Buggered Amp!

Guest42 · Joined: Feb 15, 2005 Posts: 9 Location: NJ, US

Heyo. I wasn't really sure where to post this, but I've having a problem that I'd like to resolve as quickly and cheaply as possible, so hopefully someone can help me out.

I have a GK (Gallien-Krueger) 700RB bass amp. It was working fine for a few days, but after a few hours of having a five string bass plugged in, it started cutting out. Sometimes completely, and sometimes just making the sound barely audible, except for an occasional peak in loudness, which causes the woofers to make a popping sound. It did this even when I went back to the four string.

Turning it off and back on seems to temporarily halt the problem, but it always starts back up, now after just maybe 15 minutes. I took apart the head to find that the fan inside of it does not spin, and I'm thinking it might be over heating. But I'm not sure why it would work sometimes and not others.

Any ideas as to what's wrong and how I might go about fixing it? I am of course going to replace the fan, but I'm not sure that will totally fix my problem. However, maybe someone has had a similar problem which they were able to fix.

Thanks for your time
-Guest42

Blue Hell · Posted: Sat Jan 21, 2006 10:28 pm Post subject: Re: Buggered Amp!

Uncle Krunkus · Posted: Sun Jan 22, 2006 5:17 am Post subject:

I had a problem like this once which was heat related, fix the fan first. You could also find that the heat has made one of the solder joints go dodgy. Without proper ventilation power transistors can heat up to the point where they melt their own solder joints on their leads. If one of those goes intermittent you'll get sudden drop out, and then the transistor will cool, this will move the lead, it will connect and the whole thing starts again. It's relatively easy to check for this as you can unbolt the power transistors from their heatsink and give them a nudge with the power on. If your driver pops (keep the volume down!!) that's your problem.

Guest42 · Joined: Feb 15, 2005 Posts: 9 Location: NJ, US

Thanks for the help : )
As far as it being a thermal cutoff, I had thought of that... but then wouldn't it stop playing completely instead of just sounding like a dying old man? I opened up the head, as I've said, but I'm not really sure what a thermal cutoff looks like. Is there a link so I can see what I'd be looking for?

paul e. · Posted: Sun Jan 22, 2006 8:11 pm Post subject:

tube or solid state ?

Guest42 · Joined: Feb 15, 2005 Posts: 9 Location: NJ, US

It's solid state.

elektro80 · Posted: Mon Jan 23, 2006 1:02 pm Post subject:

That figures.
The best would of course either let a skilled service engineer solve this for you. If you get a servicemanual it would probably be easy to the most likely spots in the circuitry the problem is located. Isn´t there some sort of basic schematics inside of the cab somewhere?

deknow · Posted: Mon Jan 23, 2006 1:20 pm Post subject:

...i have blown the "expensive" part of an amp trying to figure a problem like this out...so procede at your own risk.

1. you will need to have a safe way to have the amp on and disassembled (remeber, there is wall current in there).

2. it is probably either a bad solder joint, or a failing semiconductor (transistor, ic, etc). first look at the bottom of board (solder side). if you are sharp (and lucky) you might see the bad solder joint (generally, it will look like there is a ring around the lead (this is actually a crack in the solder, and can fail intermittently with temperature or being knocked around). look at transistors or voltage regulators (3 wires) and any large integrated amp chips (ic's), or any connectors that might see strain (input/output jacks, panell controlls).

2. if you can't see cracks in the solder (use a bright light and magnifying glass), you can get some freeze spray. run the amp until it starts failing, and spray one component at a time with the freeze spray....it should start working again when you get the right component. at that point, you can try resoldering that one component (it might be still be a solder joint), and if that doesn't work, replace the component.

warning, not all transistors of the same type have identical pin outs, so figure out which is the base, collector, and emiter and double check the replacement before soldering it in.

good luck...it feels great to fix gear...but shitty to trash it by screwing up. bring it to a tech if this all scares you, or if you can't afford to mess it up more.

deknow