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Old October 29th, 2011, 12:19 PM   #1 (permalink)
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Why did my caps blow?!

I need some insight and understanding! The other day, I plugged my Acoustic B20 bass amp into my usual outlet. I then plugged a SX bass into input 1, then turned on the power. The blue light came on and immediately there was a weird groan, pop, and then nothing. So, I took the amp apart, spot check showed the slow-blow fuse had blown. (fuse reads: T315 mA 250 V P). Researched internet, found I could get new fuses at Radio Shack. Bought fuses labeled as 3.15AL250V, slow blow. Popped one into the amp, put it back together and turned it on. This time I did not have the bass plugged into the amp. It Worked! But only for about 1 minute, then same noise, pop, and smoke. Well, I took it apart and saw that two capacitors had blown up. (One had cover completely off, paper shredded all over!) Both were listed as 2200uF 25V. The newly installed fuse was ok.

So, can anyone help me understand why this happened? (Besides my ineptitude, no posts regarding this please. ) I have had this solid state amp for about a year and a half, very rarely used or plugged. Thanks for any help in understanding the electronics of this experience.

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Old October 29th, 2011, 12:30 PM   #2 (permalink)
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If they worked in the past, and were never replaced, they might have been no longer able to handle the voltage, or there's a failed resistor somewhere that let too much voltage pass which exceeded the casps voltage rating.

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Old October 29th, 2011, 12:32 PM   #3 (permalink)
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Your fuse was blowing because there is a problem with the caps, not the other way around. If everything operates perfectly, the fuse is just a wire and is not needed. When there is an electrical problem drawing too much current the fuse will go. That it took a minute doesn't really matter. Time to visit the shop.
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Old October 29th, 2011, 12:37 PM   #4 (permalink)
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There is also the possibility that they were installed backwards.

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Old October 29th, 2011, 12:44 PM   #5 (permalink)
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There is also the possibility that they were installed backwards.

Winnie
I assume you are talking about the fuse. I was told at Radio Shack that it did not matter which direction these fuses were installed, polarity or flow did not apply with these fuses. The capacitors are factory installed.
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Old October 29th, 2011, 12:52 PM   #6 (permalink)
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Also, now that the caps are done, can they be replaced? I'd like to home fix this one as much as possible. More for the learning experience than any other reason. Worst case scenario, I would like to convert this amp cab for a possible extension cab for a 5W tube amp.
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Old October 29th, 2011, 03:02 PM   #7 (permalink)
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Get some new caps with the same or close uF capacitance and same voltage rating or higher voltage rating (600 volts is better than 400 volts.)

justradios.com, mouser.com, tubesandmore.com

Solder them in where the old ones were.

Pay close heed to where the + and minus are supposed to go.

IMPORTANT: Do it with the amp unplugged and the power filter caps drained.

google that.

note: caps drain from the positive terminal to ground.

Or they discharge at once thru your body.
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Old October 29th, 2011, 03:08 PM   #8 (permalink)
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Short in the the PS?
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Old October 29th, 2011, 04:13 PM   #9 (permalink)
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Good call Fezz.

If after you replace the bad caps, and the new caps blow, you may find that your problem is a short in the Power Supply.
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Old October 29th, 2011, 07:49 PM   #10 (permalink)
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Just curious - did you replace a 315 mA fuse with a 3.15A fuse?

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Old October 29th, 2011, 11:22 PM   #11 (permalink)
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Just curious - did you replace a 315 mA fuse with a 3.15A fuse?

steven
Yes I did. I figured the 315 "mA" translates to 3.15 "A" fuse. Right?
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Old October 30th, 2011, 02:13 AM   #12 (permalink)
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Yes I did. I figured the 315 "mA" translates to 3.15 "A" fuse. Right?
Not quite. 315mA is .315A. Sounds like Fezz might be right on this one.
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Old October 30th, 2011, 03:41 AM   #13 (permalink)
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A shorted rectifier can blow the caps (places AC on them).

Make sure rectifiers are good before powering up with new caps.
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Old October 30th, 2011, 08:34 AM   #14 (permalink)
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Sounds like the bigger fuse allowed the bad PT (or the bad rectifier circuit) to explode the caps.

Replace all the rectifer diodes and the caps, put in the proper fuse and see if that fixes it.
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Old October 30th, 2011, 10:03 AM   #15 (permalink)
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Caps exploding are almost always a result of heat. If it was violent, the that points to a short in the cap. If it was just swelling the can and leaking, that is usually a cap who's ESR has risen to the point it over heats from the AC current through it
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Old October 31st, 2011, 12:41 AM   #16 (permalink)
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glfklee,

If you replaced a fuse in the amplifier with one that is 10X bigger AND a slo-blow to boot, a shorted rectifier would provide adequate AC to blow the tops off your caps. It's unlikely that both caps failed of old age at the same instant after 1 minute of power being applied to them. My guess is that one or more the rectifier diodes are shorted.

BTW, a milliampere is 0.001 Amps, so 315 mA is 0.315 Amps as mentioned above. Bussman-Cooper does make 315 mA fuses in both slo-blow and fast-acting models. Slo-blows usually have a springy-lookin' thingie attached to the fuse element.

If the caps are originals, replacing them with 25 WVDC or a little higher (working voltage DC) should be fine. Don't try to find 2200uF at 600 VDC...you'll have heart failure at the price and it's not necessary on little solid state amps. You probably couldn't fit 'em in the cabinet, either.

Hopefully, you can locate and replace the rectifier diodes or bridge. If you feel uncertain on this, take the amp to someone who can help you and explain it a bit. There are amp and volt ratings on these devices to consider, too. Unlike the fuse, going too big won't hurt...but selecting parts that are too small will blow things up again. It sounds like 200 V, 1A diodes would be sufficient. 2 Amp diodes couldn't hurt.

Good luck,
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Old October 31st, 2011, 12:42 AM   #17 (permalink)
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Here are some pics from the main board showing the blown caps.
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Old October 31st, 2011, 12:49 AM   #18 (permalink)
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Kabloooeeee.

The diodes are 1 amp units from the looks of 'em.
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Old October 31st, 2011, 01:01 AM   #19 (permalink)
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So, my previous post shows the main board. Pics are aligned left to right. Thanks for all the helpful comments.

1. So, what I understand right now is that the if everything is running properly, the fuse size would not matter. Looks like I put in the wrong size, so I need to get a slow blow with proper mA. There was a T in front of 315mA in original fuse, I understand the "T" means slow-blow.

2. Main culprit might be a Power supply short, although the other front runner is a shorted rectifier.

3. I need to find a helpful tech person locally to explain more of this to me. This I will do! Especially with the warnings of power filter caps! I needed only to see a few YouTube videos to convince me I need help!

4. In the meantime, if anyone cares to direct me to the rectifiers, power filter caps on the circuit board pics above, you are welcomed to give me some input. I'll post some pics of the power supply and fuses.

Thanks again everyone for your help. Between these posts and my research, mainly thru internet and books, I am learning some cool stuff.
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Old October 31st, 2011, 01:17 AM   #20 (permalink)
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These photos show the power supply, the board with new (wrong) fuse, and the original fuse (shows a springy wire).
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