Looking for a M8330 or a couple of SN74194's

Tony Duell ard at p850ug1.demon.co.uk
Wed Apr 2 17:00:40 CDT 2008

> On Mon, 31 Mar 2008, Tony Duell wrote:
> > 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 

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....

> 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 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.
> 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.


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