horrible buzz/noise/hum on a crate gx15

[Andres]

Hey y'all:

I was hoping someone could help me with a problem i have with my crate gx15.

Here's the thing:

I turn it on, and all i can hear is this loud hum-like noise that won't go away, not even when i turn the volume down or plug the guitar in.

I'd really appreciate any help or suggestions on how to fix it myself.

Thanx in advance.

[Enzo]

Inside on the heatsink will be a five legged thing looks like a transistor. It will have a number like TDA2040. It is bad. Disconnect the speaker and see if there is 20-30 v0lts of DC between the speaker wires.

[Andres]

Inside on the heatsink will be a five legged thing looks like a transistor. It will have a number like TDA2040. It is bad. Disconnect the speaker and see if there is 20-30 v0lts of DC between the speaker wires.

Excuse my obvious ignorance about the topic but...

Could you explain me what exactly is the heatsink? and a transistor? and how do i "see if there is 20-30 v0lts of DC between the speaker wires"

Thanx

[Enzo]

Andres, I am sorry, I assumed you were a technical person. Most everyone here is into soldering and trying to work inside their amps.

There is a main integrated circuit - what is often referred to as a "chip" - that powers the speaker in the amp. When that chip fails, one way it can act is as your description. I'd bet my lunch money your amp needs a new one of those.

A heat sink is just any hunk of metal used to radiate heat away from something else. In this case it is just a piece of aluminum standing next to the circuit board inside the amp. There will be one electronic part standing along the edge of the board with a screw through its tab. The tab then screwed to the heat sink.

It looks like a transistor means that spmeone familiar with the standard three leg transistor shape will see something that loks the same but with 5 legs instead. It will have the part number TDA2040 printed on it. Older units might have a TDA2030, but the 2040 will work there too. 2040 is heavier duty part. The body of the part is about a half inch on a side with a metal tab sticking out the top for the screw.

WHen the 2040 fails, it will allow DC voltage to get on the speaker wires - this causes the noise you hear, and will destroy the speaker if left on for any length of time. To see if this is the case, we use a standard volt meter, touching one probe to each of the speaker wires.

To make this repair, you will need to disassemble the amp, and remove the circuit board from the chassis. All the wires can remain connected to it, we just need to get at the underside of the board. We then will unsolder to bad part and solder in a new part. You will need basic hand tools, and a soldering iron, and a TDA2040 chip (assuming my diagnosis is correct.) A voltmeter is handy to verify certain things, but one needs to know how to use it for any readings to be meaningful.

If this is beyond you technically, then a repair tech will have to do the job. For reference, in my shop, this sort of repair on a small practice amp would cost you about $40

[Andres]

Man, I really, really wanna thank you for taking the time to reply to my post.

I did everything you said and, although I´d never fixed any electronic device before, I did it!! Amp's working just fine now

Really thanx man. It's nice to see there's still people out there who have social awareness.

Thanx again and see you around

Andrés

[footsandwich]

the NTE1376 will work for this repair as well.

[Paul P]

Really thanx man. It's nice to see there's still people out there who have social awareness.

Yeah, Enzo is Da Man. I was impressed by his (your) taking the time to
spell things out so clearly but I'm also really impressed by you, Andres,
following his instructions and fixing your amp.

Bravo to both of you.

Paul P

[Enzo]

Hi foot, welcome to the forum.

I am a one man crusade against NTE parts. This is an example. Mouser sells the NTE1376 for $14.53, and they sell the real part - the TDA2040 - for $3.17 each. The NTE part costs almost five times as much. Why should anyone order the NTE instead of the real part?

And in the case of something like power transistors as used in solid state amps, the replacement part will not match the real part in thermal tracking and other things important to it sharing current, so I consider them actually dangerous.

[GT6]

Inside on the heatsink will be a five legged thing looks like a transistor. It will have a number like TDA2040. It is bad. Disconnect the speaker and see if there is 20-30 v0lts of DC between the speaker wires.

I have the same amp with the same problem. Based on the advice in this thread, I changed the power amp. I still have the hum and no guitar sound through the speaker. There is a pop through the speaker that was not there before when the power is turned on or off. Any advice on what's wrong and what to check next?

[The Dude]

1) Unhook the speaker
2) At the speaker leads measure for both AC and DC voltage.
3) Report back what your findings are for both AC and DC.

[GT6]

1) unhook the speaker
2) at the speaker leads measure for both ac and dc voltage.
3) report back what your findings are for both ac and dc.

7.6v dc
16v ac

[Enzo]

The power amp runs on two power supplies, which OUGHT to be clean DC. And without looking it up, somewhere in the area of 20v each. Oh, I looked, it is 15v each. From your description, I'd wager one of your power suppliies has lost its main filter cap. Might be a bad cap, might just be cracked solder under the cap. Look behind the reverb control to see two filter caps standing up, C28, C30. Look on their underside for cracked solder. In fact, just resolder them anyway, can;t hurt. In any case, we at least need to find smooth 15vDC across each of those caps.

[GT6]

The power amp runs on two power supplies, which OUGHT to be clean DC. And without looking it up, somewhere in the area of 20v each. Oh, I looked, it is 15v each. From your description, I'd wager one of your power suppliies has lost its main filter cap. Might be a bad cap, might just be cracked solder under the cap. Look behind the reverb control to see two filter caps standing up, C28, C30. Look on their underside for cracked solder. In fact, just resolder them anyway, can;t hurt. In any case, we at least need to find smooth 15vDC across each of those caps.

Re-soldered both caps (both looked OK) and measured 15V DC and 33V AC across each cap. Also, the heat sink for the power amp got quite hot while I was measuring voltage (speaker not connected).

[Jazz P Bass]

If you truly have 33Vac across a filter capacitor, then it is in need of replacement.

[aardbei2000]

Hi all. I just add my problems to this long forgotten chat, so that related issues is bundled together.

I have also an GX-15 with an enormous buzz. After trying the solutions above, I noticed that the buzz comes from the transformer.
But I have no idea the type of the transformer.

Can somebody point me into the right direction to replace the transformer. Or if you have other thoughts, please share.

Thanks.