blown amp

can someone help me out ? i have a onkyo m504 i bought some elevtro-voice pa speakers and hooked them up to my amp and they played great for a day. my wife turned it on when i was not home she said it played for 20minutes then shut off. i took back the cover off the amp. I noticed one of the fuses were blown in the amp. i replaced the fuse that was blown and now the amp when turned on nothing happens. I took off the back of my speaker off and saw the cross over was bad. i plugged my amp into an outlet and the fuse blew as soon as i turn on the amp with nothing hooked up to it. I have checked the transformers with a volt meter and they read the same. I dont see anything blown on the boards. i need some advice thanks

Let’s see if I can flag down a moderator to move this to General Questions for you.

Moved thread from ATMB to General Questions.

When troubleshooting a busted amp I usually look at the final stage first since it does the most work. Check the power supply voltage first, then the final output devices. I’m not familiar with that model amp. Does it have transistor outputs for the final stage? Transistors usually fail short or open, and if it keeps blowing a fuse without anything attached I’m guessing one (or more) failed short.

You can find a schematic and parts list for this monster at http://www.eserviceinfo.com/downloadsm/49448/Onkyo_M504.html

However, some poking around shows that this amp is probably worth the price of having it repaired professionally. When big power amps fail, the failure often cascades down the line, so to speak, and you can get into a chase of fixing the obviously blown transistor, then at the next power-up, other parts that were overstressed decide to fail and take the new transistors with them. :smack: A good amp tech will know what to look for, or even know from prior experience that a particular collection of parts should be changed out.

I would concur with the above. This isn’t a simple amp, and clearly isn’t cheap either. Unless you are knowledgeable enough to be able to read the schematic you have zero chance of being able to fix it, and if you could, you wouldn’t be asking the questions you are.

This isn’t a small amp either, the voltages inside, not just the mains power, are dangerous.

The fact the the amp died is worrying - it has a significant amount of internal protection circuitry, and it should be pretty hard to damage. Exactly what is meant by a “bad crossover” is worrying too. These are extremely hard to kill.

You are going to need a professional to fix it, and you should be quite clear about the entire chain of events with him. This might include getting the speakers looked at. If the crossovers are indeed “bad” they need sorting as well.

Came back to add - This amp has some self-protection ability more advanced than a couple of fuses. From over here, I’m wondering if the protection circuitry has also failed - no idea if it failed such that it doesn’t do anything, which enabled some problem with the speakers to blow the output transistors, or if it failed in “crowbar” mode where it prevents the amp from powering up.

And, as Francis notes, crossovers are almost impossibly hard to damage. On E-V speakers, I’d expect them to be actually impossible to damage. In 25 years, I’ve encountered exactly one “bad” crossover and it wasn’t even a true component failure - the speaker had been overdriven so hard that the potting compound melted and seeped into the tweeter’s level control. Once the adhesive re-solidified, the goop disrupted the sliding contact and cut the highs.

Exactly how loud was your wife running the amp that day?

She said she was listening to talk radio and that she did not have it turned up loud. She did tell me yesterday she smelled what she said was a hot plastic smell but could not figure out what it was until the amp cut off. i shook my head and asked her to never touch my stereo equipment again.

Which fuse was blown? F702, F801, F802, F803, or F804?

I also agree with other posters that trying to fix the amp is not amateur hour. You need to know your way around a schematic and how to use test equipment. If you don’t know what you’re doing you could cause even more damage.

Having said all of that, there’s likely a short someplace, and I’m guessing proper troubleshooting will uncover one of the following:

  • Shorted power supply bypass cap. (Electrolytic cap.)
  • Shorted power transistor.
  • Shorted winding in a transformer.

Someone also said that failures can cascade. Very true. Just because you find a shorted transistor, for example, does not mean the transistor is the root cause of failure. It may be a secondary failure.

What do you mean you say you “saw the crossover was bad?” Were there burn marks on the crossover’s PCB, and/or components that looked fried?

There you have it; she let the smoke out! :smiley: I concur, never let her touch your stereo again for listening to talk radio.

You may want to double check the impedence load your putting on the amp with those speakers. It looks like that amp is rated at 165W per channel @ 8 ohms.
If your speakers are 4 or 6 ohm you may be blowing fuses.