blowing fuses on a ss bass combo

[black_labb]

hi guys
i picked up a second hand 100w ashdown bass combo a no. of months ago for teaching my girlfriend to play bass on for the trio i got together (she sings mainly).

when we were having a practice a few days ago the fuse blew as we were finishing up. i picked up some fuses today and the amp blew the fuse again instantly (or close to being a slowblow). this is the fuse regulating the current from the mains. it seems to me that it is probably a shorted power transformer (the only transformer being ss).

another idea was that it may be the filter caps leaking current to ground, as i opened it up thinking the fuse looked ok and saw some cream/pinky stuff dried around some of the larger electrolytic caps. i wasnt sure if this was the dielectric leaking out or if it was just something to hold the caps in place like silicone or similar thing for electronics. it was dried hard against the board and the cap. the fuse shorted out within a second of me turning it on, so i dont think this is the problem, but thought id ask about the unknown stuff.

would you guys recomend me ordering a newtransformer for the amp and giving it a go, or do you think that it could be something else?

would a company be likely to send a tranny to someone like me who is a hobbyist with tube amps? there is a local music shop that carries ashdown amps that would probably be able to order it for me, but maybe not as they dont repair amps there.

thanks for any advice

[Alex R]

- the stuff they use to glue caps in place is either silicon or hotmelt usually. Sometimes it gets a bit hard and brown, but it's kind of like glue if you know what I mean. If what you have is chalky and dusty then that's probably bad.

If the power transformer has shorted it will blow fuses as soon as you turn the amp on. If a cap has fully shorted - and leaked dialectric on a modern cap might well mean that has happened - then that will blow fuses too. So you can't tell from the way the fuse blows what has shorted.

A good tool to equip yourself with at this stage would be a light bulb tester. Some arrangement with plugs, flex and a socket on a bit of wood featuring a light bulb, say 60W, in series with the live mains lead. This will stop most if not all of the damage an amp can do to itself if it has a short, and will allow you to put in a new fuse and check for shorts without the fuse blowing again. The bulb lights brightly if there is a short, bright-then-dimmer if all is well.

The procedure would be to gradually disconnect all the transformer secondaries, turning the amp back on again between disconnections to look at the bulb. If the bulb is still bright once the secondary circuits are all broken, then you have a shorted mains transformer. If the bulb changes to normal dimness at one stage then you know which circuit had the short. If those caps look dodgy, pop them off the board and see if the short goes away.

[Alex R]

Don't know about Ashdown's policy. They do keep spares.

[black_labb]

- the stuff they use to glue caps in place is either silicon or hotmelt usually. Sometimes it gets a bit hard and brown, but it's kind of like glue if you know what I mean. If what you have is chalky and dusty then that's probably bad.

If the power transformer has shorted it will blow fuses as soon as you turn the amp on.

A good tool to equip yourself with at this stage would be a light bulb tester. Some arrangement with plugs, flex and a socket on a bit of wood featuring a light bulb, say 60W, in series with the live mains lead. This will stop most if not all of the damage an amp can do to itself if it has a short, and will allow you to put in a new fuse and check for shorts without the fuse blowing again. The bulb lights brightly if there is a short, bright-then-dimmer if all is well.

The procedure would be to gradually disconnect all the transformer secondaries, turning the amp back on again between disconnections to look at the bulb. If the bulb is still bright once the secondary circuits are all broken, then you have a shorted mains transformer. If the bulb changes to normal dimness at one stage then you know which circuit had the short.

great advice, thanks for that. probably wont be able to do it for a couple weeks as im visiting family in canada on monday for nearly 2 weeks.

[Alex R]

Have fun.

[Enzo]

I seriously doubt it is the power transformer. Is there some reason you think it is bad?

Power transformers do fail, of course, but on the list of likely causes for fuses blowing, the transformer is always way down the list. I the power transformer first on your list if a tube amp pops fuses? I'd be looking for a shorted power tube first. And on this amp, I'd be looking for shorted output transistors first, myself.

SHorted outputs is almost always the very first thing I check on a SS amp blowing fuses. On the rare occasions it is something else, then I look for shorted main rectifiers - the ones for the 30-40 volts (or whatever) that the power amp runs on. And in the even rarer case where that isn't it either, THEN I look at the filters.

Your filters most likely did not vent all that stuff, it is more likely glue. COuld be hot melt or silicone, or could be contact cement even. They want to keep them steady so as not to crack their solder.

And way at the bottom of the list is the power tranny. If you want to test the tranny, disconnect all the secondary wires from the board, now power up with ONLY the primary wires connected. If that blows fuses, then a bad tranny is likely.

Go back to the heat sink and test for shorted output transistors. Please.

[black_labb]

I seriously doubt it is the power transformer. Is there some reason you think it is bad?

Power transformers do fail, of course, but on the list of likely causes for fuses blowing, the transformer is always way down the list. I the power transformer first on your list if a tube amp pops fuses? I'd be looking for a shorted power tube first. And on this amp, I'd be looking for shorted output transistors first, myself.

SHorted outputs is almost always the very first thing I check on a SS amp blowing fuses. On the rare occasions it is something else, then I look for shorted main rectifiers - the ones for the 30-40 volts (or whatever) that the power amp runs on. And in the even rarer case where that isn't it either, THEN I look at the filters.

Your filters most likely did not vent all that stuff, it is more likely glue. COuld be hot melt or silicone, or could be contact cement even. They want to keep them steady so as not to crack their solder.

And way at the bottom of the list is the power tranny. If you want to test the tranny, disconnect all the secondary wires from the board, now power up with ONLY the primary wires connected. If that blows fuses, then a bad tranny is likely.

Go back to the heat sink and test for shorted output transistors. Please.

thanks, will do when i get a chance.