11/23 clock issue

Noel Chiappa jnc at mercury.lcs.mit.edu
Thu Feb 5 06:44:26 CST 2015

So I'm debugging a dead PDP-11/23 board (M8186). The first thing I check is
the clock; specifically the output of the first stage buffer-inverter (pin 16
of E4, a 74S240, upper right corner of page 2 of the prints), which is
distributed to a number of places. Nada. Totally flat-lined. So I checked the
input (pin 4), and it's sitting at about 2V. Also flat-lined. This does not
seem good... :-)

So here's my question: what's the likely fault here? The only thing upstream
of that gate is the 13MHz crystal itself. So the fault has to be either the
crystal or the S240 (which seems to be some sort of octal 3-state inverting
buffer). Any guesses as to which?

As usual, thanks in advance for any help!


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