Finding the short...

[StudentSA]

Good Day,

I have a old circuit that was probably build in the 1980`s that splits an stereo audio input in to 5 frequency band components that them drives a LED equalizer display.

The circuit was powered using a 7812 and a 7912 to attain +-12 V. I believe that the circuit was damaged by either and over voltage supply or a reverse voltage supply at some time. I am currently separating and testing the different sections of the circuit.

Ive narrowed the problem down to a single circuit board, where the positive and ground rail are shorted... this board contains 4 LM324 IC`s and lots of resistors and capacitors.

What component is most likely to malfunction as a short?

Regards,
StudentSA

 

[ericgibbs]

Good Day,

I have a old circuit that was probably build in the 1980`s that splits an stereo audio input in to 5 frequency band components that them drives a LED equalizer display.

The circuit was powered using a 7812 and a 7912 to attain +-12 V. I believe that the circuit was damaged by either and over voltage supply or a reverse voltage supply at some time. I am currently separating and testing the different sections of the circuit.

Ive narrowed the problem down to a single circuit board, where the positive and ground rail are shorted... this board contains 4 LM324 IC`s and lots of resistors and capacitors.

What component is most likely to malfunction as a short?

Regards,
StudentSA

hi,
Capacitors >>> IC's

 

[StudentSA]

Thanks,

That brings me to my next question... In the fry of many capacitors how do I find the problematic one? A visual inspection for bulged or cracked caps did not help as everything looks good. I guess remove and test until I find the problem?

Regards,
StudentSA

 

[ericgibbs]

Thanks,

That brings me to my next question... In the fry of many capacitors how do I find the problematic one? A visual inspection for bulged or cracked caps did not help as everything looks good. I guess remove and test until I find the problem?

Regards,
StudentSA

I would start with tantalum's, then electrolytic's
Just lift out one leg and use your ohm meter to measure..

If faulty, its possible they only breakdown at their normal working voltage.

 

[KeepItSimpleStupid]

I have a unique instrument for tracing shorts. A tone milliohmeter.

 

[canadaelk]

A 78xx/79xx will shut down under an overload conition. Apply a variable voltage/high current source on the output of the regulator and slowly increase the voltage. Watch for smoke and measure temperature on various components (caps and ICs).
I am reckless and just connect it to a 12 volt car battery, if a pc trace burns you get a good idea as to where the short is. E

 

[Mr RB]

I have a unique instrument for tracing shorts. A tone milliohmeter.

Very interesting!

Can you please describe it and the way it is used?

 

[Mikebits]

One way, get a meter with milliohm accuracy, like 1.000 ohms, then put the ground lead on big ground planes and move red lead along the plus path, the lowest resistance is most likely where the short is. Other way, use a high current supply, set to correct voltage, current limit off then connect power source to circuit, (Do this after the regulator, Your using power supply ) if your lucky, the short will produce smoke and there is your bad part. Note: last method can burn traces if short can handle more current than your traces.

 

[canadaelk]

I found an article (see attached) from a 1983 mag. Mabee this will help. E

 

[KeepItSimpleStupid]

I bought a commercial one many years ago. It was called Short Squeak. Here is a schematic of one that you can build. Build An Audible Milliohmmeter - Nuts and Volts

I searched for "tone milliohmeter"

A standard milliometer could be used.

The idea is as you get closer and closer to the short, the resistance (tone) goes down. So, if you place one probe on the common side, and start searching for a short to (+) the tone will change.

 

[Mr RB]

Thanks for the information. I was curious about finding the short by measuring ohms.

I have a fair bit of experience in finding shorts and almost always use the method of isolating sections and narrowing down the fault area. In my experience many shorts (especially blown semiconductors) have a much higher ohm resistance than the copper tracks, and also the tracks are rarely linear and usually branch in so many nodes and directions (especially power tracks) that finding the short by ohms would be almost impossible.

But I still want an audible milliohmeter!

 

[KeepItSimpleStupid]

The reason why it's nearly impossible is because of resolution of a normal DVM. There are 4 wire DVM's and ones with Zero controls, but they are expensive.

I also have one of these as well: Keithley 195A | Used Multimeter | Used Test Equipment | Refurbished Test Equipment (see specs) which has a 100 micro-ohm resolution.

Here is a tone milli-ohmmeter "LeakSeeker 82B Short Locator by Electronic Design Specialists"