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Recapping a PSU -choosing the right replacement caps

Discussion in 'Troubleshooting and Repair' started by TTL, Jul 3, 2017.

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  1. TTL

    TTL

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    Oct 24, 2013
    I've decided to replace all the capacitors inside a 30+ year old computer's switched mode power supply, and I've understood that it's not simply a matter of getting the same uF values and same (or higher) voltage ratings and of course ensuring that their physical dimensions will fit, but what exactly should I be looking for in the specs?

    Is it "low ESR" (low internal resistance) I'm after, and a high temperature rating?
    I'll be ordering from Farnell, and among their reputable brands they have capacitors from Panasonic, but which series should I go for, for this type of application?
     
  2. kellys_eye

    kellys_eye

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    Low ESR and (typically) a temperature rating of 105C is what to aim for.

    Ichicon are a good 'name' but if purchasing from a reputable stockist you should be getting a decent product anyway - certainly Panasonic should fit the bill.

    Have you checked to see if replacement NOS (New Old Stock) power supplies are available?
     
  3. Cannonball

    Cannonball

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    May 6, 2017
    If it isn't broke don't fix it.
     
  4. Arouse1973

    Arouse1973 Adam

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    Are the capacitors faulty?
    Adam
     
  5. kellys_eye

    kellys_eye

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    After 30 years I'd be changing them as a matter of 'good practise' regardless! Dried-up electrolytics are now a major cause of equipment problems/failure.

    I've come across this issue many, many times and would be doing what the OP is suggesting without batting an eyelid.
     
  6. TTL

    TTL

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    Exactly!
    I've been warned about how turning a blind eye to aging capacitors can cause irrepairable damage to other parts of the circuitry so I'd better be safe than sorry. The PSU is working fine as far as I can tell (I've checked its voltage which is all within specs) but I'm no expert and believe I might have to test for unstable power, ripple and such. Not quite sure how to do that (are there any good and easy to understand guides available on that?).
    Desoldering and replacing capacitors is something I can do, and an easy fix against a ticking timebomb.

    No, I don't know of any NOS replacement PSU for this computer, but I've already opened it up to take notes of capacitor values and physical dimensions, so it's just a matter of buying the right replacements. After posting here I heard someone suggest Panasonic's FR-series. Does that sound right?

    Kellys_eye: are you sure you didn't mean Nichicon (not Ichicon)? That for sure is a well known capacitor brand, but last time I checked I couldn't find a lot of the capacitance values I needed, which is why I looked towards Panasonic. But Farnell do sell Nichicon caps. Which series from them are best suited for power supply applications such as this?
     
  7. kellys_eye

    kellys_eye

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    Yes 'N'ichicon is what I meant lol.

    The values aren't as critical as you'd think. Many electrolytics have a basic tolerance of +10/-50% (or some such ridiculous span) so the 'next nearest' value is often a simpler way to go.

    Only other advice I can recommend is to check the capacitors on the motherboard (many have onboard DC level shifting circuitry) and replace there too if necessary. Obviously this is more involved due to the multi-layer properties of the PCB so appropriate care needs to be exercised.
     
  8. TTL

    TTL

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    Oct 24, 2013
    Fortunately I got myself a cheap (but very nice) desoldering iron (a 40W soldering iron with a hollow tip and an integrated desoldering pump) which does wonders.
    Yes, I thought about replacing the motherboard caps as well -these aren't as critical as the ones in the power supply, are they? I mean, can I use just about any (quality brand of course) capacitor as replacements there?

    By asking for the Nichicon series of choice for recapping power supplies I meant to ask which series are the low ESR/high ripple capable ones to go for? As I understand it I can't just replace them with any type capacitor just by looking at their values.
     
  9. kellys_eye

    kellys_eye

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    The series you want will be the one that physically matches the mounting requirement - presumably the radial ones. Low ESR and the 105C spec are all that you need to know along with the value/voltage but, as mentioned, those aren't entirely critical to the task, especially in a simple SMPS of that age.

    In modern motherboards the onboard voltage level switching circuitry uses 'quality' capacitors in SMPS's to derive the 'silly' 1.8V (etc) supplies so would need care in selection however I think your motherboard age may preclude there being such - only you can tell! Either way, considering the fairly minimal cost difference between the low ESR/105 capacitors and standard 85C versions there's no real point in NOT using the better quality ones. I think keeping 'old' mobo's working is worth the effort.
     
  10. TTL

    TTL

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    No, there's no "exotic" voltages on the 30 year old motherboard and the PSU which outputs +12V, -12V, +5V and -5V, and it's probably of a simpler design than the ones of today as you say.

    I've had problems finding low ESR 400V capacitors (which there are two of in the PSU), but found out from another thread that those components are used in the input stage of the SMPS, so it's not affected by the high frequency/ripple which I understand the rest (past the transformer) of the PSU has to deal with. Knowing that makes things easier (without thinking I assumed ALL electrolytic capacitors in an SMPS had to be low ESR types)!
    The low ESR caps I'm getting are all Panasonic FR-series so I should be good to go :)

    Regarding electrolytics on the motherboard (there are a couple near the power supply connector and also a few smaller ones here and there). Should I ideally look for low ESR types there as well, or just "regular" ones (but long lasting and with a 105C temperature rating if possible)? I agree that getting the higher temperature ones (and longer hour ratings) are worth it while I'm at it, instead of having to replace them again in the not too distant future. Another thing is that if I use the same life expectancy caps for the entire computer/PSU while recapping it's just a matter of recapping them all again if/when needed and not trying to figure out which caps are the oldest.
     
    Last edited: Jul 6, 2017
  11. kellys_eye

    kellys_eye

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    Out of sheer nosiness :D what make/model PC (or motherboard) are you servicing?

    30 years seems to point to a 386 or 486 (maybe Pentium)? Do you have an application for the PC or is this a nostalgia trip?
     
  12. TTL

    TTL

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    It's an Atari ST.
    I still use it for MIDI (music making) purposes where it still works great :)
     
  13. shiekh

    shiekh

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    Oct 11, 2010

    Mother board caps need to be as good as if not better than those on the power supply.
     
  14. davenn

    davenn Moderator

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    You also need to make sure they are the high temperature ones 105 deg C

    ESP is a consideration but not as much as the 3 things above


    they were a great computer MC68000 micro. I owned and serviced many for people from my home based workshop
    during the late '80's and through the early to mid 1990's

    is your one the 520 or the 1040 ?

    I stacked ram chips on many 520's for people to bring them up to 1040 (1GB) of memory

    Dave
     
  15. shiekh

    shiekh

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    Oct 11, 2010
    ESR (equivalent series resistance) can be quite a factor when the ripple current is high, as heating can be a big issue.
     
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