2 Apr
2008
2 Apr
'08
3 p.m.
On Mon, 31 Mar 2008, Tony Duell wrote:
I rarely use a 'scope when debugging digital circuitry. For a long time I
used a cheap Radio Shack (!) logic probe -- I got my PDP11/45 and PDP8/e
running with just that (and a VOM to check the power supplies). After
spending a weekend sorting out a Whitechapel MG!, I bought a LogicDart
and never looked back.
I do have a larger (and better) logic analyser, which has proved very
useful on occasions, but the Dart is a useful handheld instrument that'll
help with 99%+ of faults. Pity it's no longer made....
I don;t know what test gear you have, but I find
a simple logic probe to
be more use than a 'scope for this sort of problem. The former can easily
Yes, logic probes are very handy. They are small and easy to use, and I have found faulty devices most of the times just with
the logic probe. The
seven-segment display show transitions and illegal voltage levels (or no
voltage at all), very useful e.g. for dead totem pole outputs (in this
I've seen that too. Particularly in 74Hxx devices for some reason.
Many logic analusers only detect 0 or 1 (they have a single threshold on
the input). A logic probe (and indeed the LogicDart) is better for this
sort of fault becuase they also detect illegal levels.
case the voltage of the signal will be determined by
the input stage(s)
and lies around 1-2V).
I think ticking PSUs in displays are a sign of a shorted HOT and/or
flyback (had this case several times).
Yes, you're right. I'd mentally assumed he'd run the PSU on dummy load,
but I guess not. I know the VT300 series are famous for bad flybacks,
dunno about the VT420.
-tony
I also
return all systems to working order. Apart from a couple of
VT420's with ticking SMPSU's everything I have restored runs. (Three
Any reason you';ve not fixed those? Ticking SMPSUs are often quite simple
to fix, maybe just dried-up capacitors.