how to check ic chips

HAVASUGAMEMAN

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what is the best way to check and see if a chip is bad? I want to check chip 7402 (12a) on the cpu board it is in the spec coil switch input circut .this is in my batman mach.
 
Datasheet at:

http://www.ti.com/lit/ds/symlink/sn7402.pdf

Quad, 2-input gate.

Using your logic probe, check each input, and each output. Use the logic table to determine what your output should be based upon the input.

In this case, if either the A or B inputs are high, the output should be low. If both inputs are low (at the same time), the output should be high.

If the inputs are pulsing, you'll probably see pulsing on the output.

Given where the chip is located, I'm assuming you know what coils it should be controlling; if you do whatever would normally cause that coil to trigger, see the input change on the chip, but see no change in the output, you're on the right track to suspect that chip.

Good luck!
 
-Desolder it and put it in a TTL IC tester. However, not many people have such a tester (I don't) and once you've desoldered the damn thing, a new one is only like $0.50 so you may as well just replace it at that point rather than bother with testing it.

-Use a logic analyzer. Few people have access to one, and few still the knowledge to use it. I am not among either group.

-Use a logic probe along with a the truth table from the datasheet for the IC. Not fool-proof, but useful for finding "stuck" lines. This is typically the approach I take.

-There's a semi-ghetto method of using a DMM in diode-check mode to test for bad lines. I don't really know the details of this method, as I usually use my logic probe. Go here, then scroll down to "testing chips" http://www.pinrepair.com/begin/index.htm#howdmm

-In some cases, some clever test jumpering (study the schematics) may allow you to determine if a TTL IC is bad.

-Damn the testing... if you have good reason to suspect it, just replace it.
 
DarrenF;1769896 -Damn the testing... if you have good reason to suspect it said:
Exactly. You're talking about a $0.39 part, save yourself the headache and try a new one.

Chris
 
-There's a semi-ghetto method of using a DMM in diode-check mode to test for bad lines. I don't really know the details of this method, as I usually use my logic probe. Go here, then scroll down to "testing chips" http://www.pinrepair.com/begin/index.htm#howdmm

Randy Fromm covers this in his beginning arcade repair video as well - my question - has anybody actually found this useful? I'd tried it a few times in the past and it really didn't help me isolate which chip was bad, so I'd kinda blown it off. Essentially, you're using your meter in diode testing mode, and looking for similar results (.4 to .6) on the inputs/outputs. I've yet to have this identify a bad chip, although Randy notes in his video that he doesn't even bother using other methods any more as it works so well for him...

I too have gone with the logic probe, and reviewing schematics to help identify likely areas of failure.
 
Randy Fromm covers this in his beginning arcade repair video as well - my question - has anybody actually found this useful? I'd tried it a few times in the past and it really didn't help me isolate which chip was bad, so I'd kinda blown it off. Essentially, you're using your meter in diode testing mode, and looking for similar results (.4 to .6) on the inputs/outputs. I've yet to have this identify a bad chip, although Randy notes in his video that he doesn't even bother using other methods any more as it works so well for him...

I too have gone with the logic probe, and reviewing schematics to help identify likely areas of failure.

This is an excellent method, and one that I use frequently. What you NEED though is a good board reference - something to compare your results against, otherwise the numbers don't mean much.

Chris
 
This is an excellent method, and one that I use frequently. What you NEED though is a good board reference - something to compare your results against, otherwise the numbers don't mean much.

Chris

I actually found a bad 74XX chip that was bad using this method. I was in a pinch since I didn't have a probe.....
 
Exactly. You're talking about a $0.39 part, save yourself the headache and try a new one.

Chris

Sure, if it's socketed, you have a replacement on hand and you really suspect the IC is bad. Something that's not socketed I would definitely be taking 30 seconds with a logic probe to make myself feel better about digging into the board. Testing simple gates with a logic probe takes very little time. It's probably a good idea for a lot of pinball repair people to take a bit of time and actually learn how this logic works and what it does in the context of a pinball circuit board anyway.
 
Sure, if it's socketed, you have a replacement on hand and you really suspect the IC is bad. Something that's not socketed I would definitely be taking 30 seconds with a logic probe to make myself feel better about digging into the board. Testing simple gates with a logic probe takes very little time. It's probably a good idea for a lot of pinball repair people to take a bit of time and actually learn how this logic works and what it does in the context of a pinball circuit board anyway.

Well, I wasn't the first one to suggest this, I was agreeing with Darrenf (the guy who first said it) and, additionally, pldoolittle (another guy who agreed with Darrenf).

Chris
 
Well, I wasn't the first one to suggest this, I was agreeing with Darrenf (the guy who first said it) and, additionally, pldoolittle (another guy who agreed with Darrenf).

Chris

Yeah, I get that. Not trying to single you out. My advice is for everyone and will remain the same regardless of who might agree or disagree.
 
Some boards are very forgiving, and easy to remove IC's from, whereas with others, it's very easy to lift/damage traces - even if you're very experienced with soldering. It happens. I've been soldering for 20+ years, and I still occasionally end up screwing up a trace.

If you can be confident that the chip is bad before you remove it, then you have less chance of replacing chip after chip "because they're cheap", and ending up damaging the more expensive PCB.

Sometimes a shotgun approach is all you're left with, but if you have an alternative, go with it first.
 
We can all do it different ways and still be right.

I will agree that there are many ways to fix the game. Some are more direct than others. Some are basically just guesses. Some could arguably be called "better" than others. Whatever gets the game working could be called "right". Still... you're rarely going to see me call shotgunning the right method. When you understand what you're working on you'll almost never need to shotgun.

There are many "right" methods but there's only one best method. Opinions will differ on what that is, obviously.

It really depends on what you want out of the game and what you want out of pinball all together. If you want to learn how the thing works in intimate detail and be able to troubleshoot and repair any problem on your own then learn how it works and troubleshoot everything until you're absolutely sure of what you're doing. Learn as you go and accept no shortcuts or bullshit. Eventually there will be no pinball problem you can't solve. If you don't give a shit about any of that then shotgunning until it works is a totally acceptable method for you. Sure, you might waste a bunch of time and money but it will work eventually and in the long term you're probably not going to be repairing many anyway so it's not a big deal.

Believe it or not I totally understand that there are people who have no time, desire or ability to learn and understand this stuff. I respect that. For those people just getting the thing working is all that matters.
 
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