Peavey Decade schematic

[nickb]

Mouser also offer USPS, $4.95 <=1lb.

[Amp Kat]

While I agree that it is best to make sure the chip is the problem I have never found one of these that hums where the problem isn't that chip. That being said it is possible it's something else I just haven't seen it. Unless you have the experience to put a signal in and check it with a scope at the chip and then see if it's at the speaker outs I would take the chance of changing it. Think Enzo made a typo on the CA part # as it's TDA 2004 not 22004. Mouser has the TDA-2004 also but it's $10.30 so you may want to look around at Mouser or Peavey which is also a great source and their shipping is very reasonable. The NTE part # is 1396 and will work also but at least with Peavey you know you are getting the right part.

[runicen]

Having said that...

Welcome to the forum. But let me suggest you open with your amp's problem. The original thread is about a no sound amp, you say yours is dead, but it isn't dead, it makes noise. Dead means it does nothing.

The TDA2030 or in your case TDA2004 is a power amplifier IC (integrated circuit). It has pins for power supply , input, and output. SImple. They donl;t fail "over time," they can last forever, or they can fail. They don;t usually "wear out."

Good point and, again, thanks Enzo and Amp Kat for the feedback on this. I really hate to let this thing go.

As to the problem, I can't recall the first time it came up, but I would assume a few years ago. At the time, I just picked up a small Roland Cube which became my "heavy use" practice amp and periodically broke out the Peavey for kicks. As I got more into using the Peavey over the past two or so years, I noticed that I would get a fixed period of use out of it before it sounded like someone was pranking me by rolling the gain knobs down rapidly. A slight crackle usually accompanied the effect, but it never sounded like a blown speaker. After that happened, the amp would be powered on, but emit no signal whatsoever.

At this point, when turned on, the amp will actually "play" for about ten seconds before signal output fades to nothing and stays there.

Sadly, it's been kicked around pretty savagely through various moves, been kept in storage, and so on, so a certain level of abuse may factor in as well as its age. In fact, I need to secure the brass box that hangs from the electronics portion of the amp (would you call it the "head" in a combo?) as it's hanging from one tab that seems to want to bend and break off. I assume this part is a power converter or conditioner based on the wires feeding it and its shape.

Oh, and regarding the suggestion to run a signal in... I DO have a fairly old signal generator kicking around that belonged to my grandfather, but I haven't the first clue as to how to use it. Any suggestions there?

Finally, should I (in the interest of propriety) move this inquiry to a new thread to keep things tidy, or are we alright to continue here?

[Enzo]

Yes, that was a typo, sorry.

You don't need to use a signal generator, though that is fine too. In my shop I mostly use music, I have a line out from my cheap little Technics shop stereo receiver tuned to a music station. I do have a pile of generators too, and they have purposes, but in general, I just grab some music. A lot easier to listen to music for a half an hour than it is to listen to 1kHz sine waves for that long.

[Amp Kat]

Yes, that was a typo, sorry.

You don't need to use a signal generator, though that is fine too. In my shop I mostly use music, I have a line out from my cheap little Technics shop stereo receiver tuned to a music station. I do have a pile of generators too, and they have purposes, but in general, I just grab some music. A lot easier to listen to music for a half an hour than it is to listen to 1kHz sine waves for that long.

Absolutely what I use too. I use the tone for when I want to see the symentrics or crossover notch.

[runicen]

Yes, that was a typo, sorry.

You don't need to use a signal generator, though that is fine too. In my shop I mostly use music, I have a line out from my cheap little Technics shop stereo receiver tuned to a music station. I do have a pile of generators too, and they have purposes, but in general, I just grab some music. A lot easier to listen to music for a half an hour than it is to listen to 1kHz sine waves for that long.

Ok, so how would I go about using that to troubleshoot the amp and isolate the problem? I gather that, with a signal generator, you use that in conjunction with a multimeter, follow the signal flow and measure the signal coming down the line. This is a loose understanding and probably incorrect, but I don't quite understand how you would feed music into the amp and follow it through the various components.

A while back in the thread, the possibility of a dried out electrolytic cap was raised. What would the symptom of that failing be vs. the IC dying?

Thanks for bearing with me. I'm attempting to speak intelligently on something I have precious little knowledge about. =)

[Jazz P Bass]

I think Enzo was alluding to test running an amp on a music signal.
After it is fixed.
For sure the man uses a signal generator for troubleshooting purposes.

[Enzo]

Well no, I use it for all manner of stuff.

A signal is a signal. If i am working on some PV solid state power amp - in the 1980s it would have been the rear panel of a head, for example - if I need a go/no-go signal, I'd just usee my finger. Hum is as good a signal as anything else if all you want to know is whether the thing amplifies sound or not.

Yes, without a scope, you can trace signal presence with a meter. A sine wave tone is stable, for sure, but music is usually measurable too. A half a volt of music is still a half a volt. I can read that on my meter. If I am looking for a signal that goes away completely or doesnl;t make it through the amp all the way, then I don't care if my meter won;t cover the entire audio frequency band, it is one of those go/no-go situations. If in doubt, turn the signal on and off and see if whatever you are reading does the same, if it does, you are reading the signal

As KB said, there are things a generator is better for, like looking for clipping or setting crossover distortion. But even with a scope, I still reach for my music first, I can see the music waveform as easily as I can a sine wave.


I apply a signal of whatever sort to the input, then I step along through the amp stage by stage looking at the result, whether with meter, scope, signal tracer, or whatever technology you have. Absolutely nothing against a signal generator other than I just don't want to listen to it, plus through a speaker it gives me a quick listen at all freqs to see if any part of the band is missing.

[52 Bill]

Ok, so how would I go about using that to troubleshoot the amp and isolate the problem? I gather that, with a signal generator, you use that in conjunction with a multimeter, follow the signal flow and measure the signal coming down the line. This is a loose understanding and probably incorrect, but I don't quite understand how you would feed music into the amp and follow it through the various components.

A while back in the thread, the possibility of a dried out electrolytic cap was raised. What would the symptom of that failing be vs. the IC dying?

Thanks for bearing with me. I'm attempting to speak intelligently on something I have precious little knowledge about. =)

I don't have a copy of the schematic for your amp, but I think there is a preamp out jack. Plug a cord into it and plug it into your other amp. Does the signal from the preamp out jack fade out as well or does it stay strong? If it does stay strong, then you know that the preamp is okay and the problem is with the power amp section. If it fades out, then that leads you to check the preamp circuit.

In general, you should start by remounting the power transformer that you said was hanging. If you leave it you will cause stress on the wires and may damage it. Check the circuit board for any loose parts and check to see that any push on connectors are clean and tight.