[sdiy] OTA question

[Ian Fritz]

At 06:51 AM 9/17/2010, Harry Bissell wrote:
>WHAT ??? are you saying that the circuit did not work on a perfboard, but 
>WORKED
>on a SBB.

LOL!  Right.  Perfectly understandable, though, when you realize that the 
SBB strips have over 10^12 Ohm isolation resistance.  :-)


>Maybe your circuit needs a bunch of stray parasitic elements coupling 
>God-knows-what
>and intermittent connections everywhere to work...

Could be ... even at DC, I suppose.  But I never have seen any intermittent 
connections on my SBB, and it has been in service for well over 30 
years.  Sounds like you've been bitten by some cheapos.  Oh wait ... didn't 
we had this conversation once before?  :-)


>seriously, was it a copper clad perfboard ?  Sometimes there are faults in 
>copper-clad
>where there are subterranian short circuits (contamination in the laminate 
>that can
>join unknown circuit points).

It's a pad-per-hole phenolic board.  Sounds bad, but I've never had any 
kind of trouble with them.  And it looked clean, but I pulled it apart, so 
I'll never know.


>I have seen this ~once~ ... a metal whisker in the fiberglass that shorted 
>out two
>adjacent points.  It was unbelievably hard to find as it was invisible.

I've seen that on PCBs where I cleaned off some flux using a dental probe. 
The probe can shave off an extremely fine strip of metal.  This was on a 
cheap Advanced prototype without solder mask and with long parallel 
traces.  What a mess finding that one!

   Ian