You’ll Flip for This 7404 IC Motherboard Fix

We often lament that the days of repairable electronics are long gone. It used to be you’d get schematics for a piece of gear, and you could just as easily crack it open and fix something as the local repairman — assuming you had the knowledge and tools. But today, everything is built to be thrown away when something goes wrong, and you might as well check at the end of a rainbow if you’re searching for a circuit diagram for a new piece of consumer electronics.

But [Robson] writes in with an interesting story that gives us hope that the “old ways” aren’t gone completely, though they’ve certainly changed for the 21st century. After blowing out his laptop’s USB ports when he connected a suspect circuit, he was desperate for a fix that would fit his student budget (in other words, nearly zero). Only problem was that he had no experience fixing computers. Oh, and it takes months for his online purchases to reach him in Brazil. Off to a rocky start.

His first bit of luck came with the discovery he could purchase schematics for his laptop online. Now, we can’t vouch for the site he used (it sure isn’t direct from Dell), but for under $5 USD [Robson] apparently got complete and accurate schematics that let him figure out what part was blown on the board without even having to open up the computer. All he had to do was order a replacement IC (SY6288DAAC), and solder it on. It took two months for the parts to arrive, and had to do it with an iron instead of a hot air station, but in the end, he got the part installed.

Unfortunately, it didn’t work. He was sure of his diagnosis, and the chip appeared to be installed well enough, so he couldn’t figure out where he went wrong. Upon closer inspection he realized his mistake: he ordered the wrong part. He needed SY6288DAAC, but he bought SY6288CAAC. The difference was his chip was enabled HIGH, while the appropriate part was enabled LOW.

Ordering the correct part would have meant another two month wait, so [Robson] decided to take the nuclear option. He searched in his parts bin until he found a 7404 hex inverter, which he put between the SY6288CAAC and the motherboard. With the inverter in place, LOW becomes HIGH and vice versa; saving him from having to wait on the correct part. He now has functioning USB ports, and learned a few valuable lessons about SMD work and making sure you pay close attention to those model numbers.

This is isn’t the first time we’ve seen [Robson] work around shipping limitations in his part of the world. He recently put together a low-cost PCB mill so he doesn’t have to wait months on professionally fabricated boards whenever he wants to make a prototype.

33 thoughts on “You’ll Flip for This 7404 IC Motherboard Fix

  1. Good focus for a quick fix. If the drive from the initiating gate was able to handle the gate capacitance for the speed/duty cycle I’d have used a sot23 2N7002 or equivalent with a SMT resistor to act as inverter. Though I do have lots of ttl parts it’s what’s available that often counts first and it worked :-) Cheers.

  2. Who buys surface mount parts, and waits months for them to be delivered? You know that huge pile of old hard disks and video cards you just can’t throw away, but haven’t yet realised why…SMT Bonanza!

          1. Digikey has 1.3 million different semiconductors alone
            Not to mention there are very few useful chips in a hard drive or video card
            … Granted you can get some epic mosfets from blown ebay video card VRM

          2. @Olsen Yep, those processors are actually just the same three dies repeated over and over. There’s no way we could make a die just for one chip, that’d be ridiculously wasteful

    1. SMD codes in such packages are often just 2 characters long, often the same one is used across multiple variations
      I rarely harvest anything in SOT packages, it’s just not worth the risk of putting the wrong part in and killing your project

      1. Yep, you gots to start with a schematic diagram, or know what you’re looking for, though buying those online seems a bit tragic also. Then you go to the datasheets for all the manufacturers of the part you’re looking for, and they tell you exactly how they mark their individual product. Now that you have a list of parts that will do the trick, it’s out with the loupe, and if you’re lucky, you’ll be in business within a few minutes…and sometimes not.

    2. Sometimes things have a long lead time. Could be because they’re only manufactured when they have a big enough order, and if you want one or two then you have to wait for a big enough order to come through to get tacked onto that.

  3. I have boards loaded with mod work because of misordered parts
    Different packages, minor single letter differences in part numbers, totally unlabled differences, different manufactures changing one little thing
    One of my biggest problems is not putting the full, sometimes miles long part numbers on schematics and specifying venders in the BoM

  4. OMG it’s 2018! I started this stuff in 1960 when dad had me plug in a tape recorder and I nearly electrocuted myself trying. I was in kindergarden!

    I’ve got a parts stock of my own that goes all the way back to the 1970’s…. and my Dad taught me electronics pretty much by 8th grade and USAF loved me when I joined, made me a tutor. I still have Dad’s stash too and it is replete with pin tubes and anything needed for an old color tv repair. Is there anybody that needs like 10,000 1N914’s? I could supply 8 hobbiests that do. The point isn’t the parts… it’s the space and weight, and having moved 8 times. My gosh… I still have magnetic core memory boards on hand that I thought I would use!

    The probability that you can actually use these parts to pull off a repair nowadays are low but still possible. Now… take a look at that time passes, same as for me as for you. Oh, by the way, anybody still need an old B&W crt monitor? Hacking? Yah, you can do a “lil”bit more with these parts… cough…

    Let me give you a tip. Large collections of today’s latest and hottest hobby items, you buy what you learned with and at the time seemed like a smart buy, and it was smart! It actually was! But…. all the latest and greatest hobby projects are all the latest and greatest parts and that progresses day by day, month by mont, year by year, and omg in my case decade by decade.

    I can still do the old stuff, and collected all the info to do it with…. had intentions to make xyz about 200 different times. But… What I want to do is today’s stuff…. all the old parts have sat and gotten in the way and been moved from one household to the next over 1970’s to now.. lovingly cared for.

    I am trying to save you money and time….. and space.

    Tools! Those were always a worthwhile buy. Parts…. ahhhmmmm…. not at all so much. At all… EVER.

    1. Hey, I am the writer of the linked article.
      If you were being serious about the 1N914s, I may have use for some stuff (not dozens of thousands though haha).
      If you are willing to let go of some components to help a young hacker, please send me a message. You can use the contact form of my website or at “robsoncouto at outlook dot com”.
      Thanks!

  5. The days of opening the back of an old radio and finding the schematic stuck on the back of the lid are long gone and for sure we have been through a dry patch where schematics were not available anywhere but of late all the nice little dodads that are coming out of China without any sort of manufacturer never mind schematic can be sorted with Google because the Chinese nearly always copy some open source thing on the net. The dummy loads that are $10 on ebay and the Arduino LCR meters to mention a couple but for sure most of the little schematics for stuff like that can be found.

      1. Fun appliances don’t. I was trying to repair a dashboard clock on a 1998 Honda CRV when I found that Honda’s schematics showed internal wiring up to the IC level. All I got was a pinout for the clock. Luckily the problem was a few surface mount caps that had developed cracks in their solderjoints. They probably shouldn’t have used SMDs when working with a VFD. Nothing that a little heating and flux couldn’t fix!

        1. It should be a law that everything coems with schematics to save on landfill for those that can be bothered to fix them.

          If commerical electronics were designed for repair there wouldn’t be so much of a problem with e-waste.
          Yes things would be more expensive. So why is that a problem?
          People might not think of things as disposible and then consider the concequenes as they do not do at the moment.

      2. Not all of them. I have a big commercial grade power inverter (24=>230V, 3kW) from a european company which cost close to $2,000 and the company refused to send me the schematics when the thing blew.
        I had to partially reverse engineer the thing to get it working again.
        I will NEVER EVER buy another appliance from that company and I will not stop to tell my friends about that.

  6. He might have been better off searching for an alternate. Most of the little stuff like this is knock-offs of existing parts. I did some quick digi-key fu and found TPS2001 from TI which looks pretty close. I assume he could get that in .br faster than 2 mos.

  7. Ah! I fell backwards in my chair, hand over my face saying “aaaa… I’ve made that exact god damn mistake once!”
    So relateable for anyone who has possibly designed USB-power delivery. I did the same fix to get the prototype working while waiting for the right part.

    I didn’t have to wait two months though… great job :)

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