In article ,
"robb" wrote:

the only buttons that do not work are the 3 groups of two on the right of
picture .

So, whatever is connected to the 12K resistors that connect to the
traces joining those buttons and the 4 on the left must work, if the 4
on the left work.

Check continuity between the 4 on the left and the 6 on the right where
those traces are - just in case there's a problem with the traces. If
that's good, then the problem is presumably either with the 3 traces
leading to the 75518, or with the 75518 itself (or something beyond it?).

Couldn't hurt to try remelting the solder on every pin in the path of
the problem, in case one was marginally cold-soldered and failed over
time, but it's not overly likely, just an easy thing to try.

A note on schematic style - rather than the semicircular jumps,
schematics are much easier to draw and clearer if you observe the
following two rules - No 4-Way junctions - if a line crosses a line, the
assumption is that it does NOT connect. If something "would look like" a
4 way junction, make it two 3-way junctions by moving the end of one
line along the intersecting line to make 2 adjacent connections. That in
itself will almost do it, but the second thing is to put a solid dot
where things _are_ connected. But don't do a 4 way with a solid dot -
that starts to beg the question "is it a connection without a dot, or is
it not connected?" with no 4 way connections, all crossings are not
connected, end of story, clarity. Not my idea - one of many useful,
simple things from Paul and Win's book.

--
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