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Tell stories about annoying faults in either repair or protyping/etc in electronics!

Discussion in 'Members Lounge' started by fezder, Jun 27, 2016.

  1. fezder

    fezder Well-Known Member

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    Got idea of creating this thread, as I encountered annoying fault. Circuit was simple; arduino sends i2c data to TEA5767.
    It wasn't obvioulsy working. I started with simple blink sketch, and it worked, so atmega wans't dead. Next, using serial monitor, I checked that A5/A4 pins read. A5 was constantly held high, and when I pulled it to gnd, arduino reset. WTF. A4 in other hand, worked like charm.

    The issue was just very TINY strip of foil left behind between A5 and reset pin. Well, reset was held high from start via pull-up resistor, so that wasn't problem.
    Issue was that due that tiny strip of foil, when reading via A5, reset was pulled to ground. And, when i2c clock started (A5), this clock pulled reset to gnd too!
    And, what was more annoying, multimeter wasn't much help since this was so annoying fault. And thanks to 10k pull-up what reset was using, it wasn't direct short.
    If you look pinout of atmega328, you see that reset and A5 are opposite sides of chip.
    Phew!
     
  2. JoeJester

    JoeJester Active Member

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    You make the case for using pull up resistors.
     
    Last edited: Jun 27, 2016
  3. fezder

    fezder Well-Known Member

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    Yeah, thing is, pull-up for reset is needed, unless tying it to vcc but then reset would need power failure since pulling reset to GND would cause short.
     
  4. dave

    Dave New Member

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  5. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Stories?

    I could write an encyclopedia set on the odd stuff I have came across in my life time while doing day to day design/redesign or repair work. :p
     
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  6. fezder

    fezder Well-Known Member

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    Hehe, goog thing is that mostly these teach when happen, 'specially if component that goes "boom" is expensive. Luckily, most expensive component I've killed so far was arduino UNO, which was scenario when I killed a wire (wire smelled much worse) when 'scope ground loop occured....*zap*
     
  7. killivolt

    killivolt Well-Known Member

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    I know I've told this story a while back in another "Thread" but I'll tell it again.

    It's how I got my nickname, Killovolt. The name was taken when I came to the site so, I just changed it to Killivolt.

    Back when I was working in the TV repair shop, we had a new "Technician" he was in the EE program at the local University his first day and I had a five disk Kenwood CD player on the bench, now the problem is we couldn't figure out why it wouldn't read the disk and nothing anywhere and no one had found the problem in the country or other shops, no documentation from the manufacture or elsewhere on the net. So, we decided to purchase a new laser.

    I have the laser ready to install when suddenly the new Tech walks up and ask's me what I was doing, so I explained why and what I was going to install.

    He paused for a moment and then asked me, I've never seen a laser before where is it? I said, the laser is in the back left corner. He ask's well how does it work, so I plug it in and load a CD to demonstrate the operation and fault. He watched as it tried to read each tray and then it came to the CD and passed it and I said, you see the laser is at fault and the laser's eye has no movement. But, then he ask's well where exactly is the laser? and I said, that round thing in the middle, and again he said where I don't see it?

    Now, I haven't put my Ground strap on my arm. The shoes I'm wearing I have to say, I shouldn't have worn but, I decided to point about three inches away from the lasers eye and said that is the eye of the laser and it's not moving like it should. He said, exactly where? so I moved my finger a little closer when suddenly a Blue Arc Jumps between my index finger and the lasers eye, literally shocked by what just happened and in fear I hurried and unplugged the unit I turned around to the window just behind me to look at my boss who was sitting working on a camera, he turns and looks at me and turns back to the camera, I'm thinking shiz, shiz what do I do now. I'm scared, this machine retails at $500+ still under warranty 3 months old.

    The new "Tech" is now looking at me and I at him and both of us with blank stare at the machine in wonder.

    To late now:nailbiting:

    I thought? plug it back in to see the damage I caused, the CD still in the tray it lit up and was working, I just pass my first hurdle so, I ejected the tray whew, well that works so I reloaded to begin a new operation. It now begins searching each tray and I'm baffled beyond belief and exhilarated at the same moment, I'm thinking "Your kidding me" as it reaches the Disk my hopes and fears relieved as it Display's the tracks. I explained what happened to my boss and we moved it to the Test Bench and ran it for a Month without any other issues.

    We sent the laser back, and charged the customer a shop fee and cost of restocking of the laser. It never returned to the shop 10 years later, we closed the store. I went to work at a shooting range company and worked in R&D as Qc, and Qa but thats another story.

    Notwithstanding, a shock story when again another new Tech asked a similar question and this time it wasn't my finger but his and he found out the hard way what Magnetron is and what it can do to an index finger. The arc jumps from the terminals to his index finger and makes a path through his hand to the case from his thumb. 2 white holes on the thumb and finger :eek: lucky to be alive I laughed at him and asked if he had respect for high voltage or wanted to try that again :p

    What an idiot, he didn't last 3 days.


    kv
     
    Last edited: Jul 2, 2016
  8. fezder

    fezder Well-Known Member

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    Ouch! :D I had funny moment just few minutes ago, started desoldering connector and spark flew, I wondered why, since DUT wasn't plugged in....reason was 20v/1000uf capacitor that I forgot to de-energize, amateur error!...
     
  9. JimB

    JimB Super Moderator Most Helpful Member

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    Earlier this week I was working on a VHF radio transceiver (145MHz 15 Watts) for a friend of mine.
    The symptoms were very low power out on transmit and very poor receive sensitivity.
    This would indicate a problem with the antenna connector/ Lowpass Filter/ Antenna Changeover relay.
    After some initial probing around, there appeared to be something intermittent with the antenna changeover relay and its associated wiring, so I removed the screws holding the c/o board to the chassis and wiggled it around, while at the same time watching the RF power meter and putting the thing into transmit mode.

    Bad mistake!

    As soon as the thing went into transmit, there was the most horrible searing pain in my thumb which was making contact with the track side of the c/0 board.
    Those of you who have ever had an RF burn will know exactly what I am talking about.
    Those who have not, I suggest that you take care to avoid it in the future. It is very painful.
    It does not shoot up your arm like a simple DC or low AC shock, it just hurts like hell at the point of contact.

    As for the radio, the problem turned out to be bad contact between the antenna connector on the radio and the corresponding connector on the radios mounting cradle.
    Otherwise it was all good, no ill effects from transmitting into an open circuit antenna.

    JimB
     
  10. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I'd suspect the burn would be much different. I only worked on a few CB radios as a teen and a 1000 W 13.56 MHz industrial transmitter for work.
    The potentially lethal 100 kV and 15 kV (10 kW and 20 kW respectively) supplies you have to be at your very best. I was careful around the lamp that required 22 V at 45 A operating and a 45 kV pulse to start.

    The nastiest jolt was probably a spark plug wire (lawn mower). I had just changed the oil and the throttle cable. a) I forgot to put the cap on the oil filler and 2) The throttle cable was the wrong one and didn't activate the stop circuit. I pulled the plug boot. Ouch.

    I guess I could have wasted a shirt (spurting oil) and smothered the intake - maybe.
     
  11. tomizett

    tomizett Active Member

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    Here's something that had me perplexed today.

    The attached image is a snippet of a schematic for a piece of equipment I was working on. I was interested in two signals entering at TO-U5-4 and TO-U5-4. From my general understanding of the circuit, I was pretty convinced these should be generating two sepperate outputs, and couldn't see why they would tie together at TO-U5-6. Furthermore, I knew that the outputs from the circuit as a whole appeared at B23 and B25, but couldn't for the life of me work out how any signal arrived at U6B. I'd searched all over the schematic for other references to B23 and B25... nothing.

    Essentially, the schematic didn't correlate at all to the circuit I was expecting to see... I didn't understand how it could possibly work.

    Now, the astute reader will have noticed that the inverters are marked HEF4081, and noted that the HEF4081 is a quad-AND, not a hex inverter. I'd looked up the part number and thought either I must of mistyped it or that it was something obscure that just happened to have "4081" in it.

    So what's going on here?
    It turns out that the pin numbers correspond to a 4081, but the symbols correspond to a 4096 hex inverter. That is, if you installed a 4096 on the PCB then you would have exactly the circuit shown - but the PCB has a 4081 on it, which makes it a different circuit. And the different circuit makesa lot more sense! I've made a rough modification to the schematic illustrating what you get.
    Just to add to the fun, it also turns out that components with no marked values where not populated - the designer had hedged their bets, allowing the output from each gate to be router to either output.

    Why did they do this?
    It could be a last resort if you found your layout software didn't have the 4081 in its library... but I suspect this was to throw unsuspecting would-be coppiers off the scent. It certainly threw me for a while!
     

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  12. tomizett

    tomizett Active Member

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    And another recent one...

    I couldn't make head nor tail of how this transistor was supposed to do the thing it was supposed to do...

    Until I realised they had drawn it upside-down!
     

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  13. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Finding a 10 A rectifier where at least a 50 A should have been. Company replaced the transformer and rectifier for free.

    I used to do the more complex audio repairs at a shop as a sideline. One amp came back 2x before they gave it to me. One of the amp hybrids was operating too close to the absolute max ratings. Fixed with a couple of ZNR's.

    A commercial amp with components that "puddled". The metal film resistors melted into a puddle. 5 shops refused to fix the amp.

    One crazy system had wire-wrapped boards in it. The system operated for 10 years and suddenly a problem appeared, One color was missing and it was absolutely needed for the instrument to function. The company soldered the power pins except on this one IC.

    A thermocouple scanner operated near some 300 W halogen IR heaters kept breaking. The schematic indicated no voltage regulator was populated. A transorb fixed it.

    I got tired of sending boards out for repair for a computer (Pre IBM PC ERA). Added an ISOBAR and a ONEAC Power conditioner. Problems then became mechanical and the power supply. Later that system was replaced with a Macintosh Centris computer. 17 years later, the same hard drive, the same program was running on it (I managed the development and did the hard sections), there were only two issues. Dust and the floppy drive.

    A later upgrade continued to use the same conditioner and two others were purchased for critical systems. They didn't need blackout protection. It cut repairs - big time.

    A possible 420 electrical outlets were defective. Disassembly worked. The ground was brass to a plated portion made only by asymmetric pressure. Variances of the molds made some outlets not work. One ground would "let go" if you plugged two cords in and wiggled one. Some got replaced without question. Others got tested and replaced as the value of the equipment plugged in went up. Figuring out the "easy test" without removing the outlet was hard.

    Two PHd's one a student and one an employee spent 40 hours, combined to try to figure out why something was arcing. I came in on a Monday morning and the problem was identified in 5 minutes. One outlet on an outlet strip had a bad ground.

    A seasoned AC technician had diagnosed an air to water heat pump as being bad, They put a new one in and it still would not work. He asked for my help. I spent about 10 minutes with him and told him to replace the tstat wire. He really didn't believe me. I worked and he was big time happy. I saved his butt. No one else needed to know. the contractor doing remodeling scraped the wires on the metal studs because there was no protection.

    We had just purchased a $40,000.00 USD used e-beam evaporator at an auction and when received all of the wires were cut between the controller and the vacuum system. The connectors were "hidden". They could have been unscrewed too. The high voltage lines were also cut and not removed 6" away. Our director was betting that I would be unable to get the system up and running.

    Except for the "mess", the other problems were minor. The wrong power was installed. It was the first ever piece of equipment that fused the neutral of a 3 phase 90 A 3 phase 208 VAC input. Most issues were loose connections (locktite 222 to the rescue) and the power HV regulator tube fell out of it's socket during shipping. We did learn from the company that the three hearths had a design flaw, but they were never fixed (too expensive).

    I had to replace a 1 Megohm 200 Watt resistor. Anyway, this HV power supply had a history of breaking about every 6 months and my predecessor's solution was to replace said transistor. I get tired of doing those sorts of things, so I fixed it right. It really was loose connections in the HV divider (a bunch of 625K resistors in series), so a few were stressed and the HV bleeder (the 1 M resistor) got stressed.
     
  14. djsfantasi

    djsfantasi Member

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    Much simpler. Hobbyist goof. I had developed a prototype servo control system for an animatronic. On the bench, it worked fine, as it did when loosely connected in the frame for a test. However, after permanently mounting the circuit board, everything went haywire.

    There were two rows of servo connections. When the board was mounted permanently, it was rotated 180 degrees. I had connected the cables backwards!
     
  15. tomizett

    tomizett Active Member

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    ^^ Done that one too!

    Someone I know damaged a rather expensive geartrain by doing exactly the same thing. The worst of it was, it wasn't his fault - he'd meticulously marked up the equipment and re-assembled it in the same way that it was supplied. The problem was, a previous would-be repairer had already made the mistake.
     
  16. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    My most recent one was tractor related air conditioning.

    It's haying time here now so I have been getting things tuned up and fixed this last few days. Anyway if any of you deal with old tractors you know they never seem to hold their AC charge over winter and ours are no different.

    This year our old MF 2745 gave me recharging fits, high side blew the pop off and the low side never made it over 10 PSI, so I figured I had nothing to loose being that more than likely the whole damn system was going have to come apart due to an apparent plugged line or hidden mystery filter that was plugged someplace on the high side. :mad:

    So first things first I cracked the low side and got an at best puff of refrigerant from it then cracked the high side and proceeded to weep the frosty liquidy stuff for about half an hour before everything drained down. That side was definitely full up and then some. :eek:

    Once it was drained down I started opening fittings on everything I could find them to see what part of the system was open and what wasn't. One thing I found was that MF built those tractors with those stupid self-sealing couplers between the cab lines and the engine bay lines. I always wondered why those couplers were so dang oversized. Now I know. (Really hoped they were some sort inline filters.)

    Given that I traced the plugged line to being someplace in the cab which I hate working on that things heating and cooling systems with a passion being the whole ceiling of the cab has to be ripped apart to get into it. The first attempt to clear it out was to try and force starting fluid through it with compressed air to dissolve the plug. No good.

    Nothing would pass forward. Second attempt was to blow through it backwards. Surprisingly once I got it up to around 100 PSI with air it started to bubble the starting fluid out the high side line. Given that I loaded up the return line with as much either as I had and sure enough bit by bit the blockage started to open up. Unfortunately, I ran out of starting fluid and the the only thing I had on hand for solvents to work with was gasoline so I figured heck why not? The things probably going to need to come apart anyways so let's give it a try so I loaded the line full and put the air to it and wooosh!

    A major blast of rusty fluid shot out the high side line!:D
    It was clear! Given that, I hit it about 10 more times with the gasoline flush until it consistently flowed through clean in both directions then did the same for the condenser and other lines plus the accumulator canister as well. Major rust removal and then some from everything. I didn't do the compressor being I didn't want to mess with the oil in it.

    About a hour and a gallon of gas later everything was flowing free and clean in both directions so I put everything back together and put the vacuum pump to it.

    It pumped down good so I put about 50 PSI of R-134a back in then drew that down. Did that twice times to make sure there was no gasoline vapor or residue in the system before starting the tractor and doing the normal recharge.

    End results were impressive and then some! High side pressure leveled off at around 140 PSI at full charge. Low side stays around 15 PSI with the blower on high and blowing icy fog out the vents until it pulled the cab humidity down. It also frosts the return line all the back to the compressor and the compressor isn't burning hot and rattling like it's coming apart either. :cool:

    I know, by every professional AC repair techs standards I did it all dangerously wrong and then some but dang I got some good AC now even though I am using R-134a in an old tractors system and sure, maybe the compressor will seize up in the first hour or two I use it but heck I have been expecting that to happen for two summers given the noise it always made before. :p
     
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  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Maybe when I put my 82 R-12 system in the car from a box, I did it right. Zippo fingerprints. Lots of vinyl gloves. 14 hours of work. Aluminum lines, so any fingerprints would probably be bad.

    Vacuum pump was an old car AC compressor, a behemoth from about 1968.

    So, every 8 years, like clockwork it would leak at two crimped hoses. So, new dryer and new hoses. The 17th year, the compressor clutch started to slip. Bought a used compressor and installed it and it was bad. One of the reasons to get rid of the car.
     
  18. fourtytwo

    fourtytwo Member

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    Oh my your AC stories make me jealous, right now its as hot as the UK ever gets and my car AC doesn't work, recharged it last year but gone again over the winter :( it's either compressor seals or a leak somewhere, sadly I don't have the kit and repair places charge a fortune to even look, then it takes a year before you know if they did they actually found the leak! Joys of a 10 year old car :) never mind it's only this hot for a few weeks a year.........
     
  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I have a old thermal leak detector that I got when we upgraded at work/ It has to use NiCd AA batteries. Regular AA's won;t fit.
    The company wont even talk to me as a consumer., so it's pretty decent.

    Usually, the deal is to use Nitrogen to test a new system's integrity. Under N2 pressure you can use "bubble stuff". Smaller leaks require a small Freon charge. The integrity test, is to pressurize and wait a few hours and watch for a pressure drop. You can do the same with vacuum. Vacuum it down, turn off the pump and wait. If this was a house AC, lines would be soldered under N2 flow.

    Usual car leaks are hose crimps and the evaporator. Leaves like oak + water can mean corrosion. Replacing the evaporator (the inside coil) isn't fun. The dryer should be replaced whenever the system is opened.

    The biggest difference with R-12 and R134a is oil compatibility. O-rings need to be greased in the right oil.
     
  20. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Being your car is only ten years old it more than likely uses those crappy snap together quick connects for everything in the AC system and they are on of the worst designs ever for leaking.

    A basic refrigeration manifold is pretty cheap these days compared to any auto shop bill. Same with a box of automotive AC system O-rings and the tool that unlock s the quick connect fittings from your local auto parts store.

    As for DIY basic AC system service ideally having a vacuum pump is handy but if you don't mind doing a light charge and purge a few times to displace the air out of the system once you have had it open recharging your AC is not hard to do.

    For a reference to how easy it is to do an AC system fix and charge just look at the DIY videos on YouTube Vs the pro guys videos . According to the pro's everything needs to be done in as expensively way as possible ( and only they can do it of course) but as the DIYers like me will show its pretty basic work if you know the theory behind the procedures.
     
  21. fourtytwo

    fourtytwo Member

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    Thank you so much for your encouragement everybody, yup I have no tools but I shall definitely have another look at this :)
     

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