11/23 clock issue

Holm Tiffe holm at freibergnet.de
Thu Feb 5 14:08:15 CST 2015

Noel Chiappa wrote:

> 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!
> 	Noel

Hmm.. it looks to me as if the Board is missing the jumper W1 and the clock
is disabled. Look at this website (mot mine)


The jumper next to the crystal can seems to connect the clock from the
crystal oscillator to the 74S240. check the clock on both posts of W1.



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