Piggybacking 74LS logic chips to confirm a suspected fault

[Adrian Graham]

On 22/12/2015 20:50, "Terry Stewart" <terry at webweavers.co.nz> wrote:

> 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 did this last night with my latest PET-under-repair, in this case an
8032-SK that I bought 15 years ago and am only just getting round to fixing
now. I wasn't getting any display but the video pins LOOKED ok, I could see
the flashing cursor on pin 1 for example. Shortly before giving up for the
night I remembered a post elsewhere that said if the H signal wasn't spot on
20Hz the monitor wouldn't get any HV so I put my logic analyser on the
74LS86 that does the output to the video pins and one pin was stuck low.

Piggybacked another one over the top and I got a display...

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?