[sdiy] a little knowledge is a dangerous thing !

[Scott Gravenhorst]

"Lanterman, Aaron" <lanterma at ece.gatech.edu> wrote:
>
>On Apr 25, 2012, at 6:58 AM, Roger Mote wrote:
>
>> Are you sure? Check at 8:25 the cap he says is bad is close to 
>him. To determine this, he's probing nowhere near its leads when 
>he makes the 48 ohms statement. 
>
>
>I am skeptical for three reasons:
>
>1) A few moments prior to that he makes a reference to checking every part with his meter.

And without reference to a schematic.

>2) 47 is a common cap value. 47 is a quirky number, unlikely to 
>pop out of someone's head, so I suspect he read it off a cap. 

Good point.  And while 47 is a common value, for a capacitor, it is never ohms.

>3) Looking at typical tube amp schematic (including, say, that of 
>a Fender Princeton Reverb), I'm hard pressed to find a cap where 
>I'd expect to measure 47 or 48 ohms across it, based on looking 
>at the other resistor values. (There's also the issue that he 
>never makes any reference to any documentation of any sort, let 
>alone a schematic, let alone some record of resistances measured 
>on a working amp). 

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.  

Schematics, documentation and learn more about electronics.  The resistance to this is something
I've never understood except that their rote technique fixes 95% which they feel is good enough and
after all, what's more important, fixing that amplifier or their ego?  

-- ScottG
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-- Scott Gravenhorst
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