Help, I blew up my friends combo amp

[Gregory East]

It stopped working one night when we had my 15" bin as extension cab under it at a jam. Now it puts out enough sound for jamming with an accoustic guitar and that's all it's good for. Any ideas what failed that would let it keep working at low volume?

[Radian]

Crapped-out volume pot?

Blown output cap?

Who knows...

[Gauss]

I'd guess the power amp was shot and that you're hearing the preamp, but who can say from here? I'd bite the bullet and take it in. Some areas of DIY take a lot more time and money than others.

[Greg Plouvier]

What brand of amp?

[Gregory East]

Thanks for the suggestions. I'm pretty sure the output is working to some degree.

It's a Laney 150W tiltback with quite a few bells and whistles in the preamp.

[Ron K]

Probably chucked an output can or two.Most transistor amps will continue to function even with a blown can but at a reduced level.Basically you would have to remove the cans and check each one individually to see if you have an open can.It's possible to check each one in circuit using an octopus and a scope buts that's far beyond where I am willing to go in this response!

Any shorted cans would cause the main power supply rails to draw very high current and would blow the fuse or trip the breaker.Once a can shorts it often just blows open and the amp responds by protecting itself during the short or until the short becomes an open!

[Gregory East]

Output "can" is a big capacitor right? Google's no help with that.

If I'm understanding properly you're saying one or more could have briefly shorted internally, without tripping a breaker or fuse, and then gone "open circuit" and the amp coninues to work with remainder.

[Ron K]

Output "can" is a big capacitor right? Google's no help with that.

If I'm understanding properly you're saying one or more could have briefly shorted internally, without tripping a breaker or fuse, and then gone "open circuit" and the amp coninues to work with remainder.

A lot of amps use TO3 style BiPolar Metal Case Transistors commonly called cans by repair guys hence the term "can". Not caps! LOL.

http://www.parts-express.com/pe/showdet ... er=MJ15024

Sorry for the confusion.

[Gregory East]

Thanks, and cheap enough to replace without spending more than the unit is worth on a tech.

[Ron K]

Thanks, and cheap enough to replace without spending more than the unit is worth on a tech.

Depending on the manufacturer they may or may not be that cheap! That's just an example of a common one used in Peavey's CS800s not the Laney Unit you described.

With that Laney unit it's best to try and get a schematic and a parts cross reference chart.

I have never seen one apart so without any prior knowledge about their units it's pretty hard to say much more about repairing one.

[Gregory East]

It's a Laney made in PRC so I imagine it will be on the cheaper end of the scale!

[Rune Bivrin]

Does the volume control still work over the same range as before, only with lower output volume?

If so, it could be the output transistors that have blown open. That happens, and if so you're hearing the output from the driver stage of the amp.

It could also be the output capacitor which some power amps use (more common in olden days) that's broken down.

There are quite a few soldering points that could cause the same symptoms, one being the ground leg of the feedback network going open.

[Gregory East]

I'll have to check up on that. The thing is he isn't very keen to dive into it as it's in daily use for his practice and he goes direct to PA for his gigs so he doesn't need to fix it.

I feel a bit bad about it because I convinced him running my 8 ohm cab off it was all good, and so it was for several gigs, but the minute he was absent it took a dump.

[tcorder]

For Laney spares and service manuals try here. http://www.majelectronic.co.uk. Good luck.

[Harley]

try here. http://www.majelectronic.co.uk. Good luck.

I can see why you said good luck
Anyhow, Gregory lives on the opposite side of the world to this nutter.