Crate GFX 120 - Constant hum

[Roshihyde]

Well, there isn't really a tech near where I live that I know of, and any guitar shop I've ever gone to with an amp that needed repair the first thing they tell me is that I would spend less on a new amp, or that they just wouldn't be able to do it. I paid $80 three years ago for this amp and its the only one I have right now that's anywhere close to 100% functioning but I'm not paying more than $20 to fix it. Plus I'd rather accidentally blow it up and then find something on craigslist that is not as freaking heavy. I know I dove in head first on this one, just help me swim.

[J M Fahey]

I'm not paying more than $20 to fix it

Sorry, but that's not a realistic goal.

[Roshihyde]

Well I'm not paying 1/4 of what I've already invested in it to fix it, and I'm absolutely not paying more than that. Like I said, I'd rather blow this one up and go get another one that works instead of paying to fix this one that is too heavy and bigger than what I need in the first place.

[J M Fahey]

The "25% of cost " limit is meaningless if you paid almost nothing for it.
Suppose you buy another in Graigslist .... will you get it for $20?
Do you have any real guarantee that it will work?
What about shipping?
Or gasoline spent on picking it up?

[Roshihyde]

It's the point is I really don't have money. I only work 1 to 2 days a week right now because my type of work doesn't really get any business this time a year. I'm a musician on a budget where things like bills, food, and life in general get in the way. The cost of getting an amp repaired is way out of my budget, and I really want to learn to do this myself. The only amp I have right now is my xbox when I play Rocksmith (which was purchased with my second to last decent check) and my tv speakers don't serve well enough to us as an amp, and if I want to jam with people I can't because I don't have an amp thats fully functional. I paid $80 for this amp because I can't afford an only slightly used one much less a new one. If I get another on craigslist, I can go test it out before I buy it, and its true there is no guarantee it will work, but this is what I have to work with because its what I can afford. I just want to fix this one.

[J M Fahey]

I hear annoying hum when you turn the amp on, what happens if all controls are set to 0?

[Roshihyde]

Same thing. It stays there.

[nashvillebill]

It's not easy to teach somebody how to repair electronic equipment through the Internet, especially if they don't know many of the basics. This isn't a personal attack on you, just a harsh reality. When somebody says "test the 40V rails" and the answer is "I have no clue what you're talking about" well that makes it difficult to get much accomplished. Nevertheless, we will try to help as best we can.

You need a piece of test equipment called a multimeter, sometimes called a digital multimeter (DMM) if it's got a digital display (the old ones had an analog scale with a pointer needle), sometimes this can be called a VOM (volt-ohmmeter). It measures voltages and resistance. Voltages can be either AC or DC and this can help diagnose a major hum problem. A good DMM can cost a lot of money, the cheap $10 ones at Harbor Freight won't help much IMHO.

You mentioned in post #42 that you had re-soldered the filter caps. This alone could be the source of your hum. If the solder connection is poor, or if a filter cap has a broken lead, or the trace on the printed circuit board (PCB) has been burnt through, then the cap won't be able to do its job. A power supply filter cap takes the pulsating DC voltage that comes out of the rectifier and smooths it into an almost constant-voltage DC. The DMM can help diagnose this in two ways. If you measure both the supply rails (with your DMM set on DC volts), one should read +40VDC and one should read -40VDC. You can measure right at the + and - connection on the capacitor.If a filter cap is not connected that rail will likely read off. A second test would be to set your DMM to AC and measure the power supply rails. Significant AC voltage on that rail means the filter cap isn't smoothing the DC.

Note that these caps have a polarity, a + and -. on the + rail, the - side of the cap goes to ground. On the - supply rail, the + goes to ground. Soldering that cap in backwards will make it fail.

[Roshihyde]

Ok my girlfriends dad has a multimeter. When you say supply rails Im assuming your talking about the leads. I cheched them by touching the leads of the filter cap and the doing the same again only switching which test lead was touching which filter cap lead. Testing one way, it read +40, but the other way kept bouncing from -39 to -40 and back. From what you have said, this seems like a logical explanation of why the sound isnt full and I get a pop and crackle. So Im assuming the next step would be to order another filter cap. I've found several of them online but they dont look to be the size of the one I have. Can you give me a part number or link me to one?

[nashvillebill]

I wouldn't jump into changing that cap quite yet. Your meter may not be precise enough (if the actual voltage is 39.5 and your meter doesn't read into the tenths of a volt, the meter reading will jump from 39 to 40). Take your meter and turn the setting to AC volts. See what the AC reading is on that cap. Hopefully your meter reads into the millivolt range, if not you need a better meter.

Check the connection on that cap. Are the solder connections solid? I cannot stress how important it is that these solder connections are good. If you aren't used to soldering, it is very easy to have a cold solder joint which would result in your hum.

Are the copper traces on the circuit board in good shape around both caps? Visually follow the traces on the circuit board. Going one way, they will go back to either diodes or a bridge rectifier then back to the transformer. Going the other way, they likely go to some sort of connector. Make sure the connector is tight, make sure the wires on that connector haven't broken or getting intermittent.

[Roshihyde]

Well I should also say that for a couple seconds the meter would kinda go crazy and jump from 50 to 10 and anywhere in between. It did that twice and the I couldn't get it do do that anymore. I think the meter I'm using may not have the quality I need because it doesn't show anything out to a decimal. I'll test the connection on AC but would it be a good idea to remove the existing solder and just resolder it with fresh solder?

[nashvillebill]

Yep, resolder that cap.

[Roshihyde]

Ok I have to get a desoldering pump or braid. I just dont know anyone who has it. I should have a check coming in Friday Ill get some then if I haven't yet.

[g1]

Seeing as you are fairly limited as far as equipment goes, might as ask what kind of solder and iron you are using?

[Roshihyde]

I have your standard 30w radio shack pencil iron. As for solder Im told anything 40% tin and 60% lead with a rosin or flux core is supposed to be the best you can use. The brand Im currently using is alpha metals and its a .062 diameter.