IC's showing high and low on logic probe

dyno

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Working on a board and following the schematics there seems to be alot of the ICs showing high and low on my logic probe. Also ic pins that are suppose to be low are high. Is this a Ic problem or should I be looking at something else. None of the ics are getting hot to touch.
 
Working on a board and following the schematics there seems to be alot of the ICs showing high and low on my logic probe. Also ic pins that are suppose to be low are high. Is this a Ic problem or should I be looking at something else. None of the ics are getting hot to touch.

If it's showing low AND high, then it's probably pulsing. If it's supposed to be low or high and you have other chips are that high when they're low... trace it back to a point where you have the proper readings, then check the next chip in the circuit really well. If it's supposed to be something like an AND gate (simple example) and you have 2 highs going in, and a pulse coming out... then you know something REALLY goofy in there. Same where if you have 2 highs in, and a low coming out... then again, something goofy.

It takes a lot of back tracing sometimes, but it's usually worth it in the end.

I used to have STACKS of printed data sheets laying around here (literally a stack about 3' tall total) until, thank God, I finally found my TI TTL data book... It takes a lot of digging and remembering stuff... but it's well worth it when you finally figure out that a few cents in a NAND gate is all you needed to get your board running again.
 
The ics that are showing high and low are pulsing so it good to hear that they are suppose to be showing high and low, thanks for that info one less thing to worry about. I will have to go through my ttl cook book again and do some back tracing like you said. Thanks for your help :)
 
What's this TTL Cook book tell you? [Details of the book]

Depending on when it's made (mines a little old, but new enough), it'll have the details and operations of pretty much every major chip you would run across on an old arcade board.

Book1.jpg
Book2.jpg


Just nice to have on hand to keep you from having to look up datasheets for every chip online. REALLY handy when you're out "in the field".
 
So I got one of my boards up and running, using the logic probe I found that pins 8 and 9 on K1 which is a 7474 weren't pulsing so I replaced it and now the game works:) Now I have a second board of the same game and I am having the same issue that pins 8 and 9 aren't pulsing on K1 so I replaced it but still they aren't pulsing, everything upstream from them is pulsing like they should including the clock and inputs for that ic but 8 and 9 won't. Any ideas?
 
So I got one of my boards up and running, using the logic probe I found that pins 8 and 9 on K1 which is a 7474 weren't pulsing so I replaced it and now the game works:) Now I have a second board of the same game and I am having the same issue that pins 8 and 9 aren't pulsing on K1 so I replaced it but still they aren't pulsing, everything upstream from them is pulsing like they should including the clock and inputs for that ic but 8 and 9 won't. Any ideas?

If you put sockets down (hopefully) before you replaced the chips... move the chip from the working board to the non-working one. Might have a new dud... never know. The next chip (assuming the lines from 8 and 9 go to the same chip) could be fubar'd and holding them low or high... worth a check either way.
 
Take a look at the data sheet:

http://maven.smith.edu/~thiebaut/classes/364/sn74ls74arev5.pdf

This is where a good data sheet comes in handy. Motorola datasheets are generally great for learning how chips work where some others such as Fairchild are lacking. If you don't find the data you need, try a different manufacturer's data sheet.

What's the clear pin (CD*, or pin 13), clock pin (CP, or pin 11), and data pin (D, or pin 12) doing? Are they working similar to what you are seeing on the working board?

This is where a logic analyzer comes in handy. You can look to see if the inputs are timed correctly to give you the outputs desired.
 
Did some more poking around with the logic probe, comparing to my board that is working and the Q pin 9& Q pin 8 aren't pulsing which they are on the working board, they go to pin 1 which is C which pulses on the working board but not on the bad board which in turn pin 5 Q & pin 6 Q aren't pulsing either but they are suppose to be. I even swapped out the ic with the one from the working board but still no pulse.
 
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