troubleshooting method for micro-controller board ?

I use a IR Laser guided Temp probe. that works nicely.

A couple years ago I had a PCB with about 30 TTL chips on it that the

While normally a very bad idea, a trick I've used with success once or twice was to bypass the fuse and feed a few volts with reasonably high current capability into the thing while feeling what gets hot. This needs to be used with great care though, you can burn traces off the board or in some cases do further damage, but if 32V got put into the 5V rail, you've got little to lose in trying.

Yep. I'm projecting my own skills (or lack thereof, when it comes to desoldering ICs) to the op. :-)

Tom

If you have a photocopier or flatbed scanner available to you, and the board is not too big, I sometimes find it helpful, where no schematics are available, to scan and print out the underside of the board, then take a few minutes to draw on an overlay of the major components. Coloured felt tip pens or highlighters can then be used to trace round critical signal and power tracks. Makes it a lot easier then when metering for the problem. I have found a digital ESR meter - such as the Dick Smith - which is capable of accurately measuring and displaying very low ohms, can be useful for tracking down dead shorts on rails that go all over a board.

Arfa

Thirded (thirded ?!) on the caps. A few years back, Panasonic used to use some purple 10u's in everything they made, and they were swines for going short. Often used as decouplers around signal processing ICs, so you can imagine what it was like with whole signal processing stages stopping working, because a cap on an IC pin, decoupling some internal bias rail or whatever, had failed ...

These days, I see a lot of shorted electrolytics of all manufacture, on supply rails in amps and home cinema rigs, and so on.

Arfa

that

L298s,

without

next

look

and the board

schematics are

then take a few

felt tip

signal and

problem. I

is capable

useful for

thanks Arfa, for helpful ideas. i used a slightly cruder method last night. i used a green fine tip sharpie to draw directly on top of the PCB resist 5V tracks because they zig-zag back and forth accross the board front to back and i kept loosing track (pun intended) of where i was testing

the green sharpie about same color as resist shows up as shiny line an dnot to unsightly

i like your idea as it will help keep track of test points tried as i can record the test values next to points

thanks again for the ideas/help robb

next

look

the connection

a short across

at least easy to

hello graham, no shorts on any of the power regulator chips 5v L387 or the L298s

and still get 76 ohm continuity between 5v an 0v ref. there is only one place i get a 619 ohm continuity check 5v-0v test but every where else is 69/63/73

thanks for help graham iguess i just need to keep looking for the bad components

my de-soldering bulb / iron makes clean quick work of it.

robb

Ummm, what's wrong with that? I = E/R: 66mA. This (static) current isn't blowing the fuse.

TM

oh yes the fuses blow.

ther were 3 bussman fuses on different lines and such and they are all blown

right now i am trying to track down the short(S) so i ca do better static tests on various components without removing and then try and power the board back up and look for other problems

at least according to the advice i have recieved etc. thanks for taking time to look and offer some help , i sorely need it, robb

I wasn't denying that the fuses are blowing - I was pointing out that

What resistance do you get on the other rails?

TM

the

they

and

problems

out that

and

resistance

apologies, i did notintend to come across as snippity :( i wasn't sure if i forgot to mention the fuses

i am a bit perturbed by this event and am not looking forward to removing all ICs and then i have to worry with ROMs too

although i am an amateur i understand your point. but, i do not realize the significance due to my lack of experience

so 73 Ohms is not a short where 0 Ohms would be but my amateur question would be... is 73 Ohms good value between

thanks for your help and ideas, robb

On 7 Nov, 20:40, "robb" wrote: ...

As someone else pointed out, 73 ohms at 5V means a static draw of 5/73 = (about) 68 mA, which for a digital circuit sounds reasonable imho. It is definitely not a short. What's the fuse rating on the 5V rail?

Regards Francesco

"robb" wrote in news:13j1fghd5a5t9d7 @corp.supernews.com:

Since you have an "anal" scopes, Google for RCA curve tracer. There are several different versions and plans for building one.

There is a simple device that I have used for years for checking ICs, transistors and diodes--- in circuit!

Basically, it consists of a transformer that gives some ac voltage (12 or so will do fine), current limited with a resistor. The voltages goes across the part to be tested and the scope monitors the voltage across the resistor on the vertical axis The scope uses the voltage across the leads that go to the part to drive the horizontal trace.

You get a nice sharp knee for a good diode. You get a slanted line for a resistor. You get an oval for a capacitor. You get multiple knees for some of the good ic gates.

After a while you will recognize a bad gate by the rounding of the knee.

A dead short is a vertical line An open is a horizontal line.

You can often see reverse breakdown (zenering) for good ic junctions.

You want your series current limiting resistor large enough so that your peak voltage from the transformer will not damage a good junction.

the 30V rail to 0V is 1.28 MegOhms

robb

of 5/73

imho.

rail?

the fuse was a "T 630 mA 250v" so slow blow bussman type fuse

originally the 5v fuse blew i checked transformer out put then replaced and powered and then all three fuses blew the 5v 12v 30v

maybe i need to take a closer look at the power regulators again

thanks for help, robb

Yes.

I thought it was a SHORT ! Not 76 ohms ! Looks like there may be hope yet.

Graham

the L298s

be hope yet.

apologies graham and anyone else who thpught i knew what i was saying :{

well i did say i was an amateur, i guess everyone forgot to ask me if i plugged it in :}

ok, well i thought any thing less than couple of 100K Ohms was bad news ( rather a short )

i said short because my meter (fluke 177) has continuity setting that measures anything less than 250 Ohms as having continuity (beep) and i thought this was a bad thing between high/low that is 5v and 0v rails

so does this new light change my tactics any ?

maybe all are to upset with my mis-use of "short" ?

still in need of help, robb

I have an idea for you. get your self a low voltage auto tail light lamp. use it in place of the fuse that blows. start disconnecting things until the lamp glows dim or not at all. this will allow you to keep the circuit on, you can then track down some heat traces which won't be much.. actually, you'll be able to do a volt tracking using that method.. You might have some logic issues that is causing current to exist only when the circuit is fired up.