- Published: Friday, 22 June 2012
- Written by Jon Chandler
- Hits: 4785
You may recall that my Hantek DS-3X55 signal generator met with disaster when I was testing out the Sony-Tek o'scope I got at Goodwill when I had a ground issue with my bench power supply. I ended up shorting the power supply vis the USB ground connection which resulted in no output.
I found a picture of the Handtek internals on the EEV Blog. The output of the microcontroller has an op-amp buffer chip. Since everything worked but I had no output signal, my conclusion was that my power supply short had destroyed it. The signal generator still responded to USB commands and the sync output operated normally which supported this conclusion. The board is shown below. A large microcontroller, a few support chips and a lot of empty space.
The output buffer is the SOIC-8 in the center - an LMH6702A, a low distortion opamp.
The question is how to remove the dead chip and replace it with a new one ordered from Digikey. It's hard to get both sides of the chip hot at the same time with a soldering iron; a large tip will help with this. I could have went after the chip with an iron and soldering braid.
A hotair rework station would be an easier method, but I don't have one of those. However, a reasonable substitute is available at low cost. Wagner sells a hand-held air gun for paint removal and crafting that gets plenty hot enough to remove solder. I positioned the nozzle opening about 1/2" (10cm) over the chip and let it heat for about 15 seconds. All of the solder melted and it was easy to pick up the chip with a small pair of needle nose pliers.
In removing the chip, one of the 0603 SMT resistors was moved off of its pads. I moved it back into position and directed the hot air at it again to melt the solder.
I put the replacement chip into position (being careful to orient it correctly) and heated it with the hot air, using a loupe to verify position. This got the SOIC tacked down, but some of the leads didn't have clean joints. I used my fine-tipped soldering iron, which I tinned heavily before each joint, to reheat the joint and add just a touch of solder. The entire job took about 5 minutes.
The question was whether this quick and easy replacement actually repaired the signal generator. The picture below is the proof that it did! Yes, my 'scope still isn't a thing of beauty, but it does the job. It was a great help in trouble-shooting my recent problems with the shift module on the 18F14K50.
The Wagner heat gun has many clones in the crafting market. This pretty purple one for embossing has the same power draw as the Wagner and works just as well.
Tools like this don't offer the temperature and airflow control of a hotair rework station or the various nozzle sizes but will d the job if some care is used. These are available for @20 or less. You might want to practice on a junk board before working on something you're trying to repair.