Subject: PDP-8/M/F front panel

From: [email-masked] (Douglas W. Jones,201H MLH,3193350740,3193382879)
Date: Tue, 19 Jan 1993 02:45:26 GMT

I've just gotten down to work debugging the front panel on my
second hand PDP-8/F.  3 of the LED's won't go out (and one more
won't go on) in the data row.  The problem is in the 74L54 chips
that are used to multiplex the data for the data lights.  I can't
get 74L54 chips, but I can get 74LS54 chips, these have the right
pinout, and more importantly, three of the 74L54 chips have
already been replaced with 74LS54 chips.

So, I verified that the problem is indeed in those chips by
replacing one, and now I have to wait for 3 more (since I bought
the last one in stock from the electronics place near my office.

But, my question is, what is the problem with the 74L54 chips in
the 8/F front panel?  I'm replacing 4, and three others have been
replaced some time ago.  Did DEC screw up when they selected this
chip?  Is there something dangerous about the way it is driven
that tends to burn it up?

By way of information, the symptom I'm getting is an output that
goes from 2.5 to 4.5 volts as the inputs change, instead of 0.3
to 4.5 volts (more normal).  This suggests a blown output
transistor, as if the chip tried to sink too much current at some
time (difficult to do, considering that it only drives a 7404
chip).

A second question is, does anyone know of a source of 74L54 chips?
The 74LS54 seems to work, but it would be nice to use the right
part if I could get one.  The 74LS54 loads its inputs just slightly
more than the 74L54, and one of the inputs is the DATA bus, so this
could matter in a fully configured system.  Any experience or
warnings about this would be appreciated.

				Doug Jones
				[email-masked]