My friend with the scope came by this evening. What he found leads us to suspect that U10E, the 26LS30, has gone bad. It's receiving a healthy TTL signal from U8A, but the signal it emits varies between -3.85V and -4.2V or so only. Channel 1 in this screenshot (yellow) tells the story:

https://photos.app.goo.gl/ulNyJN32gExRLXE13

It looks like replacing U10E would be a good idea. Even though it's an obsolete part, it's not too hard to find on eBay. Still, I thought I'd seek the wisdom of experts to be certain---does this sound like the right thing to do? For what it's worth, there's no sign that U10E's output pin is getting shorted somehow...

Thanks,

--Tom

On Mon, Oct 30, 2017 at 9:31 PM, Tom Stepleton <stepleton@gmail.com> wrote:

Hello again,

Since I don't know for TTL... :-)

...I tried a different strategy. Unless there's a difference between the Z8530PSH that came on the Lisa and the Z8530PS (no H) that I found for cheap somewhere, the SCC was not the culprit after all.

Back to the original strategy: probing! I'll update if I have any discoveries...

--Tom

On Thu, Oct 26, 2017 at 1:37 AM, James MacPhail <gg_@sigmasevensystems.com> wrote:

When no data is transmitted, U9C pin 25 shows 3.66v

For TTL signals, the logic "high" threshold is 2.0V, and "low" is 0.8V.

So, 3.66V is within the specification for a high level TTL signal.

Anything between 0.8 and 2.0V is not high nor low... which is to be expected for a signal in transition but may indicate a problem for a steady state circuit.

High signals are often much closer to 5V, and low signals much closer to 0V; it depends on the parts involved.

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