Grizzly G0720 Motor Power Supply Repair - Part 2

Saturday, January 18, 2020

Right before Christmas, my three-year-old Grizzly G0720 (AKA SIEG SX4) milling machine's main controller board released some of it's magic smoke. In the previous post I went over the initial troubleshooting steps that led me to believe that my failure was due to a blown output transistor. This appears to be a very common failure point of these brushless motor controllers and the replacement boards are very costly. Since the cost of output transistor replacement is less than 1/10th of the replacement board I decided to attempt it before buying a new board. If you find yourself in a similar situation, this post will guide you through the replacement process.

Warnings and Disclaimers

What I'm about to describe below is potentially dangerous. If you decide to follow this post to repair your brushless motor controller, make sure to read the post about the troubleshooting process first, especially the "Disclaimers and Warnings" section.

Risking to sound like a broken record, I will repeat again that there is high voltage connected to the board that can seriously injure or kill you. Be especially mindful of the capacitor bank that can hold a charge for hours after the board has been powered off. If you let your skin touch the leads, at beast the word "pain" will gain a deeper and more personal meeting; worst case scenario it can stop your heart.

I got a few passionate comments and emails that transistors should be tested with a proper transistor tester, etc. I'm fully aware that the troubleshooting methods I described in the last post is pretty rudimentary, and strictly speaking, "incorrect". Doing it "the right way" would involve fancy test equipment that 99% of the people reading the post simply don't have access to. Given that BLDC motor controllers are a common failure point on Chinese machines, and the replacement cost is pretty high, my goal was to help fellow G0720 owners find and fix the most likely issues using the tools that they already likely have or can easily access.

Quick Recap

Following the BDLC motor controller troubleshooting steps described in the last post I ruled out the possibility of bad motor. Using an ohmmeter, I discovered that there was significant resistance difference between different output terminals. Since the circuits for the three motor phases should be identical, different resistance suggested that there was a problem with one or more of the output transistors. By comparing the resistance between different sets of transistors I found one that appeared to be a dead short across all three leads. Subsequently, after desoldering the part, I was able to confirm with a transistor tester that the part was completely dead.

Repair

At this point I've made a decision to replace all of the transistors, even though it was likely only that single part that failed. My motivation was two-fold. First of all, output power transistors can fail with age due to the demanding operating conditions, constant heat cycles, etc. While the one that failed was the weakest link in the output stage (bad specimen from the batch. etc), the rest have been subjected to the same condition and can be on the brink of failure. Having six new parts from the same batch would give me more peace of mind. Second, I'm not convinced that the parts on my board are 100% genuine Fairchild transistors. Counterfeit or relabeled electronic components are very common in China, and I was a bit suspicious about my transistors. Ordering new parts from a reputable distributor would [hopefully] ensure that I had genuine transistors that would last longer.

In addition to the transistors, I decided to replace the gate driver ICs as well (U301, U302, and U303), since in my experience a transistor that fails in "On" state often takes with it whatever is driving it. At the end the ICs were likely undamage but at $1.60 each, replacing three additional ICs was a relatively cheap insurance.

Next Steps

If you have determined that your controller is not working due to a failed transistor, you will need to decide whether or not you want to replace all of them like I did. As a bare minimum I suggest that your replace the pair, since it's likely that when one transistor failed it could've damaged the second one as well. If you're comfortable with desoldering surface mount parts, I recommend that you replace the gate driver IC(s) as well. This will involve desoldering the transistors and ICs, cleaning the holes and pads, cleaning up the heat sink and the thermal pads, soldering the new transistors and ICs, reapplying new thermal compound and bolding the whole thing back together. Fair warning: some of these operations are pretty fiddly, but with some patience they are very doable even if you have minimal soldering skills.

If you are not comfortable with doing the rework yourself try searching for an electronic assembly company in your area that specializes in small batch prototype jobs and rework. If you can bring the board and the parts to them and can be flexible on the turnaround time they will likely be able to replace them for you for a very reasonable price.

Required Tools and Materials

To do the replacement you will need the following tools and materials:

  • Six ON Semiconductor/Fairchild FGL60N100BNTD transistors
  • Three Infineon Technologies IR2101STRPBF ICs
  • Soldering iron that can reach at least 400C/750F. The board assembled using Lead-Free solder that has higher melting temperature. Combined with beefy parts and large pads/traces, a basic inexpensive soldering iron might struggle to melt the solder. If you don't own a good soldering iron, I recommend Hakko FX-888D that can be purchased on Amazon for around $100.
  • No-clean solder wick. I use Chemtronics Braid with Rosin flux, but any similar product will work.
  • Large desoldering pump*. I prefer the Soldapult from Edsyn; the small pumps are less expensive but work less effectively, especially for larger solder joints.
  • Sn/Pb 63/37 or Sn/Pb 60/40 solder with no-clean flux core. The exact alloy is not important but make sure it's the kind designed for electronics, not plumbing and is marked as "No-Clean".
    I use Kester 44 Sn/Pb 63/37 solder, but unfortunately it's sold only in 1lb spools.
  • New thermal compound. You should be able to find some at a store that sells computer parts, but make sure it's not electrically conductive. There is no need for expensive silver-base compounds; simple silicone compound is more than adequate.
  • Sturdy tweezers and/or needle-nose pliers

* The pump is needed if you want to preserve the parts. If you're replacing them outright it's options (more details will be provided in the "Transistor Replacement" section below)

Disassembly and Cleanup

Start by separating the board from the heatsink. They are held together via ten Phillips head screws:

  • Three are in the upper right, lower left and lower right corners respectively. The one in the lower-left corner is underneath one of the daughter boards, but the latter has a hole that allows access with smaller screwdriver.
  • There is a larger screw holding the large rectifier bridge in the upper-left corner. It can be accessed via a hole in the PCB (that might be plugged with some RTV)
  • There are six screws that hold the transistors to the heatsink. They are accessible through small holes in the PCB, bit mine were slightly offset, so getting a good grip was a bit difficult.

Once all of the screws are undone, carefully separate the transistors from the heatsink. A few of mine were stuck pretty well, so I carefully pried them with an Exacto knife. Be careful not to lose or damage the rectangular silicone pads. They are needed to insulate the metal pad on the underside of the transistor, which is connected to the base of the transistor.

Once the heat sink is removed, clean the old thermal compound with some isopropyl alcohol. Do the same to the thermal pads and the transistors.

Transistor Replacement

I will admit that desoldering large through-hole transistors is a major pain in the neck and requires a lot of patience. If you decide to outright replace the transistors, you can simplify your life by clipping the leads off first and then clearing the holes individually. If you go this route, the process is as follows, clip the leads half way between the transistor body and the PCB, leaving ¼" or so sticking from the board for grip. In the steps below you can skip step #1.

Old transistors desoldered from the board

Start by setting the soldering iron to 400-450 degrees C (750-850F). Then, for each transistor that you will be removing do the following:

Hold the pump as close to the pin as possible
  1. Prime the desoldering pump
  2. Heat a solder joint until the solder melts
  3. Add a small bit of fresh solder to it
  4. Heat the joint for a few more seconds
  5. If you cut the transistor leads earlier, nudge the lead down a but with the tip of the soldering iron. It should just fall on its own. If not, pull it gently using needle nose pliers from the bottom of the board. Skip to step 7.
  6. Press the nozzle of the desoldering pump as close to the molten solder as you can and press the release button. This should pull most of the solder from the hole, but the lead will likely still be attached.
  7. If there is a lot of solder left, apply some fresh solder, heat the joint a bit longer than the last time and try sucking it away again.
  8. If there is just a bit of solder left, use the wick to get rid of it and press the lead away from the side where it's stuck using the tip of the iron. Once the solder melts you should feel the click; remove the soldering tip immediately.
  9. Using the needle-nose pliers or tweezers check that the lead is moving freely. Repeat steps 6-8 as needed. Do not apply any significant force, since that way you can pull a trace or damage the hole, which will require more rework.
  10. Repeat steps 1-9 for the rest of the leads until the transistor can be removed
  11. Using the solder wick remove any remaining solder from the holes so the new part can be inserted
  12. Repeat steps 1-12 for the other transistors
Transistors have been removed and holes cleaned

Replacement is much easier:

  1. Prepare the parts by bending the leads similarly to how the old ones were bent
  2. Spread a thin amount of thermal compound on the silicone insulating pads on both sides and place them on the heatsink
  3. Attach the transistors to the heatsink but don't fully tighten the screws yet
  4. Attach the board to the heatsink. This is the fiddly part of the process, since you will need to get 18 pins to align up with the PCB holes. It's easier to have another person lightly press the board and the heatsink together while you use the tweezers to wrangle the leads from the underside.
    New transistors lightly attached to the heatsink and ready for re-assembly.
  5. Solder the pins and add a bit extra solder to the diagonal trace between the transistors. Check that the solder flowed to the underside and that the fillets wet both surfaces and look concave.

A few things to keep in mind while you doing the above:

  • The board is assembled using Lead-free solder alloy. Besides having higher melting point that Tin/Lead alloys, Pb-free alloys have nasty reflow characteristics. If you have trouble sucking out the solder from the holes, add more Tin/Lead solder. This will do two things: add fresh flux to the joint and lower melting point.
  • When a pad is hot, the adhesive that is holding it to the fiberglass substrate is weakened. If you apply any sort of force you stand a chance of ripping a trace or damaging the hole. It's not the end of the world, but rework will be pretty involved, so do your best to apply as little force as possible with the tip and avoid rubbing the tip on hot traces.
  • Stating the obvious, the PCB and the solder wick will be scolding-hot right after you lift the iron, so be mindful of where your fingers are.
  • When soldering the new parts heat the pad and the lead first and then touch the solder to the lead, not the tip of the iron. When the solder flows you know that both surfaces were hot enough, ensuring no cold solder joints. Add enough solder to have a nice fillet on the underside but don't dwell too long or you will overheat the joint.

Gate Driver IC Replacement

Note 1: If you decided to replace the gate driver ICs, it would be good to have a magnifying visor or a lamp with built in magnification glass, since the pins on those ICs are pretty small and you don't want to create solder bridges. Alternatively, although less convenient, you can take a picture using your phone and magnify it to inspect the joints.

Note 2: The part is directional. Before removing the old parts note the orientation of the ICs (the IC should have either a small round indentation next to the first pin or one of the sides is chamfered).

Gate driver ICs are the small 8-pin surface mount components right next to each pair of transistors. They are relatively easy to desolder if you have a wider soldering iron tip as shown below. The tip needs to be wide enough to be able to touch four pins simultaneously.

The desoldering process is actually pretty easy, but since there are tall parts all around, getting to the part at the right angle will be a bit challenging.

"Knife-style" solder iron tip can cover four pins at once
  1. Add a bit of fresh solder to one side of the IC
  2. Cut off used wick from the spool and place the new clean end across the four leads
  3. Press the tip of the iron into it as parallel to the board as you can so all four leads can be heated at the same time
  4. After a few seconds you will see the solder being wicked.
  5. Heat the leads for a few more seconds
  6. Without removing the tip of the iron, grab the body of the IC with tweezers close to the side where the iron is
  7. Gently pull the body up a bit and remove the soldering iron. Don't apply a lot of force, since you don't want to rip off the pads on the other side; just bend them a small amount.
  8. If you did this right you should see a small gap between the newly desoldered leads and the pads.
  9. Now heat the other four leads, add a bit of fresh solder and pull gently pull the part off
  10. Use solder wick to clean any remaining solder from the pads

Replacement process is much easier:

  1. Add a small amount of solder to one of the pads
  2. Hold the IC in place with tweezers, making sure the first pin indicator is facing in the right direction. Align the pins so they don't overhang the pads.
  3. Press the tip of the soldering iron to the pin that is over the pad to which you added solder in the first step. Hold it there for a moment, until the solder melts
  4. Turn the board by 180 degrees, if needed lightly tweak the IC to align the pins and solder them one-at-a-time
  5. Turn the board back and solder the remaining three pins
  6. Inspect the solder joints to ensure that all pins are soldered and there are no solder bridges between them
  7. If there are bridges, use the solder wick to remove the excess solder and desolder as needed

Now you can reinstall the board back into the machine and take it for a test drive.

Conclusion

BLDC controller re-installed into the milling machine

As you can see, repairing the common transistor failure on a BLDC motor controller is not that difficult. At the time of this writing the parts are still readily available and costs less than 1/10th of the replacement board. If done right, the repaired board will be as reliable as the original. In reality it can actually be more reliable since you would be using genuine parts and will be installing them with the correct amount of heatsink compound.

2 comments :

  1. Nicely done and great write-up
    -brino

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  2. Great info!
    BTW, to remove surface mount chips, I like to use an Exacto knife. Use a new blade. Place a small (jewler's) screwdriver flat on the board. Place the point of the exacto knife on the screwdriver blade. With a scissor-like motion, tilt the exacto knife down cutting through the pins right at the body of the IC. You then just take a soldering iron and sweep off the pins that are stuck to the board.

    For soldering surface mounts with tightly spaced pins, I like to use liquid flux. It's like magic. You tack the chip onto the board with the corner pins, cover all of the pins with liquid solder then just run your iron and solder quickly over the pins. It's a bit smokey but the result is that the liquid flux causes the solder to separate giving you perfectly soldered pins. I saw that on a Sony training video 25 years ago. ;)

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