it too hard :-

,i looked at the mixer,the led lights where literally full on,not moving,turned and looked at 1 of the amps(peavey cs800x),the ddt warning lights where stuck on

 

Jim

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The Peavey PV1500 front panel rocker switch is just a standby-on switch and doesnt interrupt the mains to the unit. This is standard on many larger amps, Crown, C-Audio to name two more. The switch just fires up the startup circuit which does a self-test, softstarts the transformer and finally unmutes the audio.

 

It sounds like you've blown a channel on the amp. You say you were using it in bridge mode to drive two sub woofers...what was the total impedance? Not too low?? Why not run it in stereo or parallel mono?

 

The PV1500 series are a surface-mount construction which means that all the clever circuitry is on the bottom of the circuit board out of sight. Unfortunately, when an amp like this fails, bits can blow off under the pcb and vapourize into a mini fireball between the pcb and the case...sounds like this is what's happened.

 

It won't be the toroidal, they rarely fail. It'll be the output transistors..possibly both channels if it failed in bridge (a good reason not to bridge unless you have to), the drivers, emitter resistors, pre-drivers, PSU fusibles, low-voltage rail zeners, regulators, current limit op-amps, pre and main opamps....I hope your friend is a good friend!

 

http://www.mcbia.com/auction/randy/nick/PeaveyPV1500-3.gif

Edited by superstardeejay

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Looking at that picture it looks like the live wire from the IEC goes to the power switch.

 

edit: I have just checked the schematic and the power switch is normal, ie it cuts the live, rather than a standby.

 

Jim

Edited by Dream Catchers
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Well, ok, however the last one I repaired had a small inverted board over the circuit breaker with the power relay on, driven from a connector on the main board.

 

I particularly remember it because the amp wasnt energising the relay.

Maybe they modified the design or it's different for different markets?

 

Ive just fired up my old PC and looked at the pdf, yeah Jim you're right about what you say!

However I remember (it was only a few weeks ago) having to take the daughter board out and solder a bridge wire across the contacts to get it to power up. (via variac of course). The fault I had was that after a blow-up, it wouldn't energise the main transformer because it thought it was still faulty, I could get it to work by doing this. It turned out to be a hairline crack in an LL4148 diode in the delay circuit. The owner admitted to having dropped it whilst it was 'on' from a great height.

Edited by superstardeejay

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This wasn't my normal set up I was just doing it to help out the client so I wasn't sure on how to go about connecting everything.

 

I got some advise on here and tried my best to get it right based on that but it seems I was asking too much of the Peavey as it ended up being a 4ohm load going through it.

 

It sounds like it might be a write off so perhaps I should just forget about it :(

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The PV range will actually work bridged into 4 ohms, (most amps wont) as they are rated 2ohms per stereo channel. Most amps that are overloaded anyway ought to shut themselves down safely (within reason!) if the impedance is a little too low or they get too hot, including the peavey. Are the bass bins still ok? I just wondered if they'd been overdriven and burnt out, thus giving the peavey a short circuit load which it wouldnt like at all.

 

You may just have been unlucky of course!

 

 

 

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The subs belonged to the client, he had asked me to mix and match his equipment and mine hence the unfamiliarity with it all so I don't know the current situation with the subs.

 

I will get my friend to have a look under the main circuit board to see what visible damage is there.

 

You seem like a guy in the know - would you say it's usually financially viable to repair an amp that's failed or are there too many variables to be able to say?

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You seem like a guy in the know - would you say it's usually financially viable to repair an amp that's failed or are there too many variables to be able to say?

 

It's difficult because failures like this can be random. You may be lucky and just have a component gone..though in my experience it's usually a domino effect with the PV range. I'm sure your friend will let you know the total cost before he orders anything too expensive! Factor in around 30-40 quid for the parts and a pint for your mate and you'll be ok.

:usa:

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