Piggybacking 74LS logic chips to confirm a suspected fault

[Rik Bos]

> -----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