Hi,

I was looking for an alternative to the HP 547A current probe as they're unbelievably expensive (there's one on ebay for $500+ atm!) and came across RichardO's post:

In another thread, I mentioned that I had made my own current tracer probe similar to the HP547A:
https://forum.allaboutcircuits.com/...-i-certainly-should-have-known-better.146232/

I figure I hijacked that thread enough and that I should start a new one.

Well, I actually found some documentation from way back in 1993. I have attached scans of my notes for your amusement. Be warned that the MC1350 -- which is the heart of the circuit -- appears to no longer be available from distributors.

For anyone wanting to reproduce what I built, I found a few of the MC1350's at Jameco:

https://www.jameco.com/z/MC1350P-On...3w0Ar6FyiKLCplI6_8wQmKHzbV6g7gyRoCYkoQAvD_BwE

They are pricey but only 2 are needed.

Something I should have noted is that my circuit was hand wired on a copper clad perf board. All of the ground connections solder directly to the copper clad. This is very important.

If there is any interest I will try converting the circuit to more readily available parts.


I got bored and did a simulation of the circuit using the MC1350's:


View attachment 147885
View attachment 147886

Does anyone know if Richard ever updated his design? I was playing with KiCAD the other day and thought it'd be fun to put together something using his design.

It's a shame that noone makes something similar to the 547A these days; it's a remarkably useful little device for rapid fault tracing.

 

I'm a personal friend of Rich's. Unfortunately, he died a couple years ago. I would suspect that if he made updates that he would have posted them, he was good about sharing what he was working on for others to enjoy.

I'm happy that his posts continue to be useful after his passing.

 

I'm a personal friend of Rich's. Unfortunately, he died a couple years ago. I would suspect that if he made updates that he would have posted them, he was good about sharing what he was working on for others to enjoy.

Oh, I'm sorry to hear that. I'm sorry for your loss.

I think I might go ahead and see if I can do the update myself and dedicate it to him then.

Thanks for the reply

 

$500 is a lot of money for these probes. I did not find the logic probe of much use. However, you do need the pulser in order to use the current tracer.

If you are in the repair business and you frequently encounter dead shorts across power supply rails then the pair is worth having.

 

$500 is a lot of money for these probes. I did not find the logic probe of much use. However, you do need the pulser in order to use the current tracer.

If you are in the repair business and you frequently encounter dead shorts across power supply rails then the pair is worth having.

Believe it or not, but that price was just for the current tracer, not with the logic probe (545A) or pulser (546A). I've got a simple logic probe myself but I'd normally just use my scope rather than use the probe.

The current tracer works very well by itself on old electronic hardware. A good use is to find bad ICs that are pulling a line low. If you've got a suspect address line but a whole bunch of old DRAM chips, all you need to do is sweep the probe over the pins on each chip and the unusually high current over the bad pin will light up the indicator! As it's non-contact you can even follow a trace that is pulling too much current over the board to its source, even past vias and in multi-layer boards.

I think the reason why the current tracer is so sought after at the moment is the current popularity in repairing old retro computers and consoles.

 

I was looking for an alternative to the HP 547A current probe as they're unbelievably expensive (there's one on ebay for $500+ atm!)

Keep looking and bide your time. Prices for test equipment on eBay wax and wane (outrageously high now). I think I picked up a set of the probes with the IC clip at a reasonable price 15-20 years ago.