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Thread: Recapping Pet analog board?

  1. #1
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    Default Recapping Pet analog board?

    I'm recapping my analog board as a suggestion from someone at VCF. Should I just replace all the electrolytic with equivalent ones or are there any other types of caps that I should look out for and replace?
    Wanted: Any old clunky 286-P1 machine that has some kind of working battery or replaceable with off the shelf parts. Preferred: 10+lbs 386 machines.

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    First order of business, electrolytics.
    Then tantalums, if they're failing.

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    Just out of curiosity, what's the problem that prompted the suggestion of recapping? Might narrow down where to focus.
    -- Brian

    Systems: Amstad PCW 8256, Apple IIe/II+/Mac+/Mac 512k, Atari 800/520STFM, Commodore 64/Amiga 3000/PET 4032/SX-64, IBM PS/1 2121-B82, Kaypro II, Osborne 1, Tandy 1000 SX, TI-99/4A, TRS-80 Model 4 GA

  4. #4

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    Quote Originally Posted by twistedpneumatic View Post
    I'm recapping my analog board as a suggestion from someone at VCF. Should I just replace all the electrolytic with equivalent ones or are there any other types of caps that I should look out for and replace?
    What was the actual fault? I've never "recapped" anything unless the actual capacitors are bad (high ESR, leakage, etc) OR they're the RIFA with cracked epoxy (or about to crack anyway).

    Frank IZ8DWF

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    First of all, the machine doesn't work. It made a pattern of a small dot that would bob up and down slowly then a large raster flash. I showed it to someone at VCF and they said recap the analog board. I figured it was worth a shot. I don't have an ESR meter so I'll probably just recap everything I see.
    Wanted: Any old clunky 286-P1 machine that has some kind of working battery or replaceable with off the shelf parts. Preferred: 10+lbs 386 machines.

  6. #6

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    Massive recaps usually do not pay out IMHO.

    I think I was never able to fix anything by doing recaps. I tried a few times, then I realized it was wasted time & money: unless you know a cap is failing, you're basically replacing good old caps with good new caps.

    That being said, if you can't make a proper failure diagnosis and if you're confident you won't add new issues (damage PCB tracks, solder caps with inverted polarity) then it may be worth a try... still it's a longshot.

  7. #7

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    Quote Originally Posted by spaghetticode View Post
    Massive recaps usually do not pay out IMHO.

    I think I was never able to fix anything by doing recaps. I tried a few times, then I realized it was wasted time & money: unless you know a cap is failing, you're basically replacing good old caps with good new caps.

    That being said, if you can't make a proper failure diagnosis and if you're confident you won't add new issues (damage PCB tracks, solder caps with inverted polarity) then it may be worth a try... still it's a longshot.
    This is my thoughts as well. I didn't want to keep saying the same thing over and over. It is true that tantalum caps are a common failure point. Electrolytics, used as active components in circuits and not as bypass, are a common source of failure. Electrolytics that fail as bypass usually have visible failures.
    Still, unless the person is an experienced repair person with excellent rework soldering skills, the chance of doing damage with a soldering iron are greater than one might think. Unsoldering takes different skills than soldering components in.
    There are different parts of the circuit. Replacing parts that have nothing to do with the failure are surly a waste of time and money. It is like your car radio is acting up so you replace the gas cap, the spark plugs and the thermostat because these are common failure points on a car but have nothing to do with the radio.
    There is always someone that has had success with blanket replacement.
    I agree with removing old RIFA caps. They will make a smelly failure. They do show visual potential failure if you look at them for cracks in their cases. Still, I doubt one of these is the cause of your failure. Flashes are often caused by dust or cigarette smoke deposits in the high voltage circuits. It can also be caused by a bad picture tube, with cathode to filament arcing.
    Dwight

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    My view on this is that the OP (TP) is bent on replacing electrolytics no matter what anyone says and wanted advice on other capacitors.

    So if he was going to do it anyway, what's the point in saying anything other than "yes, it's true that the bulk of capacitor failures involve electrolytics". I think I've seen a grand total of 2 ceramic discs fail, for instance, in all my years. of dinking with this stuff. I don't think I've ever seen an oil or silvered mica cap fail--ever.

    Electrolytics in power circuits that involve self-heating (e.g. filter caps, motor start/run caps, etc.) do fail quite often.

  9. #9

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    Exposed silvered mica capacitors fail quite often. The example is the capacitors used in IF transformers made in the 50's to 60's. The silver forms oxide ( or maybe sulfate ) that shorts sections of these caps. I've cleaned them on several radios to bring them back to life.
    The only ceramic caps that I've had issues with are the glass encapsulated ones used in military equipment. Still the ceramic disk with arc slots cut in them can fail as a result of dust and cigarette smoke deposits. Other than that, the only ceramics I've seen fail were mechanically damaged.
    Tantalums and electrolytics used in timing circuits can have issues. Most places that have high capacitive AC currents, as Chuck mentions, are common failures location ( LCD screen power supplies come to mind ). Old oil/paper capacitors are often bad ( not usually found in modern computers ) are know to fail.
    Tantulums used as power supply filters often fail when first powered up after having been stored for some time, although, I've had one fail on a disk drive that had been on continuously for more than a year. Since these short out power rails and stay shorted, they are not likely to be the cause of the OPs problem.
    If you think a bypass electrolytic capacitor has a high ESR failure problem, it is easier to tack a capacitor in parallel with the capacitor to see if the failure goes away than to replace it first. If beefing it up with a paralleled one makes the unit function, replacing that capacitor makes sense. Do note, this only works for ESR issues with bypass capacitors, not shorted, coupling or timing capacitors.
    Dwight

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    The most common electrolytic failure mode that I encounter is with drying out due to heating. It's not at all unusual to measure a bad filter cap and find that it's little more than an empty can. Shorts do occur, but less frequently than the former case. Ether case can usually be tested for without removing the old cap.

    If you need a bit of equipment to test components and are on a tight budget, I recommend the LCR-T1 for less than $20 shipped. The probes are garbage, but the unit itself is remarkably accurate and will test a wide variety of components. There are less-expensive versions of the same unit, since the MCU source is open, it's more a matter of cosmetics.

    I keep one on my bench for quick tests.
    Last edited by Chuck(G); June 19th, 2019 at 07:36 AM.

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