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Old 15th September 2007, 04:41 PM   #1
spooney is offline spooney  United States
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Default Capacitor replacement question

I've got a friends punch 200ix that i'm doing some simple repairs on.There is (or i should say was) three 330 microfarad capacitors directly next to the transformer near the power inputs that have broken off of their leads. I have some higher voltage rated 1000 microfarad caps laying around that I could use to replace these but i'm wondering if there would be any problems with using these caps in this situatuion.The caps are in parallel with the battery positive and negative if that helps at all.I can also provide a schematic of the amp if necessary.The schematic specifies a 330 microfarad 25 volt capacitor for that spot and the caps I have are 35 volt 1000 microfarad.
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Old 15th September 2007, 05:06 PM   #2
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That's an excellent choice. Higher voltage means higher ripple current capability, uF for uF. Since these are at the input, your ripple will actually be low, based on the capability of your battery and charging system.

105C caps are best, IMO, as are high frequency electrolytics designed for switch mode PS, but again, in this location, you should be fine.

Were you on the other side of the switch mode transformer, you have to be a little more careful about how much C you put on the output. Going up in voltage is never a problem, as long as it fits.
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Old 15th September 2007, 05:47 PM   #3
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I change 12 volt side caps all the time, especially on certain brands of amps. Those caps are there to filter hash and stop noise, There value is small so as to provide noise suppression only. Not energy storage as one might think. On some brands they build huge banks of low ESR caps in a attempt to store energy, but most people use 1 farad caps or larger for that nowadays.

They begin to leak over the years and the chemicals cause the circuit board metals to dissolve, and result in board damage.

I generally upgrade the caps myself using low to ultra low ESR, high ripple current type caps. Increasing the voltage is a good idea. Usually from 16 volts to at least 25 volts. I generally increase capacity from 2200 uf < as a example> to 6800 uf as modern capacitors have been improved over the years so that such a improvement takes no more space then the old value did.

I stock the 330 uf caps your talking about because I have seen so many of them explode on RF amps over the years, especially when the power fets get toasted by the owner. I have also seen vibration break them off at there leads like you are viewing currently.

On amps where they fail due to age and leakage I try to improve the design a bit by upgrading them as possible to prevent board damage due to leakage.

I do a lot of collectible amp restorations where the client wants his product maintained to a higher level of quality then as built. Its a fairly simple upgrade, and my clients feel better knowing they got the best money could buy as standard at my shop.

Your upgrade will not cause any harm, but I do suggest you use the best caps you can find for this service as it is hard service in this location with electrical noise coming from both directions. Oh and always use 105 degree C rated caps for car amp, they get oh so hot in this sort of service, and you get what you pay for quality wise I have found in life.
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Old 15th September 2007, 07:23 PM   #4
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When you reinstall the amplifier, run the amp hard for ~5 minutes while monitoring the temperature of the capacitors. If the capacitors get too hot to hold your hand on comfortably, you may have a problem with reliability. If the caps fail, the power supply will fall again.

If you decide to buy replacements for this (or other repairs), use FC series caps from Digikey. I used their HFS and HFQ caps until they were replaced by the FC and all have been very reliable in this application.

If you want to improve reliability, use the same capacitance with the highest voltage available.

You definitely want to use 105C caps.
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