[sdiy] a little knowledge is a dangerous thing !

[Scott Gravenhorst]

Dan Snazelle <subjectivity at hotmail.com> wrote:
>
>
>Sent from my iPhone
>
>On Apr 25, 2012, at 11:50 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
>
>> 
>> On 25 Apr 2012, at 14:44, Scott Gravenhorst wrote:
>> 
>>> Amen. But as Harry pointed out, there are people who fix 
>things with rote techniques, able to fix >> 95% where the other 
>5% completely evades them. I've worked with them as lead, very 
>frustrating to >> get them to read a schematic. I finally gave up 
>and made a flowchart. It didn't help fix more of >> them, but it 
>did reduce the time to fix their 95%. The bottom flow state was 
>"Give it to Scott"... >> "proper" Resistance charts can help as 
>an overview scan, but from there it remains to be >> understood 
>_why_ a resistance is wrong. > > In defence of this technique 
>(which was used where I used to work too) it enables companies to 
>get a lot done for the minimum budget. Our repairs department 
>consisted of several "technicians" who knew very little but could 
>hold a soldering iron by the right end and one "engineer" who 
>could actually fix stuff. Since the commonest problem was battery 
>failure, and the second commonest problem was optoisolator 
>failure (caused by lightning most likely) the technicians could 
>rapidly deal with the 95%, leaving the engineer enough time to 
>actually be able to debug the remaining 5%. Employing more 
>engineers would have made the whole department significantly more 
>expensive without really improving throughput of work. The truth 
>is most repairs don't need clever debugging techniques and 
>consequently don't need engineers. > > T. > This sounds 
>interesting...how are these charts prepared? They sound useful 
>for some situations 
>
>
>You simply test values at all points in a working unit and write 
>it down? Or is there more to it? 

To a degree, yes.  Remember that the components are in circuit.  You can have, for
example a resistor and capacitor in parallel.  Measuring resistance tells you something
(but not everything) about the resistor, but often nothing about the capacitor unless
it's shorted.  Eg. what if one of the leads of the cap is broken internally?  The
circuit won't work, but the measurement may say it's golden.  Note also that several
working units may need to be measured so that tolerance is accounted for.  There also
can be other procedures required before measurement of both the working units and
broken ones, such as discharging certain capacitors.  And static device powered down
measurements aren't really enough all the time.  It can give even better information if
the device is powered up and you measure voltages at certain points.  Oh - and God
forbid us using an oscope, but I suppose that I digress.

-- ScottG
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-- Scott Gravenhorst
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