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Old 15th July 2008, 01:04 AM   #1
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Default Murphy law applied to Electronics


I have found that into a Brazilian site...maybe some translation from a foreigner text.

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Laws of Murphy applied in electronics

A - The possibility you have that a fuse will protect a transistor is smaller when the transistor is difficult to find or expensive.
B - The stock has a variety of components, except those ones you need now!
C - When you have the equipment schematic, the most confused part printed, or erased, is normally the part were you have the damage.
D – Power semiconductors, the possibility of them to explode and become unreadable, happens more when you do not have the schematic to discover the code numbers.
E - When you need a lot of components equal, defined as N, the amount that you have tend to be equal to (N - 1).
F - The more there are components in the equipment to make it work, there are more components to make it not work.
G - The right time to find out we forgot to install a small pcboard inside the equipment happens, usually, when the equipment is almost totally assembled
H - integrated circuits dedicated serve to facilitate the life of the designer, and ruining the lives of the repair man that will have to find substitution.
I - The possibility of being burned some integrated circuit is greater when it is not easy to find.
J - Understand the switching power supplies is easy when working fine..hard is to understand them when does not operate.
K – Switching power supplies turns our live easier?..
L – When the equipment smells bad, it is easy, the guilty is the transformer, be happy, untill you will realise that cannot install other in place… they do not fit, size is always a little bit different.
M – Optocouplers (Led and photo resistance) are difficult to burn; But they use to burn when there’s not substitution to the unit and it is unreachable.
N - When something metallic fall on top of the energized pcboard, will fall exactly into the most critical point of the circuit.
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 15th July 2008, 01:05 AM   #2
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Default Second part of the text...sorry, was made a google translator version from


Portuguese...not very good.. do not know if the original is in Portuguese or in other language.

.................................................. ....

fake MOSFETs works beautifully until itself starts to believe they are not fake..then it burns alike a match on fire.
P - All electrolytic capacitors dry, but most gave the impression of being perfect, and are only identified after we have tweaked all the rest of the circuit.. then finally it is found.
Q - The chance of a grounding plug pin, founded as cutted by the user, will be proportional to the parts the repair man will touch receiving electrical shocks.
R - Resistores has two ways to burn till it turns an open circuit. One it will look perfect to drive you mad to find it…or it will be toasted, and the burned part will be in a such place you cannot find with eye observation… when you are lucky, it will reveal itself burning completelly, so entirelly burned, that you will never know the value it had before burn… when this happens, the schematic you have is
not clear exactly into this resistance point.
S - When you approach the nose to sniff a circuit board, trying to identify which component is smelling burned, the nose will automatically be attracted to a heat sink with live voltage, or overheated.
T - Tantalum capacitors has greater tendency to have the mark of polarity confusing the greater the possibility of being used in a power supply line . Connect one o tantalum capacitor reversed in a power supply and you will know what I mean.
U – When the boss is inside the laboratory, the chances to have a short circuit, with smell and smoke increases 67 percent compared to the moments that no one, but you is inside the room.
V - Fuse components are of excellent quality, as they are always the last one to burn, they use to burn after everything already burn in fire…. Maybe because it was made to avoid fire.
W – Non connected wires found are difficult to occur, but happens much
more when we do not know where they are linked.
X – Intermittent, non good contact, such kind of failures stops immediattelly in front of the repair man..will re start to present problems when the equipment returns to the owner’s home.
Y - Contacts fixed using screw, will be loosen only if you do not have the tool to fix them or when the tool you have is not long enougth to reach them.
Z - If an earlier defective equipment, already repaired was plugged to be shown to the customer. It will stop immediately to operate… wait the owner leave the shop and them it will return to operate perfectly till the owner return to pick his amplifier. Better not to demonstrate, it there’s a change to smoke, it will happens with the owner watching.
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 15th July 2008, 03:27 AM   #3
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One more law (the recent one):
after you fried all samples you had of IRFP4321 you've discovered that there is more than one version of VN2222 with different pinouts...
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Old 15th July 2008, 04:10 AM   #4
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A 100 dollar picture tube blows first, saving a 10 cent fuse.
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Old 15th July 2008, 04:32 AM   #5
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when repairing an expensive receiver for a finnicky customer, you will have N-1 cover screws.....

things which must be used together, are never shipped together..... (this is also one of Murphy's Laws of Combat)

the mechanical designer of the $10,000.00 receiver never stopped to think it may have to be repaired some day.

surround sound isn't......

you will find the open spot on the bias pot on your last little tweak that's supposed to get the bias perfect.

you always need scope leads N+1 inches long.

nothing is idiot proof, because idiots are so clever....

the expensive amp you worked 4 hours on comes back with major shipping damage, the cheap amp that you replaced a fuse on was in the same shipment, but it's fine......

backordered parts come in the day after the customer cancels the work order....

you're always out of stock on 2N2222's (the most common transistor on the planet) when you need one.

the receiver that needs to have it's setup or reset done through a remote, never comes in with the proper remote.

microprocessor based equipment has two little known processor instructions. 1>BRTP branch when technician present and 2>BROP branch when owner present....

yes, the amplifier DOES know that the warranty has expired.

the likelihood of a crack in a circuit board is proportional to the number of traces that can be broken.

the likelihood of a surface mount component failing is directly proportional to the number of pins it has and exponentially related to the pin pitch.

if you can't remember, the ribbon cable IS plugged in backwards
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Old 15th July 2008, 04:40 AM   #6
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When removing the lid off of an amp, don't drop one of the screws in the PSU caps.
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Old 15th July 2008, 07:35 AM   #7
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Quote:
you're always out of stock on 2N2222's (the most common transistor on the planet) when you need one.
I would add 1N400x/1N4148 to that list.
And I can never find a LM317T to save my life,when I need it.


One more:
When you're in need of a certain part,and you think, "Ohh,I've got a bunch of those..somewhere" ... and can't ever find them!!
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Old 15th July 2008, 07:58 AM   #8
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You take the PCB, position the part that is the hardest to reach, holding things with your tongue, teeth and all available fingers... you grab the soldering iron with the other hand... DOH!!!! forgot to switch it on...

To add to Jed's post....

If you make something idiot proof, they will simply make a better idiot...
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Old 15th July 2008, 08:39 AM   #9
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A circuit measures perfectly until you listen to it.

A circuit sounds perfect until you measure it.
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Old 15th July 2008, 08:44 AM   #10
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When u realize the SMD ICs u replaced were fake, it's too late for the copper tracks

Gajanan Phadte
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