I don't see how a power supply cap in a preamp could cause a subwoofer to blow. If you really mess up you can cause ultrasonic oscillation, which can blow a tweeter. But a subwoofer should be protected from ultrasonics by the crossover inductor. I think you had an aged amp blow on its own.
One should check the output of any amp worked on for DC out and for ultrasonic oscillation, before plugging into any speaker. DC check is fairly simple with the DC scale of any VOM or DVM. No more than 100 mv on speaker terminals. I didn't warn you on that because preamps usually have a capacitor on the output, and amps usually have a capacitor on the input, which block DC.
But ultrasonic oscillation check, you have to check with a scope, or the AC scale of a analog VOM. There is one on e-bay for $25. You block the negative input of the analog voltmeter with a .047 uf cap series two alligator clip leads, to keep it from responding to DC on the AC scale. If you have AC voltage you can't hear with a scrap speaker, then change the cap on the analog voltmeter from .047 uf to 390 pf. If the voltage is still there with the smaller cap, it was ultrasonic. I buy scrap auto speakers for testing at a charity resale shop. Usally $4 to $8. You can block those with a 1000 uf NP cap is you're really expecting dc on the speaker terminal. My best speaker the woofer is $150 and so is the tweeter driver. So I check amps pretty thoroughly before putting them on that. I won't put an amp on it that doesn't have DC out protection. Ie a single supply amp with a cap in series with the speaker, or a PA amp with a DC protector with a great reputation.
Usually in power amps, if the rail cap fails the rectifier or fuse blows. Open connections (bad solder joints) or design changes can cause ultrasonic oscillation, also certain iffy designs will do it for mysterious reasons. Small signal electrolytic cap failures cause weird distorted sound or low output. In my radio, one caused crummy sensitivity when the room was cold. Cap failure in the power on silence circuit can cause silence.
One should check the output of any amp worked on for DC out and for ultrasonic oscillation, before plugging into any speaker. DC check is fairly simple with the DC scale of any VOM or DVM. No more than 100 mv on speaker terminals. I didn't warn you on that because preamps usually have a capacitor on the output, and amps usually have a capacitor on the input, which block DC.
But ultrasonic oscillation check, you have to check with a scope, or the AC scale of a analog VOM. There is one on e-bay for $25. You block the negative input of the analog voltmeter with a .047 uf cap series two alligator clip leads, to keep it from responding to DC on the AC scale. If you have AC voltage you can't hear with a scrap speaker, then change the cap on the analog voltmeter from .047 uf to 390 pf. If the voltage is still there with the smaller cap, it was ultrasonic. I buy scrap auto speakers for testing at a charity resale shop. Usally $4 to $8. You can block those with a 1000 uf NP cap is you're really expecting dc on the speaker terminal. My best speaker the woofer is $150 and so is the tweeter driver. So I check amps pretty thoroughly before putting them on that. I won't put an amp on it that doesn't have DC out protection. Ie a single supply amp with a cap in series with the speaker, or a PA amp with a DC protector with a great reputation.
Usually in power amps, if the rail cap fails the rectifier or fuse blows. Open connections (bad solder joints) or design changes can cause ultrasonic oscillation, also certain iffy designs will do it for mysterious reasons. Small signal electrolytic cap failures cause weird distorted sound or low output. In my radio, one caused crummy sensitivity when the room was cold. Cap failure in the power on silence circuit can cause silence.
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Why don't we just throw everything away and buy new? The garbage dumps have plenty of room, also the ships across the Pacific. Follow Big Brother, the sticker says "no consumer serviceable parts inside."
I started just like this guy, with a soldering iron and a $6 VOM. Actually I was testing TV tubes at the corner store before that. No internet or schematic diagrams for hifi available then . The library reference, RCA Tube Manual 5th ed. said nothing about rubber sealed caps.
Vendors fill consumer electronics with 5 year or less life rubber sealed caps to make sure you buy a new one every six years. Makes me mad enough to block all the traffic in central London.
I've beat them though, I'm using hifi and music pa equipment 25-58 years old.
I started just like this guy, with a soldering iron and a $6 VOM. Actually I was testing TV tubes at the corner store before that. No internet or schematic diagrams for hifi available then . The library reference, RCA Tube Manual 5th ed. said nothing about rubber sealed caps.
Vendors fill consumer electronics with 5 year or less life rubber sealed caps to make sure you buy a new one every six years. Makes me mad enough to block all the traffic in central London.
I've beat them though, I'm using hifi and music pa equipment 25-58 years old.
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+1 
@ zholea
I'm sorry to read about those breaked gears... Problem with one of the lead of the cap inthe wrong via/hole (one of the two on four maybe not connected to zero volt /Gnd, just having its own island for standing the cap as said before (but not returning to the ground of the circuit) ?
Had you a smell and top breaked cap ? Or maybe a short somewhere with your soldering iron when desoldering the former caps ?
But you said you have made many pc refurbishing before, so I doubt about a soldering error but if you're a absolute beginner
@ zholea
I'm sorry to read about those breaked gears... Problem with one of the lead of the cap inthe wrong via/hole (one of the two on four maybe not connected to zero volt /Gnd, just having its own island for standing the cap as said before (but not returning to the ground of the circuit) ?
Had you a smell and top breaked cap ? Or maybe a short somewhere with your soldering iron when desoldering the former caps ?
But you said you have made many pc refurbishing before, so I doubt about a soldering error but if you're a absolute beginner
Thanks for all reply, include DF96, I am just the guy with a little clumsy hand but still could afford the consequences.
I am ok, except a little regret about my m-80, other stuff destroyed by this accident is worth it, down below is the reason:
1. I feel not so hurt about c-80 destroyed, the reason is I got this for free when I buy my M-80 because of its phono output RCA is break, but I don't have a turntable so...I just grab it for free.
2. I totally not feel so hurt about my Klipsch kg3.2, I bought them from a looks very honest guy on 200 dollars, but by this accident, I found this guy is totally a liar, after the left speaker failed I detach the whole speaker, I found that 3 way speaker just have on tweeter and one mid-range in it, the subwoofer just for looking without any magnet...what a monkey deal..., so I decided to rebuild these speakers in my way
So whatever, I will do these things down below:
1. keep working on that old c-80, the accident actually happened after I recap these two caps behind the power supply because I do the whole recap in two steps, I firstly recap those two behind the heat sink when I connect them to the amp and they are working very well. Then I recap these two caps behind the power supply, then it broken... Therefore, I will replace back these two by the old caps, let us see what happen.
2. rebuild my Klipsch kg3 with new crossover board and speakers, the reason I said, I am cheated by the seller...
I am ok, except a little regret about my m-80, other stuff destroyed by this accident is worth it, down below is the reason:
1. I feel not so hurt about c-80 destroyed, the reason is I got this for free when I buy my M-80 because of its phono output RCA is break, but I don't have a turntable so...I just grab it for free.
2. I totally not feel so hurt about my Klipsch kg3.2, I bought them from a looks very honest guy on 200 dollars, but by this accident, I found this guy is totally a liar, after the left speaker failed I detach the whole speaker, I found that 3 way speaker just have on tweeter and one mid-range in it, the subwoofer just for looking without any magnet...what a monkey deal..., so I decided to rebuild these speakers in my way
So whatever, I will do these things down below:
1. keep working on that old c-80, the accident actually happened after I recap these two caps behind the power supply because I do the whole recap in two steps, I firstly recap those two behind the heat sink when I connect them to the amp and they are working very well. Then I recap these two caps behind the power supply, then it broken... Therefore, I will replace back these two by the old caps, let us see what happen.
2. rebuild my Klipsch kg3 with new crossover board and speakers, the reason I said, I am cheated by the seller...
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Okay, great. I believe investigation # 45.
Measurement before changing parts around is important. What is the DC voltage coming out of the jack of the preamp? What is the AC voltage coming out with the input shorted? What is the rail voltage of the preamp, compared to the rated voltage of the old caps? (should be about 80%). Is there ac voltage on the rail in the preamp? How much? The problem might be something like taking the board out and putting it back, which caused bad connection on a connector and an open input to a high gain point that oscillated.
BTW, buying used speakers. Check them before purchase with a CD player and a recording of a grand piano. Previously go to a concert or church and listen to what one sounds like. At least listen to a recording on headphones, which even $30 models can sound really good. Piano is really hard to do right. Do your speakers sound like a real piano? Beethoven Appasionata Sonata is a good test track, goes from very low notes to very high. You don't have to like the piano, you just have to use difficult material to test speakers for a known result, unless you take an evaluation lab truck around with you on your shopping expeditions. The lack of low notes on Appasionata would have revealed the damaged woofer.
Measurement before changing parts around is important. What is the DC voltage coming out of the jack of the preamp? What is the AC voltage coming out with the input shorted? What is the rail voltage of the preamp, compared to the rated voltage of the old caps? (should be about 80%). Is there ac voltage on the rail in the preamp? How much? The problem might be something like taking the board out and putting it back, which caused bad connection on a connector and an open input to a high gain point that oscillated.
BTW, buying used speakers. Check them before purchase with a CD player and a recording of a grand piano. Previously go to a concert or church and listen to what one sounds like. At least listen to a recording on headphones, which even $30 models can sound really good. Piano is really hard to do right. Do your speakers sound like a real piano? Beethoven Appasionata Sonata is a good test track, goes from very low notes to very high. You don't have to like the piano, you just have to use difficult material to test speakers for a known result, unless you take an evaluation lab truck around with you on your shopping expeditions. The lack of low notes on Appasionata would have revealed the damaged woofer.
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Don't destroy your speakers. I seems clear that those Klipsch are two way speakers with passive element.
Thank you again, like I said I get back the passive speakers from the dumper and install them back, sounds much better than the pair of Rockford Fosgate subwoofer. and I learned so much by this case. But I am curious why two active subwoofers will not work as good as the pair of the passive subwoofer? They should be more powerful right?
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