24 April, 2008

Electronics 101

Location: inside my fender champ
Mood: triumphant

I am very pleased with myself right now. Yes I am. I just saved myself over 100 dollars. A few weeks ago, my 1976 Fender Champ stopped working. It just magically was not operational. This is a little tiny old Fender amplifier (a tube amp) that is great for practicing, and recording (and I've used it for gigging too, though it's not particularly powerful).

So, when it crapped out, I discovered that a fuse had blown. Now, when a fuse blows, it could be that the fuse failed because the fuse failed, or it could be that the fuse failed because any number of other things in the amplifier failed. The fuse costs about 50 cents. Other things in the amplifier can cost much more. For instance, if the output transformer is fried... you're looking at about $50 for a legitimate replacement part.

I brought the amplifier into Aviator Guitars (because I was bringing another amplifier in anyway, so I figured I'd ask them). It was immediately clear that it would be expensive for them to fix it. Because they charge 1 hour labor, minimum, just to look at it. And that's $80. So I'm looking at $80 + whatever parts they decide they need to fix it. And this was not a particuarly expensive amplifier, so does it really make sense to throw that much money at it?

Plan B, which I discussed with the technician, is to troubleshoot it myself. Here's how it works. The most likely culprits are tubes, of which there are three. From input to output, they are the preamp tube (12AX7), the power tube (6V6), and the rectifier tube (5Y3). To see if it is a tube causing the fuse to blow (i.e. if a tube is bad, or shorted, it will blow the fuse - the fuse blows when your electronic device attempts to draw too much current from the power source - the fuse blows to protect the amplifier from having bad bad things happen to it).

So it goes like this.

0. Put in a new fuse
1. Take out all the tubes.
2. Turn on the amp. If the fuse blows with no tubes, you probably have dead transformer.
3. If fuse doesn't blow, then put the rectifier tube back in.
4. Turn on amplifier. If fuse blows, you *may* have bad rectifier. But could be other stuff.
5. If fuse doesn't blow, put the power tube back in.
6. Turn on amplifier. If fuse blows, you *may* have bad power tube. Could be other stuff.
7. If fuse doesn't blow, put the preamp tube back in.
8. Turn on amplifier. If fuse blows, you *may* have bad preamp tube. Could be other stuff.

At steps 4, 6, 8, if the fuse blows, you'd want to *try* putting in a new tube of the proper type, and see if that fixes the problem. If so, then you're done. If not, then it's probably the "other stuff" which would be any number of components on the circuit board. In that case, you open up the amplifier and hope that you can see a burned out resistor or capacitor or wire, that you can easily replace. If not, then you bring the amplifier to the repair guys and pay them the money.


I got lucky. With no tubes, fuse didn't blow. When I put the rectifier back in, the fuse blew. So I went online, via E-Bay, and bought a new rectifier tube - cheapest I could find - for $13 including shipping. Arrived today. Put it in. Fuse doesn't blow.

Problem solved.

Put all the other tubes back in. Played some guitar. Sounds good. No fry.

Now, maybe it will blow eventually, or maybe I just had a wonky rectifier tube. It certainly looks ancient, so not surprising.

Aren't you glad I told you that story?

1 comment: