[sdiy] How do you find the cause of a short ?

[nicolas]

One technique I have had recommended to me, but have not tried personally ...

Disconnect the +5V line or whatever line is shorted and instead feed it with an audio oscillator output - a pulse wave swinging 0-5V p-p would be appropriate in this case - would need to be capable of a reasonable current delivery, but not so much as to blow things up.  Then listen to the board with a tape head (or probably any other small pickup coil would work).  You should be able to hear where the main flow of current is going.

Nicolas


----- Original Message -----
From: Jean-Pierre Desrochers <jpdesroc at oricom.ca>
To: synth diy <synth-diy at dropmix.xs4all.nl>
Cc: 
Sent: Sunday, 16 September 2012 9:50 AM
Subject: Re: [sdiy] How do you find the cause of a short ?

I repair lots of digital boards who have shorts 'somewhere' on the +5v supply line.
The easiest way I found to pinpoint the actual shorting devices is this:
Suppose the board you are testing is short somewhere in the +5vdc supplied circuitry,
Just remove the local +5v supply and replace it with a variable 0-5vdc 5amp external lab supply.
Start from 0v then raise the supply lets say to 3vdc. Check with your finger all the components
for some that are getting HOT or warm (CMOS IC's). These are short most of the time.
Beware, if tantalum caps are in the shorted parts team raise slower the VCC.
This way I find at least 80% of the offending parts.
JP


----- Original Message ----- From: "Dan Snazelle" <subjectivity at hotmail.com>
To: "Steve Ridley" <spr at spridley.freeserve.co.uk>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, September 15, 2012 5:21 PM
Subject: Re: [sdiy] How do you find the cause of a short ?


> That shounds like a great tool!
> 
> Sent from my iPhone
> 
> On Sep 15, 2012, at 5:00 PM, Steve Ridley <spr at spridley.freeserve.co.uk> wrote:
> 
>> 
>> I got a (cheap second-hand) Polar Toneohm - it's a low resistance meter with a tone output
>> whose frequency changes with resistance. Saves me a lot of unnecessary track cutting
>> and component removal.
>> 
>> 
>> Steve
>> 
>> 
>>> I have a couple boards which are beeping on the continuity test between the positive rail and ground.
>>> 
>>> 
>>> I cant find any bridged traces with a magnifying glass. Maybe a bypass cap blew? Or maybe......
>>> 
>>> But i have no idea how to figure out what might be the culprit.
>>> 
>>> I disconnected the power supply and the problem persists.
>>> 
>>> 
>>> Thanks for any advice!!
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