My suggestion is, have it checked out by a good technician. If that is too expensive for a $150 amp, get a new one.
Power amps have large power supplies and relatively large fuses, and it's not unheard of for a failure to result in a fire.
@mfiddles I think you lucked out on this one. Semiconductors are very unforgiving about abuse! If the amp still works, it means they were not damaged. It sounds like a resistor in the amp got really hot and burned off its coating but apparently that resistor still works.
I don't think you can trust it though. But if you are easy on the amp, it will likely serve for a while, but it is prudent to get that resistor replaced.
If the amp is still running, all you are risking is $150. I wouldn't worry about it. If your mother convinced you to always were clean underwear in case you are in an accident, then you might want to take it out of service.
You didn't tell us what type of amp it is but to make that much smoke, it must have some discrete components. Likely a resistor overheated and burned off some paint but obviously didn't fail. It probably won't fail since it passed the high current test.
If it does fail, likely all that will happen is no music will come out.
Of course if it were mine, I'd take it apart and do my own troubleshooting but I've come to understand that there are probably only 3 or 4 of us here that can do that and the rest of the members pretend they're too good to do it (sour grapes).
About 3 months ago I had to document an equipment fire at work. It was an older, but high-quality, multichannel amplifier. It did catch fire in the rack and destroyed the new surround processor directly above it, the new Parasound amplifier below it, and coated all the other gear in soot so bad that any warranties from those pieces were voided by their manufacturers.
The reasons to even consider "risking it" are two:
1) I like how the amp sounds, and it has a "use value" to me that well exceeds its resale value of <$150; and
2) if the amp blows, it seems unlikely that it would damage anything but the amp itself.
Yes I think I saw a tiny flame inside, as you said, and this would be logical given that there was smoke. Yes, I did take the cover off the amp. No shards are visible, and none of the components look burned or visually damaged.
I did play music with the amp for a couple hours today. It sounded like it always does and operated normally, no smoke, no overheating, no burning smell. Not sure what to think.
Yeah...always leave the nail in the tire as pulling it out could cause some air loss. Right? Ever see a house fire? Ever see an amp blow up? Is a fire hazard worth it? I say keep using the amp but tell everybody else in the house to move out with their belongings and make sure your will is in order.
I’d take the amp ’off-line’, open it up, and just take a critical look-see at the components, if only for an educational tour...
If something got hot enough to light up the grille, smoke, and stink....it ought to be obvious.
Go shopping for a ’new’ old amp, too.... no sense in finding out how much further you can depend on luck to keep out of trouble of a more serious sort:
"Honey? I torched the house...."
That’ll be popular...NOT.
First you said:
Just now you said:
Which is it?
If flame, then the amp cannot be trusted. If smoke and the amp is still running as it used to then it is as I described earlier.
Resistors can heat to the point that they can turn red hot and in doing so will burn off their coating, which smells like plastic burning and makes a bit of smoke. But the resistor can survive if detected soon enough and allowed to cool. Its a good idea to have the part tested, since heat can cause some resistors to change value.
It really looks like you've been smokin somethin, not amp and there may be no issue at all, but just an illusion.
Nobody has commented on the preamp and what might have caused this. You said the filter was passive, right? No active components? So the input of this filter was plugged into the power amp input, and the output was plugged into the preamp.
The two preamp outputs should be individually driven, but they may just be in parallel. I suspect that the incorrect wiring caused an oscillation which fed into the other power amp pre-out.
Could also be a grounding issue. They are all balanced connections, right? Have you confirmed the xlr connections on the passive filter are correct for your setup (+ve, -ve, ground)
Thank you for all of the helpful feedback! 😊 I appreciate it and will consider all of it. My understanding has been that, when an amp blows, it does not blow components upstream from it (e.g., preamp, DAC, etc.). It could blow the speakers but that is fairly unlikely. I could be wrong though. Of course, if it catches fire or explodes, it could "blow" everything.
And just to answer the prior poster's question, no the connections are not balanced (XLR). They are RCA. Yes the low pass filter is entirely passive, and it was reversed in the fashion you describe. The filter has a male (output) and female (input) end, and the male (output) end was plugged into the output of the preamp, while an RCA cable going to the amp was plugged into the female (input) end of the filter. I would suspect you are right about the oscillation and that the preamp outputs are not independently wired.
In my 20+ years of having separate audio components, I have made setup errors on occasion. I never expected that one as seemingly minor as this would cause such an extreme outcome (smoke, fire, etc.).
Here is a link to a photo of the inside of the amp. As should be evident, there is no visible damage to any component. I have looked at each component closely and see nothing out of the ordinary.
It is hard to tell from the pictures, but, above the two big electrolytic caps, there appears to be two burned out resistors, one leaving a big white stain from what is left of it, the other a smaller resistor right next to it that also looks damaged.
It is not worth the risk running this amp, and you have no assurance that the next failure won’t take out other components, like your speakers.
Yes, upon further inspection, I found the offending component. It is a single resistor, labeled D785, in the location described by larryi. (The component adjacent to it is a diode and does not appear damaged.) It appears that the coating of the resistor burnt off, but it is otherwise still in tact. The resistor is carbon 10ohms 5% 1/2 watt. I can order a replacement for a dime or two (literally), and then pay someone to solder it in, I would suppose. And I would imagine that will take care of the problem. What a relief! :) :) :)
Here is a link to a closeup of the damaged resistor:
Did not want to get sappy in my earlier posts. But this amp, whatever its market value, was a gift from a favorite uncle who recently passed away. I like it's sound and being able to use it brings fond memories of time with him. He was known for playing music very loudly.
@larryi and his eyes beat me to the post. That resistor below the Diode labeled "R762" and above the Diode labeled "D753" got toasted....and not in a good "hitting the bong" way!
I understand the sentimental value of gear. I inherited "Klipschorn Clone" corner speakers my late Step-Dad built around 1962 -- and they mean the World to me. Other speakers out there would run circles around these speakers -- but not by much! He was a talented Chemical Engineer (actually the top dog at Tennessee Eastman's Chemical Division during the 1970's and early 1980's), but had amazing knowledge of Electrical Engineering and acoustics. I wouldn't trade or sell these speakers for any amount of money. It means something knowing his hands crafted the Cherry and Oak wood for the cabinets, and built the crossovers and did the wiring.
Before putting this amp back in my system, however, I would spend the money to have a professional technician go through the amp and do the needed repairs. Even though it looks like a simple job, for insurance purposes I would want a paper trail showing a "Professional" worked on the amp. That way, if something does go wrong when you hook it back up and it does cause a fire, you have a greater standing with your insurance company or with the Technicians Business Insurance coverage when it comes to filing a claim. It's sad that we now live in a World where you have to think along these lines...but it is what it is.
@mfiddles D753 is a diode in series with the 10 Ohm resistor, R783.
The screen for the resistor might be visible once you clean the board.
This resistor is in the protection circuit and is not part of the amplifier circuit itself. So the amplifier circuit was undamaged in any way. However, given the state of the resistor its a good bet the protection circuit for that channel is damaged. It may fail to give protection if an event occurs in the future.
I would start by replacing the resistor and testing the associated diode for a short.
The cost of a tech’s repair might be more than the value of the amp, so that must be considered. I have several amps in my collection that would sell below $200., and they all work great. But if 1 of them failed and I could not do the repair myself, it would become a recycled item and imo not worth the investment. I predict that mcfiddles will continue to use the amp, as he sounds like he will. My best to all, MrD.
I repaired stereos for 4 years in college and I can tell you that blown amps do take out speakers. I'd give you 3.14% odds depending on how good the protection circuitry is, how it fails and how robust your speakers are. It could also cause a real fire, maybe 0.1% odds. Is running it really worth this risk? You could be your own technician, buy some large enough dummy loads, 8ohm resisters, and run your amp full power for about 30 min. You need some idea of what full power is since you don't have an oscope. If it survives, then your odds of failure went down. Also, open it up again and look really closely in the area of the orange glow with a very bright light. You might find something you missed the first time. If it's a resister, compare the value to other channel.
Thank you, Nagel. Well 3.14 x R^2 = area, so I suppose the 3.14% odds you cited are not too bad. Lol. How do I "buy large dummy loads?" Do they sell them at Target? Walmart? Do you just mean running a high-volume signal into the amp, with nothing on the outputs, for 30 minutes, and hoping there is no fire?
@mfiddles A dummy load is usually a high power resistor that won't fail when subjected to amplifier power. That way you can run an amplifier without having to listen to it. If run with no load you won't learn if the amp will hold up.
Usually you'll want to use a regular musical signal. A sine wave can be used as well, but if using such you run the risk of overheating the amp in a few minutes even if at only 1/3rd the full output power level. Most inexpensive amps and receivers of the 1970s and 1980s lacked the heatsinks to survive very long with that treatment.
But we already know the amplifier portion isn't having a problem. The protection circuit is what heated up, likely due to some sort of miswired connection.
Great. So something like two of these, one to mimic the load of each speaker, with one speaker wire going to each end of the resistor?
Or does the wattage of the load need to meet or exceed that of the amp (135w)?
You can google dummy load resistor and buy them already mounted on a suitable heat sink, but the 200 watt ones you will need are not cheap and you would still need a signal and a way to measure the output you are sending to the resistor; all of this would cost way more than the amp. You can replace the parts, but it is hard to tell if the protection circuit is working as it should. It is probably best to junk the amp.
I would unplug it. Let it sit for a day to let the capacitors drain out. After that remove the screws and try to find out what melted. The wood sides can be easily removed too. You should be able to see a brown mark near the component that heated up. After you diagnose the problem you might be able to fix this yourself with a soldering iron and a little time. Please make sure it stays unplugged and you let the caps drain out. I hope this helped.