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 Forum index » DIY Hardware and Software
Your favorite secret method for finding a short
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numbernone



Joined: Aug 16, 2006
Posts: 477
Location: new york city

PostPosted: Sun Nov 11, 2007 6:11 pm    Post subject: Your favorite secret method for finding a short Reply with quote  Mark this post and the followings unread

Ive finally gotten around to beginning to box a bunch of MFOS modules. While testing the boards for resistance between ground +/-V, 8 of 10 are giving a healthy meter reading, however both VCO modules are showing a short. I have been lead to believe this will blast my protection resistors etc...

How do you like to track them down? In the past I have always sussed them out with close visual inspection with a magnifying glass, and trickier ones with macro photos. I cant find a thing on these 2, top or bottom (these are the ones with no soldermask).

Thanks all.
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shawn



Joined: Dec 13, 2005
Posts: 209
Location: savannah

PostPosted: Sun Nov 11, 2007 8:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

If a visual inspection doesn't find it, I resort to my multimeter that has a continuity tester option on the resistors section. It beeps when it connects, works really well so you don't have to look at the multimeter, if you don't have that then resort to measuring resistance along a path.
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Rykhaard



Joined: Sep 02, 2007
Posts: 1290
Location: Canada

PostPosted: Sun Nov 11, 2007 9:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

On boards at work (I'm in the Return of Materials Authorization dept. - figuring out / doing repairs) - I always start with the transistors and diodes. (Our customer does Hipot testing on most of the boards - along with CHEAP parts. They (G.E.) usually fry things and return for warranty. Razz )

Next, I'll check the tantalum caps, removing them if I have to.
Also - I'll remove the related voltage regulator if I haven't found it anywhere else yet.

(All of this is done, after the component and foil sides have been thoroughly examined through a microscope.)

Failing the above - I then check the opamps (dual or quad) as well as the TTL -HC chips to see if any of them are shorting. (Having removed the related voltage regulator 1st, of course.)

At the VERY rare time - will a socketed chip have one of it's legs not seated properly. (Rare with this company. Much more occurant with another big customer that sockets most everything.)

Don't know if that'll help you at all, but that's what I've taught myself at work. Shorts that I have on my own modules at home, are almost always, visually findable.
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numbernone



Joined: Aug 16, 2006
Posts: 477
Location: new york city

PostPosted: Mon Nov 12, 2007 9:24 am    Post subject: Reply with quote  Mark this post and the followings unread

I can rule out badly seated ICs, as I am testing the boards without them. It seems weird to me that it is only the VCOs, nine other boards are testing out swell. I guess I will start with the trannies and diodes, and see whats up, of course i am now at work and many many hours from the opportunity to try Evil or Very Mad Evil or Very Mad Evil or Very Mad
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Rykhaard



Joined: Sep 02, 2007
Posts: 1290
Location: Canada

PostPosted: Mon Nov 12, 2007 10:01 am    Post subject: Reply with quote  Mark this post and the followings unread

numbernone wrote:
I can rule out badly seated ICs, as I am testing the boards without them. It seems weird to me that it is only the VCOs, nine other boards are testing out swell. I guess I will start with the trannies and diodes, and see whats up, of course i am now at work and many many hours from the opportunity to try Evil or Very Mad Evil or Very Mad Evil or Very Mad


Thank you for reminding me of that also! I DO test my PCBs for shorts to the power suppiy inputs, BEFORE I put any of the chips into their sockets. That has helped GREATLY in PRE-finding shorts before they occur.
I've also extended the practise to continuity testing each leg on a chip with the one beside it, to see if there's a short there, before putting the chips in. On the rare occasion, that has helped as well. Smile

One thing that I realized a couple of years ago, whilst being at work: a huge number of things that I'm wanting to do; waiting to do; etc. - start fleshing out quite well at work, whilst I'm doing routine things - that frees up my brain for other wonderings. Smile

Results: the piles of paper with notes on them, has gotten so big, that it'd prolly take me hours to find them in different areas, all around the house. Wink LOL (Heh. I've started scanning them and uploading them to my web site, so that I don't permanently loose them somewhere. Very Happy )
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germaniac



Joined: Aug 04, 2006
Posts: 200
Location: California
Audio files: 7

PostPosted: Mon Nov 12, 2007 10:36 am    Post subject: Reply with quote  Mark this post and the followings unread

I once did a simple phaser with a sine LFO that wouldn't oscillate. I beeped the circuit out with a continuity tester and finally traced the short back to a 1N914 diode that was supposedly oriented properly, but which was marked backwards! Yes, I found the one diode in the pile of 100 or so that actually had the black band printed on the positive side. My luck. Needless to say, I never solder in a diode now without first checking polarity. . . .
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RF



Joined: Mar 23, 2007
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Location: Northern Minnesota, USA
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PostPosted: Mon Nov 12, 2007 11:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Seems more than a bit odd that both VCO's show a short.
That jumps right out at me. Especially when you seem to indicate it's both + and - that show a short measured to ground.

I gotta ask....
You know C7 and C10 (The 10 microfarad filter caps) are likely going to indicate a short until they have time to charge up a tad, right?

OK - I thought so, but I thought I would throw that out to you.... Smile Now I feel bad for even asking. Embarassed

Good luck!

bruce
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numbernone



Joined: Aug 16, 2006
Posts: 477
Location: new york city

PostPosted: Tue Nov 13, 2007 8:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey RF dont feel bad, you could have even began yr message with HEY DUMMY! and I would still appreciate the input...

Anyhow, I have been working very long hard days lately, with just a smidge of time to work on MY projects, my bleary eyes may be fooling me so I will ask that somebody be my backup in figuring out my latest meter readings: Now I am getting roughly 100 ohms resistance between +V and -V, however between both +/- and ground, my meter begins to give me a value then flashes to 0.L (too high to measure?). And continues to do so, flashes blank to come back with a reading then flashes old 0.L, and on and on...

Maybe I could make an LFO out of this???? Very Happy Very Happy

All my other MFOS and CGS boards waiting to go are showing a healthy resistance between volts and ground, so Im a bit stymied here, and really really tired.

Thanks yall!!! And feel free to begin with HEY DUMMY!!!!
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RF



Joined: Mar 23, 2007
Posts: 1493
Location: Northern Minnesota, USA
Audio files: 28

PostPosted: Tue Nov 13, 2007 8:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey duhhhhnumbernone....
No - I can't bring myself to do that. Smile

That charge and recharge you are reading is the caps - don't sweat that, I don't think.

100 ohms ummm...12 volts...ummm. .12 amps does seems a tad excessive. Shocked

I hope someone can be more help than me....

Have fun

Bruce
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Rykhaard



Joined: Sep 02, 2007
Posts: 1290
Location: Canada

PostPosted: Tue Nov 13, 2007 8:43 am    Post subject:  Pictures?? Reply with quote  Mark this post and the followings unread

Could you take high resolution pics of the board for us? Top and bottom sides? That may be able to help us in seeing things. Smile

"Current will always take, the shortest route to Ground." I never forget that basic ruling. You could even check resistor values, on a whim. It's either a solder short, or a bad part ...... (an obvious statement, but I know how frustrating it is, in not being able to find a problem.)
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goldenechos



Joined: Jun 06, 2007
Posts: 201
Location: Marshall, NC
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PostPosted: Tue Nov 13, 2007 11:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Rykhaard wrote:
Failing the above - I then check the opamps (dual or quad) as well as the TTL -HC chips to see if any of them are shorting. (Having removed the related voltage regulator 1st, of course.)
.


Interesting, I do work in an RMA department as well; with our returns the FIRST thing i check is CMOS and OP-Amps Smile

Still, there are a few products where I would start with select diodes.

Then there is the matter of where the product is being returned from... that will often lead me to my initial proceedings.

Guess what I am trying to say is that every circuit and every situation will have different places which are optimal for starting points.

I would isolate the module, place it within some sort of over current/ reverse current protection (fuse, diodes, 10 ohm Rs in series with power etc...) and power it p to see what it is doing.

You might get lucky and see where the magic smoke is leaking out!

Tony
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Rykhaard



Joined: Sep 02, 2007
Posts: 1290
Location: Canada

PostPosted: Tue Nov 13, 2007 12:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

goldenechos wrote:

Interesting, I do work in an RMA department as well; with our returns the FIRST thing i check is CMOS and OP-Amps Smile

Still, there are a few products where I would start with select diodes.


I forgot about the different diode types as well. Every now and then, our 'wonderful customer' manages to fry a transorb. (How - is beyond my theory, by a million miles - when they're there to protect in the 1st place. Shocked )

(Heh. For me, being placed into RMA against my wishes 11 months ago - RMA stands for "RELEASE ME, Already!!" Hopefully - they'll accept my recent application for programming in AOI .... Shocked )

Quote:

Then there is the matter of where the product is being returned from... that will often lead me to my initial proceedings.


That fits us, as well:
- elevator company, using 15+ yr. old technology - socketed chips
- previously mentioned big ugly company - improper handling of their own boards leaving the problem open to almost anything
- small medical x-ray machine mfg comp. - improper fussyness FAR beyond medical / military requirements. Razz

Quote:

Guess what I am trying to say is that every circuit and every situation will have different places which are optimal for starting points.

I would isolate the module, place it within some sort of over current/ reverse current protection (fuse, diodes, 10 ohm Rs in series with power etc...) and power it p to see what it is doing.

You might get lucky and see where the magic smoke is leaking out!

Tony


That (10R's / 22R's) to me is a very good thought, if they're not there already. The Magic Smoke syndrome can help in tracking down, immensely. Smile
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