Sometimes, you just gotta get brutal ...

Or you could track down a Shortsqueek by Global Specialties Model SQ-1.Made in the 1970s and 80s it is a handy device to keep to track down shorts on PCBs. Using a small op-amp that changes pitch depending on how low the resistance is you can find shorts such as you describe fairly quickly as long as your ear is good for tone changes of a few Hertz. Polar Devices (UK) made Tone Ohm which was a similar device (I need a probe if anyone has a spare) that I haven't been able to test (because I'm missing the probe!).

Here is the manual for Shortsqueek (470k PDF)

These turn up on eBay from time to time - I bought two for my shop and they do save hours of time (and chopping traces) every now and then...

John :-#)#

I used to use a current limited power supply and a 4.5 digit voltmeter to track the voltage drop on power rails. You would see larger voltage drops till you reached the short, and smaller ones after that. I did this at the factor on boards that cost us $8000 in components to stuff, so I had to use non destructive testing.

"Arfa Daily" wrote in news:ayv0r.137413$ snipped-for-privacy@newsfe04.ams:

Ive done the same thing on TEK scopes. sometimes,I paralleled the current limit resistor on a supply with another R to increase the current output and see what smokes or pops. those glass axial ceramic caps would pop,the dipped tantalums would smoke. Sometimes,the scope would begin working,because the current limit was right at the hairy edge. I found a series pass XSTR with a bad B-E junction that way.It affected the current limit point.

That method has located many S/C zeners for me.

On Mon, 20 Feb 2012 17:38:14 -0000, "Arfa Daily" put finger to keyboard and composed:

Nice work!

- Franc Zabkar

"Arfa Daily"

Often shows up short circuit bypass ceramics and regular pigtail tantalums too.

How come you did not have the matching transmitter ??

I always insist on customers including them - more than once I have been given a mic and receiver pair that do not work because they are on different frequencies.

.... Phil

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A non contact IR thermometer can be useful for such and similar low ohmic VTS , to wave over the errant board

You could also try using a good four wire DMM set to measure resistance. I use a Keithley 2000 which gives me resolution down to 100 micro-ohms.

I do this sometimes, but occasionally:

a.) The short will burn itself open, or

b.) The short is really zero point zero ohms and you start burning foil runs before any component reveals itself by getting hot.

I might try the current limited method next time, but those very small voltage differences may be a problem. I don't own such a DC supply though, I'd have to use an actual resistor.

Might also be a good excuse to buy a non-contact thermometer ;-)

Mark Z.

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I would recommend one, useful for zeroing in on failing caps in SMPS also, no probes anywhere near any HV. Use on the Fahrenheit scale for more resolution. Try along the wall of TVs in an electronics retail barn is quite an eye opener (makers never seem to specify electricity consumption so this is a good proxy). I'm surprised the cops don't dish them out to beat officers - much cheaper than flying FLIR choppers/planes to pick up skunk factories (another thing to try by walking along a street - noticeable variation in heat loss between brick & windows etc).

Behringer 24 channel M8000 Eurodesks used to regularly come in with one of the 100's (if not 1000's) of 17v rail bypass capacitors shorted. I used to stick 5 amps from a bench supply down the offending rail, and it would disappear in a puff of smoke within 10 seconds.

Although not strictly a pro repair, I always told the customer what I was going to do, how much it would cost, and how much it would cost if I had to dismantle the whole desk instead and conduct a proper search. Not one chose the dismantling route, funnily enough.

Gareth.

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If these were MLCC caps then maybe no less than .2 ohm or so , so a chance an IR non contact thermometer would pick it up with only forcing an amp through the rail, would save stressing the traces and leaving a time bomb in the works

I suspect the remaining 999 capacitors were far more of a time bomb - this was a regular occurrence.

Gareth.

Maplin didn't stock those back in the days when I used brute force & ignorance to weed out dud zeners.

This work comes from a music shop. Usually, if it seems to be an RF issue, they will send both bits to me. In this instance, as the receive unit was obviously dead, that was all that the owner brought in. They pulled it apart in the shop, in case there was a fuse to try, but once they saw that there wasn't, they just shipped it out to me, knowing that I have a decent HP generator that's good to 2.4 GHz. I do not see any that are a channel mis-match issue, because that sort of thing is filtered in the shop. The guy that owns the shop is fairly technical and can deal with testing and replacing valves. He is able to replace HF drivers, and has been a bass unit re-coner for years. But he knows his limitations, and anything beyond that, is just passed out to me.

Oddly, a couple of months ago, I had two radio mic receivers in the space of a couple of weeks, which both had the demodulator tank mis-tuned by a good half turn. Neither had been 'got-at' as far as we know, and both remained stable and correctly tuned over a soak test of several days. Neither has come bouncing back either, so it's a bit of a mystery as to why they were like it.

Arfa

I actually have a non-contact IR thermometer, and it's just about OK on biggish items like chips, but nothing like tight enough on its sensing area, to be able to detect a gnat's bollock cap getting hot !

Arfa

I made an attachment that resembles a black (non-reflective) soda straw, about 8 mm in diameter. It is fitted to the lens on my IR thermometer with an ugly mess of electrical tape. The idea is for only the light coming down the soda straw to hit the pyrometer. Works fine for measuring individual components (although the readings appear to be lower than expected).

Got $2,000 and up?

I like your "big bang" method of troubleshooting. I've used when desperate, with variable success. In one case, I destroyed a transceiver when the PCB traces decided to immitate a fuse. Other times, it clearly identified a shorted capacitor, by exploding. Tantalums are rather interesting, producing a bright red glow, and belching toxic fumes.

area,

I tried that a couple of years back and found what you found and by experiment no better resolution with or without the shroud, so went back to plain unshrouded and localising to area rather than individual SMD

Same make of unit? Sounds like a screw-up at the factory, and really weak QC. Somebody had the wrong tuning procedure, or the transmitter they were using to tune it was set to the wrong freq.

Well, that's China for you.

Jon