Piggybacking 74LS logic chips to confirm a suspected fault
witchy at binarydinosaurs.co.uk
Tue Dec 22 16:16:28 CST 2015
On 22/12/2015 20:50, "Terry Stewart" <terry at webweavers.co.nz> wrote:
> I've written up my recent third Apple II repair, this time an Apple IIe.
> 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...
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