[sdiy] a little knowledge is a dangerous thing !

[Tom Wiltshire]

On 25 Apr 2012, at 17:31, Dan Snazelle 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?

Depends on the circuit, obviously, but often yes that's all there is to it. This can either be resistances between various points or voltages at various points.
You sometimes see this on the DIY stompbox forums, where someone will post a list of voltages at all/most of the nodes in the circuit so that other people can see where to start looking if their circuit doesn't work.
I guess that's the key - it tends to give you a clue where to start looking, rather than instantly solving the problem. Still, too simple not to.

Tom