Looking for lost DIY project from magazine

[Gene Pink]

This DIY circuit might have been from the '70s or 80's, likely in Popular Electronics. The general idea was to follow currents flowing in circuit board traces by the microvolts drop, using a voltage controlled oscillator and a speaker to indicate what was going on without taking your eyes off the work, very handy

In use, you might set a pot for a 1K tone, and perhaps connect one probe to where the (+) power supply wire enters the board under test, and work your way down the (+) rail with the other probe, hearing the tone change, perhaps a full note per inch of trace if there was heavy current, as it was very sensitive. You go past a fork in the road, tone stops changing, take the other fork and the tone continues changing. That's where all the current is going. Go past a solder pod and the tone stops changing, and there ya go, the current hog.

20+ years ago, I breadboarded this, used it several times as-is on repairs, and set it aside to build properly "when I had the time". Sound familiar? Found the breadboard yesterday but things are missing.

The front end, and the concept, are similar to the schematic attached, but the resistor values are substantially different (1 ohm between probes), and there is a 4558, 2n3904 and 2n3906, the transistors are likely to drive the speaker, not really enough left of it to tell.

Does any of this seem familiar to anyone? I'd love to find the original schematic of this DIY build, and build it up proper. It worked well with power amps, along with a variac set really low, on an amp that had more than the usual blown output device problems.

I hope this post was lucid, my eyes are bugged out from 5 hours of searching Popular Electronic PDF archives, crappily rendered in Adobe.

Thanks,
Gene

 

[Gene Pink]

Try again.

 

[Gene Pink]

And again.

 

[JohnRoberts]

Did you google milli-ohmeter?

JR

 

[Gene Pink]

Did you google milli-ohmeter?

I am not sure of what point you are trying to make, but no, I already have store bought gear that can resolve to one milli-ohm, If I need to drill down any deeper than that, a kludged together whetstone bridge made of a specific size buss wire, along with a wire ohms per foot chart and a tape measure will do, but I honestly can't remember ever needing to know the micro-ohms of anything.

The point of this device is to measure current flow, and specifically WHERE the current flows, to aid in troubleshooting.

Gene

 

[JohnRoberts]

I am not sure of what point you are trying to make, but no, I already have store bought gear that can resolve to one milli-ohm, If I need to drill down any deeper than that, a kludged together whetstone bridge made of a specific size buss wire, along with a wire ohms per foot chart and a tape measure will do, but I honestly can't remember ever needing to know the micro-ohms of anything.

The point of this device is to measure current flow, and specifically WHERE the current flows, to aid in troubleshooting.

Gene

Sorry thought that would be a more productive search.

I vaguely recall something like that but never pursued it. Sounds like a sensitive VCO...

Perhaps with modern technology a sensitive IR camera?

JR

 

[Gene Pink]

Sorry thought that would be a more productive search.

I vaguely recall something like that but never pursued it. Sounds like a sensitive VCO...

Perhaps with modern technology a sensitive IR camera?

I understand now, thanks, I did try that angle. The 1/70 to 12/78 issues of P/E had about a half-dozen low-ohms meter projects, that's as far as I got in searching yesterday.

I did run across my first guitar fuzz box project, a 741 with no feedback at all, square wave output. It didn't make me sound like Jimi Hendrix, nope, not even close. Maybe he used a different brand of strings....

The one I'm searching for IS a sensitive VCO, first half of a 4558 has a gain of 100, can't tell what's going on after that, parts missing. The concept of listening to the results, instead of taking your eyes off the board to look at a meter is a good one, especially with 1 ohm between the two probes, where a probe slip to a different trace would be a very bad thing.

A FLIR is on my Christmas list, but just not this Christmas.

Thanks,
Gene

 

[Slenderchap]

Can't help with the "DIY" aspect... but we have one at work (unless I have misunderstood what you are trying to make)... it's called a "toneohm"... last time I looked you could still buy them ?

https://www.youtube.com/watch?v=2Y7Kv6RwLxQ

Very quick way of locating shorted multilayer decoupling caps....

Colin
www.audiomaintenance.com

 

[PRR]

Not DIY, and not worth DIYing at this price:

http://www.productiondevices.com/Circuit%20probe.htm

Mine is over 30 years old and working fine.

There was an app-note for the LM3909 LED blinker which did a low-Ohms tone to check your shorts.
ftp://ftp.jameco.com/Archive/Obsolete-TechDocuments/24192.PDF  page 6 (5-225)

LM3909 went obsolete some decades back. There must have been many unsold units. I see prices like $3 and $10(!!). It is the kind of thing I would not buy from mystery vendors. Most are eBay far-east. I see "Alltronics" in Santa Clara which is a Known Surplus joint has an eBay offer three for $16 shipped.
http://www.ebay.com/itm/like/160930880606?lpid=82&chn=ps&ul_noapp=true

 

[Gene Pink]

Can't help with the "DIY" aspect... but we have one at work (unless I have misunderstood what you are trying to make)... it's called a "toneohm"... last time I looked you could still buy them ?

https://www.youtube.com/watch?v=2Y7Kv6RwLxQ

Very quick way of locating shorted multilayer decoupling caps....

You have not misunderstood, the unit in the video has a button on the left called "current trace" that would seemingly do exactly what the DIY did, if it is sensitive enough. Thanks for this find.

Only problem, last manufactured 1988, and the only one I could find for sale without "request a quote" was $399 on ebay, a bit steep for me. I'll research some of their newer products.

Thanks again,
Gene

 

[Gene Pink]

Not DIY, and not worth DIYing at this price:

http://www.productiondevices.com/Circuit%20probe.htm

A handy item, but I don't think it can resolve down to following current paths by voltage drop in board traces. Can't beat the price, though. Tones are good, keeps your eyes on the work.

There was an app-note for the LM3909 LED blinker which did a low-Ohms tone

If I go that way, I'd likely start off with a 555, or go old-school with a unijunction reflex oscillator. A crude VCO is all that is really needed (and a ton of gain feeding it).

...check your shorts.

After I saw the price of the Polar Toneohm 700 in Slenderchap's video, I did exactly that.

Thanks,
Gene

 

[PRR]

> I don't think it can resolve down to following current paths by voltage drop in board traces.

It is for "dead" circuits. Makes its own current.

However, while it does resolve transformer windings, I just proved it will not resolve the difference between probes-touching and probes jumped by 18" of #28 wire. (At least with a very weak battery.) So probably not at your level of short-ness.

 

[Gene Pink]

Followup:

I am a bone head.

When I got the dusty breadboard off the top shelf a few days ago to start this thread, all I had to do is reach up there again, for the piece of paper that was under it, the original that I have spent a dozen hours searching for.

I also mis-remembered that the original article was for following currents, it is really meant for tracing resistance to find shorts.

Apparently, I have added a few things, maybe I was going for tracing currents, more investigation is needed, It sucks getting old, swiss cheeze memory and all that.

A scan of the original from a tractor-feed 24-pin dot matrix faded printout:

 

[PRR]

I bet you also have a box of the recommended steel phono needles.

 

[Gene Pink]

I bet you also have a box of the recommended steel phono needles.

Nope, ran out.

Can I borrow some of yours? ;D

 

[gyraf]

*like*

 

[JohnRoberts]

This reminds me of one of my after beer-oclock circuit designs (never finished to my satisfaction). I spent many an evening doodling analog circuit designs.

I wanted to design a "swiss army knife" of pocket testers for (live) sound system troubleshooting.

The  concept was a small battery powered (ideally a single AA) multi-vibrator with a blinking LED display. The frequency or rep rate of the blinking LED varied with the impedance it probed. A useful measurement range would be down to a short circuit, with enough resolution to discriminate between 2/4/8 ohm speaker loads, say one flash per second per ohm of load. On higher impedance it would be nice to discriminate between 150-200 mic impedance, 600 ohm ,  2k and >10k for line level terminations.

Next the probe can also inject a pulsed wave form while making impedance measurements.  The shape of this waveform pulse could contain both HF and LF content so you could confirm that loudspeaker drivers were connected and making sound. 

Connecting to mic or line inputs would both confirm termination impedance and inject signals for trouble shooting.

Over a few decades many beers passed between my lips without a good enough design for me to melt solder over... to prove.

Lots of details to work out and would be nice to test microphone source impedance without damaging the mics.
=====
Being reminded of this, years after I have gone over to the dark (digital) side, I wonder if I could do a microprocessor based design? I need to soak this in beer later, who doesn't want a universal pocket gadget that does all these tricks for troubleshooting sound systems and the like.  (could add a beeper along with LED and change the pitch of that).

JR