HD74LS48P Issue

dl324

Joined Mar 30, 2015
16,846
Connect anode of LED to 74LS48 to pins 9 to 15 one at a time. Should light each time.
If not check the output voltage with DMM or scope.
I find these handy for troubleshooting logic.
logic320.jpg
These are from the mid 70's. I started using them as a tech and still use them occasionally. HP introduced a set of replacements that will only support TTL/DTL. The set shown supports TTL and CMOS.

With the "pack clips" at the bottom of the picture, you can see all signals on up to a 16 pin DIP. Clip automatically picks power and ground from appropriate pins. They show up on eBay from time to time...
 

MrChips

Joined Oct 2, 2009
30,714
I have the same set. I used the pulser and current probe just a few days ago to locate a short across Vcc and GND.
The two are worth their weight in gold. There is no other pair of tools in the world I am aware of that will do the job.

Don't ever let go of them as long as you live.
 

Dr.killjoy

Joined Apr 28, 2013
1,196
I have the same set. I used the pulser and current probe just a few days ago to locate a short across Vcc and GND.
The two are worth their weight in gold. There is no other pair of tools in the world I am aware of that will do the job.

Don't ever let go of them as long as you live.
Ok cool but what are they ???
 

MrChips

Joined Oct 2, 2009
30,714
It is a set of test probes for debugging digital (and sometimes analog) systems.

HP545A Logic Probe
HP546A Pulser
HP547A Current Probe
HP548A Logic Clip

The only two that I use and find invaluable are the Pulser and the Current Probe.

Here is a typical scenario. You have a logic board (or analog circuit) with 50 ICs and just as many decoupling capacitors across Vcc and GND.
Symptom: There is dead short across Vcc and GND so that the board never powers up and even blows a fuse.
Diagnose: The short can be on any one or multiple ICs, any decoupling capacitor or even a short between PCB traces.

(In one classic case, the short was between Vcc and GND traces underneath one of the ICs. Really wicked one to find and that is another story.)

So how do you locate the short?

This is where the Pulser and the Current Probe come to the rescue. Without applying power to the board, you inject a current pulse into the Vcc power rail and you follow where most of the current is going using the current probe. The current probe has a miniature incandescent light bulb that flashes with a brightness relative to the strength of the sensed current. Hence you trace the path of the maximum current to find the short.
 

MrChips

Joined Oct 2, 2009
30,714
I know the hard way and that is to do a binary search. You keep cutting PCB traces until you narrow it down to one component.

Been there. Done that.
Until the HP Current Probe came along.
It's not made anymore. Check eBay.
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,918
More than $200.00 :eek:
Wow...!
I do not cut traces. I found a an easier way though. All on my own. I think I can patent it.
But the probe is a fastest I think
 
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