Best Way To Troubleshoot a Short

[cominginsecond]

So I've added all the components to my board, and I do the check between pins 1 and 2 of J7 to make sure there's a high resistance there, and what I get is nothing. It says it's open. I suspect there's a soldering problem underneath one of the larger components (e.g., power connector, big capacitors) but I can't really see under the components to even know where to begin. Is there a way, with a digital multimeter and schematic, to determine where a short is? I'm sure there is, but I'm a complete newbie and have no idea how to do this.

[jimi]

open means infinite resistance - i.e. the opposite of a short. are you sure you are measuring pins 1,2? i get around 6.5 M ohms on those pins. pins 4,5 and 5,6 show open.

[cominginsecond]

The pins I checked are the two pins closest to the input transformer. That's 1 and 2, right?

[cominginsecond]

What would cause an "open" as opposed to a short? A short means a break in the circuit, right?

[jimi]

a short means a closed circuit with no resistance. imagine taking a wire and plugging one end into one side of a wall socket and the other end into the other side of the same socket (DO NOT TRY THIS!!!). you would get sparks, fire, burn up some fuses, trip some breakers, death, your house would burn down, and you would get one heck of a jolt. that is a short. an open is no connection, i.e. a break in the circuit. the reason things don't blow up when you plug them in is because they have resistance. that is why you are measuring for resistance on those pins. if it is too small, then things go horribly wrong.

[cominginsecond]

Thanks for that info. I really am not that knowledgeable in this area.

So obviously, a short is a dangerous problem, but an "open" means a non-functional unit.

So back to my original question, is there a way to use a multi-meter and the schematic to test for opens and shorts so that I can detect which components I need to resolder to fix this problem?

[jimi]

there is a way, but i'm not sure where to start. do you measure any resistances on the other pins?

following the schematic you should be able to figure out exactly what the meter should at each point on the PCB.

[tele_player]

From the schematic, it's very difficult to know exactly what resistances should be seen at many places on the board.

The terms 'short' and 'open' don't mean 'dangerous problem' and 'non-functional'; they always depend on context, that is, where you're measuring.

The resistance between J1 pins 1 and 2 should be very high, essentially 'open', maybe beyond your meter's ability to read. It doesn't indicate a problem. A reading of zero ohms, or some other value less than megohms, would indicate an unintended path to ground, which would be a problem.

[cominginsecond]

I had a tech measure the resistance on pins 1 and 2 of J7, and he also got an "open," not a very high value, like I would have expected.

So what do you guys think I should do?

[cominginsecond]

there is a way, but i'm not sure where to start. do you measure any resistances on the other pins?

No, but I didn't try every combination. Just 1-2, 2-3, and 5-6. Should I try 3-4 and 4-5?

[tele_player]

What's the max resistance your meter can read?

[cominginsecond]

This is my multi-meter:

http://www.radioshack.com/product.asp?c ... d=22%2D810

It has a range for 2M Ohms, but it doesn't say in this listing what the maximum resistance is.

[tele_player]

Your meter can only read to 2 megohms - so J1 1-2 will appear as an open. Did your 'tech' use the same meter?

[cominginsecond]

Your meter can only read to 2 megohms - so J1 1-2 will appear as an open. Did your 'tech' use the same meter?

No. And do you mean J7? That's what Tim has us check in the installation instructions.

Do you think that I have any reason not to plug it in and do the other tests?

[tele_player]

Oops, yes J7. The power conector. I was thinking of the N72, where it's J1. I've been following another discussion on troubleshooting one of those.

Me? I'd power it up. But it's your responsibility...