When to and when not to "piggyback"

Arm123

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Hey guys and gals.

I have been reading repair logs to try and upgrade my skills and find new techniques for repairing boards.

I noticed alot of people / techs using the "piggyback" technique for RAM chips and some logics chips, when a bad IC is suspect.

Does this always work ? Does it only work with RAM or certain IC's ?

How would one know when not to use this technique ?

Just wondering what others think about this.
 
I've seen people recommend this... but I don't think it's a good technique.

- In the best situation, the suspect chip has some blown output pin that is outputting no voltage at all. The piggy back chip gets exactly the same inputs but drives that broken output line.

- In the average situation, there is ANY other failure that causes the suspect chip to drive one thing, the piggyback'ed chip to drive a different value which is correct, and the two chips are fighting each other. For example, under some condition one drives 0v and the other drives 5v.

There are tools (like this) which allow you to compare two logic chips, without the chance of two outputs fighting each other.
 
Thanks man,

Yup, that was gonna be my next question. I was gonna ether buy a logic comparitor or a EPROM reader that had logic IC testing capability.
 
Like any tool, it's effectiveness is in the hands of the user and should be one of many in your arsenal. Experience will tell you when this is a good technique to use. I don't discount any troubleshooting technique if it is fast and effective.

I do piggyback chips on occasion but do it based upon experience. You'll have to gain that as well. My advice is if you find a bad chip through whatever means, try piggybacking one or more stacked to see if it helps or corrects the situation. Document and the next time you see the same problem, it is much faster to piggyback a chip than to dig out and set up a piece of test equipment.

Bill
 
Yeah, I haven't done it enough yet to list any useful rules-of-thumb.

If I can easily see with the logic probe that the input of that logic gate is pulsing, but the output is staying high, against what the truth table says should be happening... then I'm going to just replace it, rather than bother with piggybacking.

I guess I've found it useful in cases where I'm not quite sure if the IC is bad or not. Just the other day, I piggybacked a RAM (TMM2014) to positively confirm the one on the PCB was bad. There have been a couple other intances I was able to determine a TTL was bad.

Not something I do all the time... but perhaps I should try it a little more often.
 
I guess I've found it useful in cases where I'm not quite sure if the IC is bad or not. Just the other day, I piggybacked a RAM (TMM2014) to positively confirm the one on the PCB was bad. There have been a couple other intances I was able to determine a TTL was bad.

Not something I do all the time... but perhaps I should try it a little more often.

many time the next stop on the schematic could be the problem and not the chip. Not often, I would cut the pin or the trace to the output to the chip. This way you can set the input(s) high/low and watch the outputs. I don't do this often but it is a way of checking if it is that chip or the next chip.
 
It really depends on how the chip failed whether it will work or not. It sure doesnt hurt anything but it wont find all problems. I have found many bad chips with this method but you really need to have an idea where the problem lies or else you will be randomly piggybacking chips all over the board.
 
i like to clean the legs of the chip with my dremel tool and a brass brush before i do it so as to get a nice clean connection between chip legs
 
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