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Sherlock Ohms

Capacitor Problem? Replace the Contactor

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Rob Spiegel
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What's the real problem?
Rob Spiegel   9/11/2013 9:42:22 AM
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Here's another good example of why it's important to dig down to the real problem. What appears to be the problem at first may not actually be the problem. We're seeing this again and again in the Sherlock Ohms posts.

TJ McDermott
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Re: What's the real problem?
TJ McDermott   9/11/2013 1:30:22 PM
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Rob, it may be more cynically stated that "the customer always lies."  Or, at least, "the customer never tells the whole truth."  Troubleshooting such problems almost always means you have to be a Missourian: "Show me" or it didn't really happen.

akili
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Iron
Re: What's the real problem?
akili   9/12/2013 9:43:25 AM
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My pet bug-bear is the customer that rings you and says something like "I press the start button but absolutely nothing happens".  When you talk them through it like a child then they admit "well, the cycle run lamp comes on but nothing else happens".  Then they admit "well actually, there is a clunk of contactors but nothing else happens..."  Before long the "nothing" that "happens" is enough to diagnose a simple fault that needs a reset button to be pressed or a connector to be plugged in or even a fuse to be replaced.  Of course you MUST think carefully about the implications of a tripped overload or blown fuse, especially the second time round, but most customers will appreciate not having to pay for your time and travel to press the button.  Then they'll pay the bill more readily when they have a problem that costs them real time and money.

tekochip
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Re: What's the real problem?
tekochip   9/12/2013 11:21:23 AM
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Very, very true.  When I was in field service this was a constant problem.  "Don't work", and "haywire" were sometimes the only symptoms that were ever reported.  It was frustrating because you wanted to help the customer the best that you could, but sometimes they wouldn't help themselves.  At times it was the best I could do to get the customer to include his return address so I could send it back.


GlennA
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Re: What's the real problem?
GlennA   9/12/2013 1:51:33 PM
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akili;  Yes.  Troubleshooting by telephone can be very frustrating.  One simple fix was to turn ON the vacuum pump switch.  The customer refused to do this, because his machine did not have such a switch, and refused to even look at the operator panel for the switch.  The end result was a technician had to fly to the customer site, and then the next day the technician turned ON the switch.

And I learned to not ask "Does the meter show 24 volts DC ?"  Instead I learned to ask "What does the meter read ?"  That way I found out the operator had no idea how to use a meter.

And I even had an 'Engineer" ask me what was wromg with his machine, but he would not give any description of any problem.  He must have assumed that I was telepathically linked to his machine !

Rob Spiegel
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Re: What's the real problem?
Rob Spiegel   9/17/2013 10:06:15 AM
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Great point, TJ. This is a real theme in the Sherlock Ohms stories. Sherlock gets the problem with a false lead from someone who had no idea what the problem is.

vimalkumarp
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Re: What's the real problem?
vimalkumarp   9/12/2013 3:13:02 AM
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this I feel is a typical troubleshooting scenario. many a times lack of understanding of the topology of the circuit , component charactetistics may result in this. for example an opamp based  schmitt trigger may appear like a non inverting amplifier ( first impression on circuit topology) and if one has to torubleshoot a problem , lack of understandign of the topolgy may create more errors than a possilble solution.

Critic
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Platinum
More Questions than Answers
Critic   9/12/2013 8:56:26 AM
Glenn,

This story seems like it is only half there.  Did you ask the engineer why he thought the capacitor was open?  If you didn't ask, why didn't you ask?  The engineer obviously had a reason to suspect the capacitor.

When I first saw the title, I thought maybe this was a case where a capacitor is used to protect contacts.  In this case, replacing the contactor would only solve the problem temporarily, and a capacitor replacement would really be needed, also.

GlennA
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Gold
Re: More Questions than Answers
GlennA   9/12/2013 1:40:30 PM
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Critic;  Part of the missing information is familiarity with the error codes.  There is no error code for an open capacitor.  There is an error code for an open enable chain.  So my first assumption was the engineer just didn't pay attention when reading the error code.  My initial testiing of the servo-on sequence did not generate any errors.  After the second complaint, and more lengthy testing, I did get an error code for an open enable chain.  Then I tested the enable chain = series of contactors on a 24V and a 0V circuit.  The enable chain drops out after a few seconds, so I had to repeatedly initiate the servo-on sequence until I isolated which contact was not closing. 

And as to why I didn't ask the engineer for more information - he INSTRUCTED me to replace the open capacitor - there was no 'conversation'.  In my experience, Engineers give Instructions to Technicians, then the Technician (me) has to do the leg work to diagnose and fix the problem.  If you have read some of my other Sherlock Ohms items, you should recognize that pattern.

Critic
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Re: More Questions than Answers
Critic   9/12/2013 7:10:50 PM
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Thanks for the clarification.  It sure seems like the engineer wasn't very helpful in this case!

cadcoke5
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Iron
Don't be too down on the non-engineer
cadcoke5   9/12/2013 1:29:50 PM
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In general, I have learned to avoid being critical of those who are less technically inclined.  God made us all with different abilities. The ability, or training necessary, to completely describe a technical problem is not given to all.

The technician who diagnosed a capacitor problem may have been completely in error, but he also may have had had a reason to make that diagnosis. The engineer who solved the problem didn't initiate a conversation to pursue that, which may have also allowed for an opportunity for teaching. It is not unusual for an Engineer to be a person, who is weak in the area of communicating with other people. Hopefully, those who are gifted in that area are forgiving of our weaknesses.

-Joe Dunfee

Tool_maker
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Platinum
Re: Don't be too down on the non-engineer
Tool_maker   9/23/2013 1:03:29 PM
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  I have learned to never try to diagnose a computer problem if it requires anything more than a reboot. I always tell the support help that, "I know how to do what I do on a computer and all else is witchcraft. But I can push keys and follow instructions. Tell me where to start."

  I did this with a mail order computer and the tech was able to tell me how to dismantle the computer and reconnect some stuff to make it work. He was great and I was happy. A friend of mine with whom I used to work told me, "There is nothing more dangerous than the man who doesn't know that he doesn't know." I think that is true in spades when it comes to electronics.

timbalionguy
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Gold
Problem customers
timbalionguy   9/19/2013 3:31:19 PM
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One I hear frequently, especially when someone brings me a cable assembly to fix is that it 'has a short'. To the uninitiaited, almsot every failure is a short circuit of some sort.

Another time, a video engineer filed a trouble report on a TV camera, saying 'the blue tube is bad'. I looked carefully at the images, put the camera on test charts, etc. No problem at all with the blue channel of the camera, and especially the pickup tube (a plumbicon, for those out there who remember those!). However, there was a monitor switcher on the camera control unit that allowed the video engineer to look at the individual channels of the camera on a black and white monitor. The blue channel was not working there. That was the real problem they were having. It turned out to be a pin that got pushed in too far in the camera cable connector (a monster connecter with 81 regular pins and six coaxial 'pins'). This happened occasionally and was easily fixed.

So now I tell people, "describe the symptoms. Don't tell me what is wrong"!

I can imagine phone support people are tired of people who can't figure out how to plug in a cord, turn on the power, etc. I try to only call tech support when it is a 'real' problem. I suspect these folks would rather have a real head-scratcher problem than someone complaining their computer won't work during a power outage!

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Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
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