A tale of a chip and a socket

[John Robertson]

On 04/09/2016 5:29 AM, Noel Chiappa wrote:
>      > From: Torfinn Ingolfsen
>
>      > Most likely a bad solder joint.
>
> That was my first thought, and so I carefully inspected all the pins, but
> they all looked good to me. But I suppose it might have been something that
> wasn't visually obvious.
>
> 	Noel
>
If you suspect solder try simply reheating the legs of the suspect chip 
to see if that cures it.

It could also be that the chip is drifting out of tolerance and the clip 
you put on it added enough extra capacitance (we're talking picofarads 
here) that it was happy again. The IC socket may add just enough 
capacitance again to help the chip get back into its operation band. Has 
anyone else got a similar board and that particular chip has a very 
small value cap on one or more of its legs?

I see this on my 1970s video game boards from time to time - a board 
from the factory would have an added cap on one chip that isn't shown on 
the schematics or in my meagre collection of service bulletins for that 
game. The job was obviously factory as the connection is as clean as all 
the other connections on the board.

Data books don't talk much about this bypass caps issue either, I 
believe it is part of the 'magic smoke' of TTL logic...

John :-#)#

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