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Adcom GFA-555 fuse stuck


Hi there,

I've got an Adcom GFA-555, which I use with a pair of Infinity Kappa 8 speakers.

I was running pretty loud on some low frequencies, and I lost all power from the amp. I figure the Kappas were just drawing too much current, since the woofers can dip down to around 1 ohm with low frequencies.

The power light doesn't turn on, so it shouldn't be thermal shutdown or the transistor fuses. I opened up to check, and they are all fine (I put in 4.5 amp fuses instead of the 6 amp ones spec'd to give me a little safety room). After cooldown it still won't turn on.

I didn't see any scorched components either, but I know from personal experience that that doesn't necessarily mean that there aren't any burned out components. I don't smell anything, at least.

So I'm pretty sure the problem is that the power fuse is blown. The problem is, the screw-in plastic holder is cracked in a couple of places, making normal extraction with a screwdriver impossible.

I thought about trying to super glue it, but I doubt that would hold (and I don't have any on hand any way). I suppose I could epoxy an old screwdriver on to it...

I'm pretty stumped as to what to do - I generally tackle this kind of stuff myself, although at this point I know I need a replacement holder in addition to the fuse anyway.

My only thought is to take it to a general electronics repair shop - there's no real audio repair shops in town AFAIK, and the nearest Adcom authorized repair shop is an hour and a half drive away. As long as it's just the fuze, it shouldn't be that difficult of a job - which is why I'd like to do it myself if possible.
blackbeardben

Showing 7 responses by blackbeardben

blackbeardben OP
16 posts
 
The fuse holder didn't melt - it was cracked already so I wasn't able to remove the fuse, which has presumably burned out since I don't even get the power light to turn on.

I didn't hear any clipping (or see the distortion lights come on) until just a few seconds before I lost power, but it was some bass heavy electronic synth stuff so I think the continuous bass frequencies brought the overall power consumption above the max without exceeding the ability of the output transistors.

Like I said in the OP, I have 4.5A fuses instead of the 6A spec fuses for the amplifier stage - and they were all fine. The previous owner used 3A fuses for an even bigger margin of safety, but one of those blew the first week that I had the amplifier. Those Kappas are really power hungry, and other than this the Adcom has taken it without any hiccups.
blackbeardben OP
16 posts
 
Here's a photo of it. You can see that there are four cracks in it as well as it being partially stripped. When I try to turn it, the parts just separate from each other.

http://blackbeardben.smugmug.com/gallery/11634641_vg2iQ#820262200_ZuxBx
blackbeardben OP
16 posts
 
That's a great idea. I didn't think about epoxying something else to it - I've got a spare knob that might work well. That way I'd never have to worry about using a screwdriver for it again.
blackbeardben OP
16 posts
 

Well, I extracted the whole fuse holder after the cover split apart on me - the nut and washer holding it in were brass, actually. It also turns out that there was a crack in the holder body as well. It must have been overtightened last time the fuse was replaced. I never actually got the fuse itself out though.

The hardest part was working around all the wires - I worked from the bottom to minimize that.

I had to cut the wires that were soldered in - there was no way I was going to unsolder them as they were rock solid with wires wrapped around very well. I suppose I shouldn't expect anything less for a power source that's supposed to handle up to 1000 Watts.

I'll go to Radio Shack in the morning and pick up a new one and the 10A fuses I need. It shouldn't be too hard to solder it back in - I'm used to soldering on delicate PCBs rather than thick wires.
blackbeardben OP
16 posts
 

Well, I went to Radio Shack today and they indeed had the right size fuse holder. It cost me $3, so my total repair cost was under $7 including new fuses.

Installing it was a snap - there was just enough wire left on the leads to strip and solder without making them too short.

Thankfully, it performs great now.

Unfortunately, for a short while I hooked up my second amplifier, a Carver TFM-15CB, and I heard a relatively quiet crackling noise from both channels.

It gets marginally louder as I turn up the volume on the amp itself, but it's still quiet and drowned out by moderately loud music. The input level doesn't affect it at all either. It runs fine at high power as well.

I'm pretty sure it's not the volume pots - I've heard that before, and it's working fine otherwise so it shouldn't be a fuse. Both output channels (A & B) show the same symptoms. I never heard it in the past, although it's been months since I've used the amp for more than a few minutes. It's just hooked up to another pair of smaller Infinity RS-5's from the same preamp.

Hmm... Maybe this is a good project to start learning audio troubleshooting and repair? I've got some experience with basic circuits, op-amps, and electrical troubleshooting, but I've never done anything more with audio components than what I did with the Adcom.

I suppose I could just take it to the electrical engineers' audio club (I'm a graduating mechanical engineer at Michigan State), they do some good work - even so far as designing and building their own preamps, amps, crossovers, etc.
blackbeardben OP
16 posts
 
Cool, that's good to know in case I ever have any trouble in the future. To be honest, I'm kind of looking at getting a pair of GFA-565s or maybe a Mac sometime in the future.

Unfortunately, I've had more trouble with the Carver (actually, now I have three of the same - picked up one for just $80 from the local used music store). I inadvertently shorted one of them causing one of the channels to go out (replacing the fuze didn't fix it), and something went wrong with one of the others too (I can't remember what it was since I've been in Sweden for graduate school since August.).
blackbeardben OP
16 posts
 
I guess I should mention that I've since moved up to a pair of Infinity Renaissance 90 speakers. Wow - what a difference they make (although the Kappas are still excellent).

I had to bi-amp them though - the 555 alone was almost entirely bass-less when powering them. I put one of the Carvers on top, and use the gain controls to match them (about 60-75% of max on the Carvers gets a good balance, which goes to show the incredible current requirements of the woofers).

That (or any potential bigger upgrade is why I'd like to try an even more powerful amp. Maybe a tube amp for the top end, like the ARC amps Infinity used for testing. I of course don't have the money now, being a poor graduate student and all.
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