RE: HP 10529A Logic comparator

I have had issues with the HP clip sometimes not seating right on the chip and giving false results. Also, sometimes the chip legs can be dirty and not make a good connection.. both things to check.. Try wiggling the clip a bit on the chip and see it it changes (I do this when on) or try squeezing the clip tighter.

But, as was saif before, this will not tell you if that chip is bad - only that something connected to that pin is bad.. (most likely that chip.....)

Hope that helps.

JB
-----Original Message-----
From: owner-techtoolslist@flippers.com [mailto:owner-techtoolslist@flippers.com] On Behalf Of Marc Alexander
Sent: Monday, March 29, 2004 6:12 PM
To: Martin White; techtoolslist@flippers.com
Subject: Re: HP 10529A Logic comparator

I have found that often the best way to use a logic comparator
is the other way around than you may expect-
Allow it to show you which chips are good!

I found it to be faster and more useful to rely more on the
positive results it gives.

You will then be left with a short list of IC's that
may either be faulty, or giving false faulty results, and
you can work on those alone.
I have an oscilloscope on my bench so I can back up a faulty
reading with the CRO probe straight away.
As I test IC's in a section I use a paint marker pen or
liquid paper pen to place a dot on the pin 1 corner to indicate
I tested it and it is ok. (handy if you come back a week later :) )

First thing I do with a faulty reading is look at the
pin on the good IC and the same pin on the test IC with
the CRO, and see if they is either too fast or radically
different.
Apart from voltage offset differences and such caused by the
comparator circuitry, you can at least see if there is a similar
waveform on the IC pins that test faulty.
Narrow spikes and really fast data that give false results
can be seen easily.

Then depending on the IC type I will either decide it looks
ok with a CRO inspection using a pinout/block diagram of the IC,
or pull it to test or replace.
(eg fast 74161's in video sections, and some other IC's
    in video output especially run too quick for the comparator)

Anyway hope this helps,

Best regards,

Marc

----- Original Message -----
From: "Martin White" <martin@guddler.co.uk>
To: <techtoolslist@flippers.com>
Sent: Tuesday, March 30, 2004 8:50 AM
Subject: Re: HP 10529A Logic comparator

> Yup comparing like for like.
>
> One of the guys forwarded a link to me about the bug trap comparator, and that
> suggests the 157 should compare okay. At the end of the day i'm not overly
> fussed about the fact the comparator showed two of the outputs as being bad.
>
> They both drive the WE lines of two ram ICs each, so i assume they must be
> good or the graphics would be totally up the creek? Ie, if the WE wasn't
> letting the ram be written to at the correct times then i guess i'd be seeing
> a fair bit more of a problem than just the wrong objects being displayed on
> some levels.
>
> Was only really asking as this pcb is starting to drive me nutty!
>
> My Fluke doesn't seem to want to know with the boardset, so that's out (ie, i
> can't get it to run UUT). I've looked all over for stuck bits and can't see
> any. Then for good measure this evening i've just checked the junction drop
> of every pin of every IC on the board. Nothing.
>
> Starting to scratch my head a little now! All interconnects are also good.
>
> Admittedly, the one thing that i haven't done which i'd like to is shotgun the
> object ram's socket's. Sadly they're the size in between eprom size and ttl
> size and i don't have any. I guess i could split some normal ones in two, but
> that's for tomorrow. Had enough for tonight :O)
>
> Not sure if i already said or not as i got confused about reply destinations
> earlier(!), but it's the video generator pcb of an MCRII set. Tron in fact.
>
> Martin.
>
> On Monday 29 Mar 2004 22:49, Phillip Eaton wrote:
> > One more thing to add... you are comparing like with like?
> >
> > I don't know from experience, but if you compared an 74LS (Low Power
> > Schottkey?) with a 74HC (High Speed ???) then you would probably get output
> > comparison glitches caused by the speed differences.
> >
> > I don't know if this would tigger an output error flash, though.
> >
> > Welcome to analogue digital electronics :-)
> >
> > Phil.
> >
> > > -----Original Message-----
> > > From: owner-techtoolslist@flippers.com
> > > [mailto:owner-techtoolslist@flippers.com]On Behalf Of Martin White
> > > Sent: 29 March 2004 20:19
> > > To: TechToolsList@flippers.com
> > > Subject: HP 10529A Logic comparator
> > >
> > >
> > > Has anyone ever made a specific list of 74 series chips that this cannot
> > > successfully diagnose?
> > >
> > > Or does it depend on the circuit in which they're placed (what the inputs
> > > / outputs are tied to)?
> > >
> > > I seem to remember being told once that it can't always successfully
> > > diagnose certain logic chips. I don't have enough field experience to
> > > know when i use it if the results I'm getting indicate a faulty IC or
> > > just an IC it can't reliably diagnose. In fact, so far i don't think it's
> > > been of any use to me whatsoever.
> > >
> > > Obviously i'm ignoring those with more than 16 pins!
> > >
> > > More specifically, i was looking at some 74LS157's the other day that
> > > are part
> > > of a 2 to 1 multiplexer prior to some ram ICs. The one that had the WE
> > > lines as two of the outputs was showing a fault on both of them, but
> > > replacing it made no difference, i thus assumed the comparator was giving
> > > a false positive (or negative!).
> > >
> > > Was this right, or should i really continue to suspect that IC? It's
> > > definately in the part of the schematics where my problem lies.
> > >
> > > Please excuse if it's a bit of a dumb question to some, but i'm merely
> > > trying to familiarise myself better with some of the tools that i've
> > > picked up over the course of the last few years.
> > >
> > > Thanks,
> > > Martin.
> > >
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