Piggybacking 74LS logic chips to confirm a suspected fault

[Jonas Otter]

 >> -----Oorspronkelijk bericht-----
 >> Van: cctalk [mailto:cctalk-bounces at classiccmp.org] Namens Terry 
Stewart
 >> Verzonden: dinsdag 22 december 2015 21:50
 >> Aan: General Discussion: On-Topic and Off-Topic Posts
 >> Onderwerp: Piggybacking 74LS logic chips to confirm a suspected fault
 >>
 >> Hi,
 >>
 >> I've written up my recent third Apple II repair, this time an Apple IIe.
 >> 
http://www.classic-computers.org.nz/blog/2015-12-22-appleiIIe-no-video.htm
 >>
 >> One interesting aspect of this repair is that piggy-backing a logic 
chip helped
 >> confirm a faulty one. I'd only ever used this technique with RAM.
 >> I'm sure it only works if the chip has a particular type of fault, 
but it worked this
 >> time for me.
 >>
 >> Terry (Tez)
 >
 > I use a HP 10529A logic comparator on regular basis for fault finding 
in ttl circuits.
 > It compares a reference chip with the CUT (Chip under test;) it's a 
very nice diagnostic tool.
 > It's a bit like piggy backing just a little more sophisticated :0
 >
 > -Rik

In my experience, the type of socket you used is quite unreliable and 
will tend to develop bad contacts. There is a reason they are cheap... I 
would use a turned pin type socket instead, e g an Augat socket. They 
will not let you down. More expensive, but considering the work involved 
in replacing a socket and the risk of messing up the PCB, the cost is 
negligible.

Jonas