Help me diagnose my guitar amp problem

I have a fender princeton chorus that I bought, I’d say at least 10 years ago. So it’s not exactly new, and I think I’ve even had the same problems before and got it fixed, but I’m not even sure. If I did it was some time ago and since I’ve rarely played the electric in recent years I’ve kind of neglected the amp a great deal. But now I’ve got some important performances coming up in a very short time (namely for a local theatre production of The Rocky Horror Picture Show (I plug when I can)) and I sure could use some quick advice.

Here’s the problem: The amp seems OK at first when I turn it on. After playing for a short time (10 - 20 min.) it develops a loud buzzing hum (which in the distortion channel is simply unbearable; in the clean channel it is present but not terrible). The more I continue to play the worse it gets.

The questions: What is the problem or the possibilities? How much could I expect to pay to have it repaired? Is it something that a complete layman who’s good at following directions, like myself, could possibly fix himself and save some cash?

What type amp do you have, make, model etc…?

Fender Princeton Chorus. I’m not quite sure what other info I should provide, but I can look if you tell me where to find it.

If you turn on the amp for 10-20 minutes with the volume turned all the way down (and don’t play) and then turn it up will you get the hum immediately or does it still take 10-20 minutes of playing to get it?

The Princeton is solid state, so you don’t have a tube problem. But there is probably some electronic part that is failing when the amp comes up to operating temperature.

Amp repairs tend to be expensive. If you’re a complete layman, you probably shouldn’t try to fix it yourself, because you could electrocute yourself (even if the amp is unplugged). The only way to know for sure what’s wrong or how much a repair will cost is to take it to a shop for an estimate, but they will charge you for that.

There might be some amp techs on this board that recognize the syndrome and can hazard a guess, but it will still be just an educated guess without examining the patient.

I’ll let you know in 10 - 20 min.

It sounds like a bad electrolytic capacitor in the power-supply filters. It’ll be one of the larger ones in the unit. Sometimes they can develop thermal defects as they age, and begin to leak electrically as they warm up.

Actually a bad capacitor in the power supply would be the first place I’d look regardless of the outcome of that experiment.

Results of experiment: hum was indeed present without any playing and volume all the way down for 20 min.

Interestingly, the hum is only present with the guitar plugged in and the guitar volume up. Turn the guitar volume off and no hum. But it isn’t the guitar (or cord) since I’ve tried it with others.

So, what are you guys saying in layman’s terms? Cost? degree of complexity involved in fixing problem? is it totalled or simply not worth fixing?

It’s probably worth fixing but proceed with extreme caution if you DIY. The capacitors that look like the culprit are potentially dangerous because they can hold a significant charge even with the unit unplugged. You’ll need to make sure the capacitors are discharged before working on them. Also do you know how to solder circuit boards?

If it’s a bad cap, as I suspect, it’s not terribly hard to fix. and they’re cheap enough, generally, to be worth replacing without even bothering to test them. It’ll be a cylinder-shaped part, with a value on the order of 2500 uF and probably around 300 WVDC. If you can post a photo of the guts, I can tell you exactly which parts they are.

After rereading your first post I see you haven’t used your electric in a while which even further makes the case for a bad electrolytic capacitor because they can deform over time. I think there is a way to reform them by slowly increasing voltage over time with a thing called a variac but I don’t know enough about this. It’s time to get to a specialist - call them and tell them the symptoms and have them make a guess. They’ll tell you to bring it in but ask them to humor you a little. Ask them what it would cost if it was the most likely problem.

[tangent]What is the textbook method for discharging a cap prior to performing any work?[/tangent]

I usually discharge the cap with a 10 to 100 ohm resistor. I do it in three steps: 1) connect resistor across cap leads for 10 seconds, 2) connect resistor between positive cap lead and chassis for 10 seconds, 3) measure voltage across cap (and between cap and ground) to make sure it’s safe.

BTW: I agree with others who suspect one or more electrolytic capacitors are bad. The “hum” is likely the result of the capacitor’s inability to filter the ripple current. This is usually the result of one or more of the following: 1) Leaky capacitor (i.e. too much parallel resistor), 2) Open capacitor (which is very common), 3) very low capacitance. It is possible to test the capacitors, but I would just go ahead and replace them.

I suspect the caps to, but also blow out dust, look for spills and dead cats and stuff inside that might be shorting it out. I ounce had a bad scream in a cassett deck in my car, turned out that the output transistor was shorted by the zinc oxide heat sink compound. You can check caps by hooking a vom up and watching it charge, then reversing the leads and watching it discharge, of course it has to be disconected (discharge first).

OK, perhaps I’m even more of a layman than I let on. I don’t know what an ohm resistor is.

Where do I buy this capacitor? Are they very specific to each amp or are they sold in standardized shapes and sizes for a variety of uses?
I’m not quite sure I’m going to pursue the DIY course of action, but I’m thinking that perhaps if I buy the part, walk into a repair shop and tell them “this is the problem and here’s the part”, they would charge a great deal less than if I just walked in and left everything up to them.

The ones being used in the power supply are thee to filter out AC hum from the DC produced by the supply. The don’t have to be an exact match, but should have a ratiing of at least about the same microfarads (uF - typically about 2500 for power supply filtering), and equal or great working voltage (WVDC). They can be had from electronics parts suppliers like Jameco, Allied Electronics or DigiKey. Don’t worry about discharging the caos too much. Simply leaving the unit off and unplugged for a couple days should allow them to self-discharge to a safe level, even if there are no bleeder resistors present.

Moe best thing to do if you don’t want to attempt it yourself (though it sounds like Q.E.D could help you) is to call a local music shop and act like you know what your talking about. “Yea man, I think I gotta blown cap in my power supply dude. Bummer cuz I gotta gig.” :stuck_out_tongue: They’ll probably charge less than if you went in acting like you had no clue.

Yeah, that was the basic idea of my last post. And if I buy the part myself I could avoid the markup that the repair guy might add.

Is there a type of electronics store, like a Radioshack, that I could just walk into and get it at? I’d have to order it online at the places you listed **Q. E. D. ** which I wouldn’t mind if time weren’t such an issue (It probably wouldn’t take too long if everything went perfectly, but allowing for shipping problems, perhaps ordering the wrong part by mistake, ya know, all the things that happen to people like me). Ideally I’d love to be able to do it all Thurs., the one free day of the week I have.