Fail Of The Week: Always Check The Fuse

[Tomas] at Umeå Hackerspace in Sweden had some broken audio equipment, including a Sharp CD player/amplifier. What went wrong when he tried to fix it is a fail story from which we can all learn.

The device worked – for about a second after being turned on, before turning itself off. That’s a hopeful sign, time to start debugging. He took the small-signal and logic boards out of the circuit, leaving only power supply and amplifier, and applied the juice.

Magic blue smoke ensued, coming from the amplifier. Lacking a suitable replacement part, that was it for the Sharp.

On closer inspection it emerged that the previous owner had bypassed the power supply fuse with a piece of copper wire, Evidently they had found the fuse to be blowing too often and instead of trying to fix the problem simply shot the messenger.

We have all probably done it at some time or other. In the absence of a replacement fuse we may have guestimated the number of single strands required to take the current, or used a thin strip of foil wrapped around the fuse body. And we’ll all have laughed at that meme about using a spanner or a live round as a fuse.

So if there’s a moral to this story, it’s to always assume that everyone else is as capable as you are of doing such a dodgy fix, and to always check the fuse.

2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which celebrates failure as a learning tool. Help keep the fun rolling by writing about your own failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

72 thoughts on “Fail Of The Week: Always Check The Fuse

      1. Actually, it’s “n**ger rigged” and “ghetto engineered.” There are a few other, even more highly offensive, terms I could put up, but I don’t want my IP blocked. ;-)

          1. It’s not encryption. It’s called “filtering.” Many of us use filtering in an attempt to keep discussions moderately civil or are just uncomfortable using the full word. If you fault people for their attempts to contribute to polite discussion, then you, sir or madam, have a problem.

      2. As uncle would say Afro-American engineered. To be fair carelessness, ignorance and stupidity are all are equal opportunity; affecting people regardless of the creed, ethnicity, gender,, race, sexual orientation. I keep with saying engineered by a dumb shit.

      1. The etymology of that is a bit disputed, but a likely origin is “jury rigged”, as in replacing the broken main mast of a ship with one of the other masts, or holding it together with rope. Comes from “jour”, as in “day”, as in a fix that’s not intended to last long.

    1. I learned this the hard way with my generator some previous owner foil wrapped the fuse which in turn lead me to blow the 250vac capacitor on the inside of the power panel. Luckily it didn’t meltdown any wiring going to the field windings on the stator or the stator itself. Actually I’m hoping it didn’t I haven’t tested it yet since I replaced the capacitor with one I salvaged (and hope works) from an old washer or stove that was sitting out back of the repair place on my town. I go there often and strip out copper wiring plus copper wound motors etc… And any circuit boards with lots of relays which I then breakdown further for the gold and silver contact buttons. I also keep some of the other components that don’t on the surface look like they have shit the bed leaving a brown stain on the board a sure way to tell if they released all their magic blue smoke and are now truly dickered. Anyway as boltr would say keep your dick in a vice.

  1. Fuses suffer metal fatigue from thermal cycles and worse if their operating current is high. Even if its not, the issue of surge re large caps of worse a large toroid in the load can exacerbate thermally induced metal fatigue add to that the availability of cheap fuses not terminated properly to the end caps – and the other extremes way over-rated for the application. The adage,
    “Transistors are a great new thing in modern electronics, they are so very good at protecting fuses from premature failure” comes to mind. In this day and age can we feel safe if the fuse is replaced with a PTC resistor, for low level voltages and currents sure (with caveats) but, for anything approaching chance of shock especially DC definitely Not – option to have in place a non easily resetable physical barrier for current flow even by air/vacuum gap puts me and those I care for at ease..

    1. Designs for circuits with high surge currents usually specify slo-blo fuses. Usually. Sometimes, however, production differs from engineering because it works for a while and costs “that much” less.

      1. Pfft, really (?) why do that (?) because:-
        1. Its unethical
        2. How would you feel//respond if it was done to you ?
        3. These days much easier to be found out, how would that make you feel ?
        4. Given 3, will your professional indemnity insurance cover you, are you even insured ?
        5. If ‘someone’ was injured or even died, how would that make you feel ?
        6. If that ‘someone’ had a relative you pined for, could you handle the loss of self-worth ?
        7. Given 3, could you handle being sued for negligence ?
        8. Maintains your level of awareness re potential consequences of your actions in the stone age
        9. In either of the permutations considered would anyone ever trust you for advice ?

        Who do you work for, which location please, give us your GPS coordinates as I could do with
        a test firing of my prototype ?

        ie Pffft

  2. Sometimes in circuits putting in a short over the fuse link allows you to positively identify the faulty component. This method is not without risk, but it can be used gently when you know it will work.

    1. A better solution is to use an incandescent light bulb as a current limiter. Audio repair geeks use this to test gear that may have a destructive short somewhere in the circuit. In some circuits like a SMPS it may not allow the PSU to turn on without some additional hacking but in normal PSUs it usually works. The only definite exception is with an esoteric power transistor called the VFET.

      Repairing gear that has been mucked with is like unscrambling an egg. If the screws on a device aren’t factory tight I ALWAYS do a complete inspection before turning it on. A Variac (c) is handy for slowly bringing a device up to full power.

      1. Just last year I have built a tester with a light bulb, loosely based on 1950-ies Pop. Electronics article. Worked fine then, it works fine now. Some things just can’t be beat for simplicity and usefulness.

        1. crap, hit report instead of reply, sorry! (seriously HaD, I’m not the first to do that…that button warrants a revisit)
          It has 3 incandescent light bulbs that can be switched in for current limiting. It’s a variac, so there’s no isolation, but the bulbs have nothing to do with that.

      1. The problem is not so much killing components but destroying the PCB, which can happen when your SMD components burn out.. Using a thermal imaging camera however is a great way to find out which components are malfunctioning because you can power up the device for a few seconds and shut it off without letting it go up in smoke and check for hot areas visually.

        1. Hmm, well this might well work some of the time with many passives but, much less likely to work with active components as a ‘tried and true procedure” if you care about the value of the module, its replacement cost and economics of time value in competition with other projects since:-
          1. They were previously stressed
          2. Re 1 can then fail with negligible heat signature and often without external marks
          3. Re 2 Unless you have good working knowledge of that particular functional implementation, its inefficient and can result in more damage removing/replacing to attempt to pin down the faulty part
          4. Re 3 It opens up the potential for cascade damage of all sorts resulting in more faults
          5. Re 4 Can stress other components more reducing MTBF & increase subsequent MTTR

          In the sorts of labs I’ve worked in, if you did that, the board would be thrown out pronto and the lab manager/CTO at the least will offer you “means to advance your education” in risk assessment and “best practice” and at worst given your arching orders. In between those potentials any workmates might well review their bonus structure if they want to work with you in near future :/
          Best example of that which can have really bad consequences I’ve experienced first hand is industrial control where a 500 tonne press-brake needs to be managed by a depth controller running on an newly implemented micro-controllers Eg NSC800 (with all its static susceptibilities) the “techs” got short shift even trying anything so lame and especially so if any equipment was essential to be reliable re safety eg IR access curtains not least of which $50K tool damage or worst slicing a hand off…

          In terms of going part way to address your point in a positive direction however, the use of appropriate series lamps (to suit load variance) in conjunction with an isolating transformer and proper neutral (secondary) side grounding is by far the safest if working on live modules along with the precaution put strongly “with one hand always in ones pocket” much misunderstand by new (often female) receptionists/office assistants passing by the lab corridors or if past an open lab area, roped off, of course, I mean the area not the staff, despite value in considering that ;-)

        2. @Mike Massen
          I have repaired a lot of circuit boards with varying complexity and cost though none admittedly which were safety critical so that might explain our difference in methodology and vision.

          You address component stress as an issue and cascading damage as a potential result; one cannot typically determine the reaching effects of cascading damage so do you replace everything which is not galvanically isolated or where do you draw the line? and when do you draw the line?

          If you do not replace everything, how do you determine which active device is malfunctioning without powering up the part and if you do resort to measuring resistance (which is a poor way of testing the functionality of complex active components) across the PCB how can you determine which of the many devices hanging on your power rail is misbehaving when they are all parallel?

          If you do power up the device to take measurements do you really think your faster with a DMM / scope then someone with a thermal imaging camera? (in the case of higher than standard current usage that is)

          If you decide that removing components one by one to test them seperately is a good idea, good luck building test rigs for every component (and being paranoid that cascade damage will get the test rigs)

          You state that in the labs you’ve worked in the board would be thrown out pronto but the truth of the fact is it was already broken and switching the device on for max 3 seconds should not make the difference between a board that should be thrown out or fixed.. If that is the type of mentality in the places you’ve worked I’m glad I don’t work there. ;’)

          Furthermore how do you determine that the device was functioning in the first place when you sent it out? Either you’ll say testing or that you don’t know. If it’s the first case then the repaired board could also be tested and if it’s the latter case then why is it a bigger problem that the repaired board is in an ‘uncertain’ state as opposed to a working board?

          This discussion was not about safety critical devices such as a 500 tonne press and you trying to render my argument obsolete and wrong is invalid and your “means to advance your education” was uncalled for.

  3. Hah, not the thing I expect to make it to Hackaday with but oh well. A bit of a post-mortem: most likely the logic board was able to sense the overcurrent condition and shut the unit off, hence the one-second delay. So under normal circumstances either that logic would kick in or the fuse would blow. Of course if you remove the logic and have a copper wire for a fuse then something else is going to give..

    In the end we stripped the Sharp for useful parts like connectors, inductors, capacitors, transformers, motors and so on.

  4. I have had this happen to me. I got a load of PS3 units from a place going out of business and went to work fixing them. One of them that I took apart still had the warranty stickers on it, apparently unmolested. Upon taking the thing apart, there was “something” not quite right. I couldn’t tell what it was until I looked at the power supply. It was obvious someone had already been there because of the marks from a soldering iron that was WAY too hot for the board (scorch marks where the leads had been touched up with the traces lifting at those spots). It would turn on but about 5 seconds later shut off. All the readings were okay–fuse had continuity, voltage was fine, etc. Then, I noticed the power supply board number didn’t match that particular model or the case it was in (was about 50W short). Upon further inspection, I found out that the previous “technician” had put a small piece of wire under the fuse holder between the posts, obviously in an effort to raise the output current.

  5. When a vacuum tube guitar amp comes in for service, the *first* thing I check is the fuse. About 30% of the time it is the wrong value and when it is wrong it is always HIGHER. Had a Fender Champ come in with a 7A fuse – normal rating is 1A!

      1. That’s spooky but in reality 5A probably isn’t going to cause any harm other than the wire’s temperature rise being a little higher than spec, while still being safely lower than the insulation’s rated temperature.

      2. There were a lot of scorched outlets in an older house my dad bought. There was a complete rewire from AL to CU with new outlets and of course the contractor cut a LOT of corners. It turns out that those 29 cent outlets don’t don’t last long under heavy load.

        I was told a few years ago that for an average house as long as the wiring is 12 GA or higher, a 20A breaker is sufficient protection for the wiring because the outlets are designed for 25A short-term loads even though they are rated for 15A . Apparently (and confirmed by another electrician), they use 20A breakers because when you’re running more than one outlet, say a hair dryer and a ceiling fan, a 15A breaker will pop in no time. I’m not sure if he was pulling my leg, though. I went with another electrician because of some other things that he said, like “just slather the AL wire with Anti-Ox and use a regular outlet” after I called about rewiring everything.

  6. I must add that there is a persistent urban myth about one guy in former Yugoslavia (during the civil war) using a 12.7 mm round as a metal punch. He was using a hammer with it. Until one day …

    1. Decades ago, the dome light door switch in my Uncle’s pickup broke off, leaving the dome light on constantly and draining the battery. The broken piece about the size of a .22 caliber rimfire bullet…

  7. note: a fuse does not blow for no reason… check the fuse but then check other things before just putting in a new fuse. The car repair boards are full of fools that do the following…

    “Car wont start, checked a fuse and it was blown, replaced it and it blew right away”
    “Try replacing the 10amp fuse with a 20 amp one”
    “20 amp worked, but then something started smoking and now the car wont start at all”

    Fuses are canaries that tell you something is very wrong. dont just “replace” them and turn the device back on.

    1. Some shady mechanics in Detroit did that to my dad.
      They replaced the ECM and put 30 amp in the ECM 1 location on his truck and it broke down on him again fifty miles from home.
      After driving up I take a look at it and noticed that the wiring on one of the fuel injector connectors was obviously damaged and was shorting.
      The a** clowns who worked on it didn’t even think to check for shorts before replacing an expensive part.
      I end up fixing it by replacing two $15 connectors.
      Of course the old ECM turned out to still be good.

      1. Very true. But perhaps the following link from my former employer will help:

        On a serious note, lexicographers nowadays are descriptive, as in documenting how the language is used, rather than prescriptive, as in telling you how you should use the language. Samuel Johnson and Noah Webster were prescriptive, their successors at Merriam-Webster and OUP are descriptive. This word will have found its way into the dictionary following statistical evidence of its significant use in current English. There’s no such thing as “Not a real word”, merely “Not a word in common usage”.

        1. For English this is true. Other languages, like French, have official bodies that arbitrarily deside what is and what isn’t a word, and what is proper usage and grammar.

          1. English could use one of those! With the descriptive system that [Jenny List] mentions in place and the “anything goes” philosophy running rampantly amok, English is booked one-way on a crazy train.

          2. I don’t know. I am a francophone, langue maternelle, so I see both sides of this and despite the efforts of L’Académie française, french, even in France, is still full of loan-words. In the end, however a language is a living thing, and as such needs to have room to develop and grow, or it will die and part of the power of English is that does seem to continually simplify its grammar, and expand its vocabulary to suit the needs of its speakers, and one of the reasons it is one of the dominant languages of the world.

          3. [DV82XL] inre: dead languages…

            After John Paul the Second became Pope, the Vatican authority on Latin sent him a letter that he needed to be signing his Latin name with an “I” instead of a “J”. The letter continued, “there is no J in Latin” JPII wrote back, “There is now!”


      1. rarely. usually there’s either enough room, or a blast proof enclosure around it, or something. also, i dont just willy nilly replace fuses with solder. I never used this solution long term enough to test it. it is meant to be a testing thing.

        sometimes, if the fuses allow i simply open the ceramic or glass tube and put a small solder wire instead of the wire originally inside.

  8. Fuses are sometimes affected by alien mind control rays, and can be fixed by wrapping them in aluminum foil.

    When I was younger, dumber, and broker, I foiled my share of fuses. Slightly better than half of the time, it resulted in being able to continue to use whatever it was – sometimes for many years. Else it saved me the cost of a fuse on something which was destined for the trash anyway. Properly sized fuses weren’t always readily available or cheap at the time.

  9. I remember a 12V halogen light system I fixed some time ago (did not work at all). Someone put in 50W bulbs instead of 20W ones and then wrapped the blown fuse in bubblegum foil (with the paper side out = no contact). Sometimes it is hard to do something wrong the right way. Got me a nice torodial transformer >:D

  10. I know of a power supply in the control system of transit vehicle in a large American city where the “Fuse” is a trace on the circuit board. The manual for this power supply even specifies the size of solid copper wire to use as a repair replacement.

    I guess that’s the way they rolled in the early 80’s

      1. There are plated through holes on the board for replacement with a “0.4 mm diameter copper jumper”, so about 26 AWG. I’d be more concerned that it wouldn’t open at the right current for all possible choices of wire. It’s supposed to open at around 20 amps.

        The design is from about 1968, it’s dense through-hole with additional modules up on edge that have SMT parts on them.

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