replacing IC chip U29 on a nord electro 2 73

[dvgarcia]

I have a burned out IC 20-pin chip that needs replacing on the main board of my electro 2 73. The schematic designates it as U29. Its specifications are 74HC374 TSSOP. Has anyone tried this as a Do-it-yourself project? If so, any suggestions as how to proceed?

thanks!

[dvgarcia]

I have a burned out IC 20-pin chip that needs replacing on the main board of my electro 2 73. The schematic designates it as U29. Its specifications are 74HC374 TSSOP. Has anyone tried this as a Do-it-yourself project? If so, any suggestions as how to proceed?

thanks!

Hello dvgarcia,
Welcome to the forums
What symptoms does your NE2 exhibit?
Are you sure that U29 is defective and not issues with the contact strips of the keybed?

How much experience do you have with SM component replacement?

Cheers, Hanon

[dvgarcia]

http://www.mouser.com/Semiconductors/In ... 74&FS=True

cheap enough to give it a whirl - i'm not good enough for things spaced closer than .1" anymore.. .635mm is reeeeally small..

[OBDave]

The hard part is going to be getting the old part out. The best way to do with with a SMT device is to use a special hot air hotplate that allows you to heat up a region of the PCB until the solder flows, and then you can just lift the device off with tweezers. Your second-best choice is to use a hot air pencil. You third best choice is to use a combination of standard soldering iron and an exacto knife, but you run the risk of accidentally lifting the pads off the PCB. These are much more fragile than through-hole pads.

Once the old device is off, soldering in the new one is much easier. Seeing clearly is the hardest part. (I use a stereo microscope for this.) You first clean of all the old solder using solderwick, then apply a small blob of fresh solder to one of the corner pads. With tweezers, gently place the new device on the pads while heating the solder blob. Once you have two opposite corners "tack welded" in place like this, it goes easily from there.

You could probably get a local electronics assembly house to do this for you for fifty bucks or less.

But you still haven't answered one question: how is it that you're certain U29 is blown?

[dvgarcia]

Thanks for the expedient replies! The whole problem started when an over-enthusiastic dancer spilled a drink onto my keyboard. I smelled plastic burning and immediately turned it off. After waiting for it to dry, I discovered that C to G of the second to highest octave played all tones at full volume when struck. The nord electro 2 service manual states, "Some keys not working or one key trigs several notes:
This problem is most likely to occur if circuits U5, U12 and U29 on the main board are faulty. Inspect
them visually to see if any of them seem to be in bad condition."

I took this as a cue to inspect the respective circuits, and discovered that U29 had a powdery white blob on it with a slight burn mark. Pablo Mastodon confirmed that this chip does indeed need to be replaced, but I am open to any other criticism about assumptions that I may have made in diagnosing the problem!

[OBDave]

Ah. Based on this explanation I think you're probably on the right track. I still recommend that if you've never done this type of repair before, you should consider having someone do it for you. If you still want to enjoy some of the adventure, you could disassemble the instrument yourself, remove the pcb, and bring the pcb and the new device to an electronic assembly house or electronics technician and let them swap the part out for you. While you're at it, ask them to clean the pcb with flux remover (which is special 99% alcohol). This should remove any residual gunk from the exuberant dancer's cocktail.

[dvgarcia]

Dude,

1) The little pile of burnt plastic ash left on top of the chip is proof positive that it was toasted and needs replacing, and
2) swapping that chip is not typically a DIY job unless you have mad tech skills -- leave it to the pros

Unless you've got Elizabeth Montgomery or Barbara Eden over there to work their magic, there's no other way...

Pablo

[mjbrands]

Find someone who repairs equipment and has the necessary skills/equipment. It is easy to do even more damage.

If you really want to give it a try, I would start on some old electronics. Find something that's already broken get out the PCB(s) and find a part similar to what you are trying to remove. Now remove that chip and solder it back again. Ideally you'd do it a few times to get comfortable.

There's quite a bit info to be found on soldering of surface mount technology. Some keywords to look for: soldering wick/desoldering braid, drag soldering, hot air rework station.

[youtube][/youtube]
(Look at around 10:00)

[dvgarcia]

I hope I haven't offended anyone by seeking out a multitude of opinions about this project. I am perhaps overly cautious due to my unfamiliarity with this type of work. Measure twice and cut once, so to speak. I thank everyone for their input, and will provide updates!

[dvgarcia]

My caution resulted in the use of my backup keyboard for almost a year at all of my gigs, until one of the bands I work with offered to fund the replacement of the burned out chip by a local keyboard tech just a few weeks ago. He did the job competently, and my board is back in action! $80 for the chip swap, vs. $1k for a new circuit board. Big price differential! Thanks for all the help!