Piggybacking 74LS logic chips to confirm a suspected fault

[Terry Stewart]

>It works if the gate is open. But if it is shorted to gnd/vcc, you're in
>trouble :)

Yea.  In my case the chip was quite cool, so I figured it wasn't shorted
and took the risk. On reflection checking vcc/gnd with a multimeter would
be a more definitive way of checking for a short of that nature.

Tez

On Wed, Dec 23, 2015 at 10:10 AM, Alexandre Souza <
alexandre.tabajara at gmail.com> wrote:

> It works if the gate is open. But if it is shorted to gnd/vcc, you're in
> trouble :)
>
> Enviado do meu Tele-Movel
> Em 22/12/2015 19:09, "Rik Bos" <hp-fix at xs4all.nl> escreveu:
>
> >
> >
> > > -----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
> >
> >
>