I think I just blew my amp...

After opening it up, the only things possibly out of place I can see are the only two wires that come of the speaker cone (one black, one white) lead to the circuit board. When they meet the board there is a sloppy yellowish gunk around the join and then there are 2 battery shaped tower things next to that that have the same yellowish gunk at the base.

Any ideas guys?

That sloppy yellowish gunk is rosin. Flux for the solder attaching the wires. Normal.

Check the amp's power fuse. If it blew you would likely hear such a fadeout as the filter capacitors in the power supply (those "battery-shaped tower things") discharged. If the fuse blows a second time after replacing, you'd better get someone who knows something about amps to check it and find out why.

Thanks Ricochet!

It seemed like the obvious thing to me to check the fuses (but I didn't because of the fadout... I assumed if it "blew" then it would pop and turn off straight away.)

As for the flux, lol, I've only ever seen red flux before and yellow seemed like a chemically discharge kind of colour to me. :)

I really want to learn about the electronics side of things to do with guitars, but it seems without going on a course it's quite hard to learn stuff.

Well, I just opened it up and surprise, surprise there's a blown 250mA 250V fuse that's blown in there.

I went down to the shop and the lowest they have is a 600mA and I don't want to risk the electronics if it's already blown once, so it means a car journey to the bigger store down the road!

Hopefully it won't blow again, otherwise I'll have broken 2 amps in 2 years.

Yeah, amp fuses are smaller capacity than one generally finds in the auto and hardware stores. A Radio Shack should have it.

Main question is, why did it blow? One blown fuse I might let go by, but if the replacement blows, something's definitely wrong inside the amp.

Yeah, as I know all too well.

I just tried 2 of the new fuses in the amp and they blew straight away, so it looks like an amp problem.

Thing is, it'd probably cost too much to get a techie to look at it and it's not even my amp, so not only do I have to buy my friend an amp, I need to get myself a practice amp now too.

:(

EDIT:

That's 2 amps in 2 years that have had this problem and in both cases it has been soon after I used headphones with them. Coincidence? Bloody hope so...

I'd suspect the power-supply rectifier diodes. The rectifier diodes will be close to where the fuse is and also where the transformer wires connect to the board. They are small black resistor-looking things with a white band at one end of the body. They might be visibly burned-looking, but possibly not. The only way to test them is with an ohm meter..they will show a few hundred ohms with the test leads connected in one direction, and nothing in the other direction if ok. If the amp is blowing fuses, then most likely one or more have shorted rather than going open. It's also possible the power transformer has shorted.

There's also the possibility, though from the 'fade out' symptom you reported rather unlikely, that you blew the power amp. If I recall correctly, those 15G's use an integrated-circuit power amp. It's the grayish-black rectangular bit with about 10 legs screwed to a chunk of aluminum at the rear edge of the circuit board. If you're curious to check further, try unscrewing the plate over it and look at the body. They will usually blow a small hole in themselves if that's what has failed, but not always.

Oh, and the yellow goop you see in there is almost certainly not flux, but epoxy or hot-glue. They use it to secure stuff like larger capacitors and wires to the circuit board. After parts are soldered to the board, usually by a wave-solder machine, they are run through a chemical bath to remove flux before final assembly. I worked in many electronics factories and assembly lines in my younger days.

Bottom line is that if it's simply the rectifier diodes that have gone bad, they are very cheap, like under a dollar, and the cost to fix it shouldn't be much over whatever the shops' bench charge is. The integrated-circuit power amp unit is about a $5 to $12 part, and the labor will be a bit more, and could easily be worth it to buy a new amp. However, if the power transformer has bought the farm, then for sure you'll be better off buying a new amp.

Most of these little low-wattage solid state practice amps like the 10 to 15 watt Fenders, Crates, Epiphones, etc are very similar circuit-wise and are built to be just good enough. The headphone circuits in them basically just switch the amps' speaker output to a big resistor, and take a sample off that through a couple of divider resistors to drive the headphones. So, when you have headphones plugged in and you crank the thing, it's the same as if you were running the amp flat out with the speaker, only with a poorer load, and they generally won't take being maxed-out for too terribly long.

Cheers!

Strat

Hmm. Thanks for the informative post, Stratman!

I still have a coupel of queeries. It's nice to know that something is different when I use headphones- which I'm never going to do again!

However, I wasn't maxing the amp out in terms of output signal (it was on about 3-4/10 clean channel). But I probably was giving it a huge input amount (4 pedals and a rack effect on all at once, including an overdrive pedal and a fuzz pedal- all about level 4/10). So my question regarding this is, are there any dangers with having too great an input signal? OR any problems with too great an output signal? As regards the input/output signal- especially if you are boosting the signal due to overdrive pedals etc?

Well, if you've boosted the signal such that what the input of the power amp sees at a volume setting of 3-4 is as much or more than what it would see with an un-boosted signal at '10', especially if the rack effects you mentioned were in the amps' effects loop, then you would effectively be 'maxing-out' the amp. Another thing with the small amps like the 15G..if they have an effects loop, it is most likely set to handle pedal-level signals as opposed to line-level signals from a rack unit. The volume control is before the return of the effects loop, which usually just passes through a buffer op-amp then directly into the power amp.

If you have a normal gigging amp, you might want to explore the possibility of using a good attenuator that has a headphone out. There are some pretty good ones here:

http://www.tedweber.com/atten.htm

Cheers!

Strat