Soldering temps can be a little bit deceiving or unimportant imho. With a contact iron you can be at a perfect melting temp to tin the motor pads on an esc. But then move over to a tiny RX and it's not hot enough to properly tin the ground pad. Grounds will always be more tedious since they can dump the heat all across the board and heatsinks. It will also make less powerful irons struggle. Meanwhile when you've got enough experience you can pretty much run the iron at max temp because you're moving fast enough not to dwell too long with heat to cause damage.

With hot air some stations don't even put temps on the adjustment dial rather something like 1-10. Also the air is dictated by the size nozzle you're using or if the nozzle is a bent one. So if you're blowing at full blast on a tiny nozzle that is bent then all that air is no longer flowing all out the nozzle. Also removing a good mosfet on a donor board will need a lot less heat than a completely burned mosfet on your bad board. So there is A LOT more play and initial trial and error on hot air stations where running it at full blast all the time isn't the ideal way to run it. Also my 350 degrees might be more like 400 degrees on your kit. As is the case with the contact irons, your tip could be in worse shape, different mass or the heater is fading so that same temp isn't constant.

It's no wonder that a popular entry level iron by Hakko is about a third of their price for their tip temperature sensor device. Then again this would be for the production guys that are stuck using leaded solder that need more exacting temps. Their upper level stations even have lockouts so the guys that actually run the iron can't adjust the temp.

Alright! So, on this iFlight Beast board- I soldered the magnet wire to the new pad and attached a wire that will eventually go to video out on a cam or (most likely) to the camera pad on a vtx that has a 5v feed and a cam trace with a nice plug to go to the actual camera, with vid in, vid out, sa/tramp, 5v, and ground.

I typically run my soldering irons at full temp and do a quick in and out- I dropped my temp down to 300 for this, hoping to keep the heat transfer to the epoxy at a minimum.

I was a little concerned that the magnet wire was running so close to the uart tx pad that I’ll be using for a vtx, until I remembered that magnet wire can rub all up on itself and other connections without shorting out as long as its coating is still intact. If it couldn’t, our motors would be a big ball of smoke.

The magnet wire is missing some of its coating from too much handling with tweezers, but it’s missing coating where it will wrap around the board from top to bottom without touching any exposed bits, so all should be good.

I tugged pretty hard on the wire I attached to the new vid in pad- it’s solid!

Checked the resistance- 89 ohms to ground. Little higher than I was expecting. Hrmm. Touched my probe tips together- they read 13 ohms. Apparently they weren’t quite screwed down all the way. Anyhow, 89 ohms minus 13 ohms puts us right where we want to be.

Since the epoxy on the replacement pad is holding well, I decided to use UV glue to hold the magnet wire in place- mainly because uv glue is easily removable in case I need to redo anything.

It’s currently curing under my ridiculously overpowered UV light. After that, I think it’s good to go.
Is it a professional, neat, clean repair? No. Is it a solid repair that I am comfortable putting into the air?  Yes!

So, on to the next.

I’ve got two JHEMCU GH743AIO 40A boards. I know from iFly4rotors (thanks dude!) that these both have the same symptoms- no voltage to the 5v pad and one toasted esc.

The esc’s are using JMSL0302AU mosfets:
https://taoic.oss-cn-hangzhou.aliyuncs.c...549000.pdf

I think Infineon IRFH5300 would be an appropriate substitute?
https://www.digikey.com/en/htmldatasheet...00pbf.html

But, I can’t find them in stock from a reputable US seller.

Okay, one of these boards may donate its FETS to give the other life until I can find a good supplier.

On to the other problem- we need to figure out what’s wrong with the 5v rail.

Monday evening we will give it some bench power (12v) and do some probing to see what is and isn’t getting the voltage it should.

Thats one large UV light source

How long can you continuously run it? Most UV lights have an auto shut of timer for about 10-15 seconds.

The entire back is one large heat sink.
I’ve ran it for a little under two hours straight when I got distracted with another project and forgot to unplug it- was still comfortably warm enough to touch without burning my hand.

Edit- This is it, in case anyone is in the market for a cheap overpowered UV light:
https://smile.amazon.com/gp/aw/d/B07JFG6...asin_title
I’ve got the 20w slim waterproof version, in case I spill my drink on it?

Double checked things on the iFlight Beast board- I’m confident that it is airworthy.

I did not start on the two JHEMCU aio FC’s. We’ll do some probing Wednesday evening and see where they’re at.

I decided to fix an easier(?) thing.
This is a JHEMCU VTX20-600. It lost its mmcx connector.

It also lost its center pin pad.

Magnifying goggle time!

Started with the easy bit- pre-tinned the ground pads. Went a little heavy on the solder. Smothered it in more flux. Took hot air to it- it attached nice and solid.

Okay, time to deal with the center pin.

What the hell is that little component I put a red semicircle around? It’s got the coloring of an smd capacitor, but with a little  square of black like it wants to be a resistor, and it seems to be inline with the center pin and connected to ground.

The last vtx that I repaired had one cap inline with the signal/positive antenna wire and another that branched off to ground- so I’m gonna assume that it is a cap, and that my center pin should connect to both it (opposite its ground) and the near side of the cap that is in line with what I think is my positive/signal.

Hrmm. Y’know, those bits are pretty close. I could probably just bridge them.

Bridged them, but then I decided, no, that’s not a solid connection, I should use some solid core copper wire to make a good solid connection. I’m real good at over complicating things, it’s one of my skills.

Over complicating skillz activate! Four tries later and it is connected with a tiny bit of copper, and is probably just as solid of a connection as it was when I initially bridged it- probably not any stronger.

Maybe this is good to go? I need to probe a known good vtx and see what it reads from center pin to ground and whatnot and compare it before I put power through it. I don’t trust myself enough tonight to say yeah, it’s good, put power through it.

If you or someone has a spare JHEMCU VTX20-600 then probing for continuity will be give us the solid answer.

I do not have another of these boards.

I was thinking that I could just probe any other vtx, but then I remembered that vtx’s are actually tuned- I don’t know enough about circuit design to know if that is going to change the resistance to ground.

I suppose I could just probe a few and see if they’re the same, and if not, at least get an average to tell me if I’m in the ballpark.

And given that it’s really that center pin cap to ground that concerns me, I should probably do the probing with an esr/lcr meter rather than a multimeter.

I’m grounded until a replacement switch set arrives for my transmitter, and my main current build is frustrating me- so, it was a good night to focus on individual boards.

It’s time to check out these two JHEMCU GH743AIO 40A boards.

The previous owner was kind enough to send both of these boards as well as a number of their other broken bits to me to play with, attempt to repair, or salvage for parts (as well as some non-broken bits).

The original owner reported that they both had the same problems- no power to the 5v rail and one bad esc, double checked by swapping a different motor.

First, we need to clean things up so that we can see what we’re looking at.
Removed all wires that were soldered to the boards.
Smothered those pads with flux paste and hit them with a clean dry (no solder added after cleaning the iron tip off in a copper scrubbing pad) soldering iron to remove any excess solder and leave nice, clean, tinned pads.

I probably sound like a broken record, because every time someone pops onto this forum and says, “hey, I’m having trouble soldering, help!” I say the same thing- get good solder and get good flux paste... My hands shake and I’m twitchy. Earlier tonight my glasses fell off of my face and hit my soldering iron mid-solder- I ended up with a big streak of solder right across a number of pins and pads. If I couldn’t just slather some flux paste across a bunch of pads and pins that I accidentally bridged and then just drag a clean iron across them to clean them up, I wouldn’t be able to work on these tiny boards. I’m a big fan of MG 8341 flux paste. It’s goopy enough for me to blob it on where I want it, and as soon as I hit it with a soldering iron or heat gun it spreads out nicely and doesn’t burn off before I’m done soldering. It’s supposed to be “no clean”, but I put it on heavy enough that it does require a scrub down with rubbing alcohol after. Anyhow.

Scrubbed both sides of each board down with 99% isopropyl alcohol and a toothbrush.
Let them dry while I smoked a cigarette (away from the super flammable alcohol).

Okay! Boards are cleaned up. Time to see what we’re dealing with. Need to inspect both boards with magnification to look for obviously burned bits, stray solder blobs, etc.
For me, that means a cheap $10 clip on macro lens attached to my phone. One of these days I’ll dig out an old hd webcam or digital camera, and 3d print a mount for that cheap macro lens, and throw together some sort of adjustable clamp type thing so that I can see a magnified live video image on my laptop- but for now, my phone works just fine for close up inspection/pictures, and my magnifying headset over my glasses works well for actual board work.

The original owner used UV glue to reinforce the usb jack. I appreciate that for two reasons:
1. Reinforcing the usb jack is a very good thing.
2. UV glue is easily removable with a pair of fine point tweezers. It’s flexible enough to grab a bit and pull, and it holds well enough to reinforce things, but it doesn’t typically rip off any smd components when you remove it.

Time to do some probing.

Board #1:
Checked all motor pads for continuity to ground. Motor pad closest to the USB port is shorted to ground.

Awesome, we found the bad esc! Let’s remove that top mosfet.
How did I know to remove the mosfet on the top of the board rather than the one on the bottom?
Skillz and intuition! Just kidding. I’m using very fine tip multimeter probes- in continuity mode, with one probe touching ground, as I dragged the other probe along the backside (side opposite of motor pads) of that mosfet, it went, “beep” on all four pins. All of the other mosfets give me 3 beeps and one pin with no continuity.

Had to crank up the heat on the hot air to remove that mosfet. If you look at the picture below of it removed, you can see why- that big square of solder is the “drain” of the mosfet. It’s what is going to the load- in our case that is the motor lead.

Okay, removed that mosfet- checked for continuity to ground again, and there is none. Great! This board needs a new JMSL0302AU or similar mosfet.

Board #2:
No shorts to ground from any motor pad. Does this mean that all mosfets are good? No. Previous owner was running motors that were near, but not over the max rating for this board. If I were to stress test this board by putting a similar load on it, I’d be willing to bet that a mosfet would fail- or at the very least, one would increase in temperature much higher or faster than the others. Because I can’t yet determine what is bad, I trust this board less than I do the one where we’ve identified what went bad.

Alright, it’s time to put power to these boards and start probing! Yay, fun!!!

Board #1 with the bad mosfet removed and board #2 are both get the voltages they should. I’m powering from a 13.8v dc bench power supply. 
3.3v pad is good. 10v pad is good.
5v pad is…. Good! I’m reading 5.002v on one board and 4.83v on the other- both are within tolerances for something rated for 5v. Traced the 5v power circuit back and everything is testing as it should. Disconnected bench power supply and plugged in usb power to both boards- all is well.

Okay!
So, what were the initial problems and what are the solutions?

Burned out esc:
With both boards, JHEMCU either over rated their esc’s or got a bad batch of mosfets. The original owner was within their ratings, and I know how he flies- he wasn’t running wide open throttle prior to having issues.
Board #1 needs a new mosfet- we’ve already identified and removed the bad one.
Board #2 will also need a new mosfet, but we need to stress test it to identify which one is starting to fail.

No power to 5v rail:
5v rail is getting power on both boards, so we need to look at other potential problems.
Both of these boards were installed at different times on the same build.
So, what are other potential and/or likely problems?

1: The combined amperage draw of all attached peripherals was higher than the 5v rail could provide- that would typically end up in a meltdown of some sort- a failure of the voltage regulator or related circuit. This isn’t the case, because we are getting the expected 5v.

2: As above, combined amp draw was more than the 5v rail could provide, but things were powered up in an order that managed to somehow not blow the 5v circuit, but also not provide enough juice to fully power up all attached peripherals.

3: Most likely- one of the attached peripherals was faulty. Without being able to test the installed camera(s), cam switcher, dvr, vtx, or rx, we can’t determine which component was faulty. 

Side note: These boards are not laid out well. There’s really no excuse for that row of pads behind the “dji” jack to not be in a more easily accessible place.
Edit- I’m gonna hafta eat my words about that bad board layout if it turns out that those solder points I just noticed in front of that jack line up with those pads I was complaining about.

Ok, I haven't been getting notications on this thread {my fault for not adding a post},
so I am a little bit behind.

Hi Sevro,

Yeah, I use a HOT iron all the time. The adjustment wheel on my iron is taped at
425 C degrees {how accurate, I don't know, but it does get HOT}. You know, I have
tried variuos tips, but the one that works the best for me is the medium conical, pencil,
tip which I use for everything. Since it does go down to a point, I can solder small pads
as well as larger things. For heavier wire and battery leads, I just lay the thing on its
side with about 4-6 mm of the tip on the pad or wire and maybe a solder bridge. I
don't have any problems with the larger stuff either.

Hi Lemony,

Ok, now I should get notificattions. By the way, thanks for the PM update.

I don't know where you are with the JHEMCU VTX, but I do have another one which
is currently mounted in the Rescue Quad. PM me some specifics on how to get the
information that you need since I don't use multi-meters very often.

Regarding the JHEMCU FC boards, it is good to know that the 5V regulator on those
boards is good. There could have just been too much stuff on the 5V rail. I did use an 
external 5V BEC and everything was fine. Also, I am now skeptical about the XM+ receiver 
that I was using so I will be using a different receiver. I was thinking about an R9MM, but 
would need to get one out of an existing quad...or...try a different XM+...just to see what 
happens. 

I am glad that you are able to repair the mosfets on those boards. 
If all goes well, then you will have two more FCs for more builds.   

I am really excited that you are able to fix those things    as I am not. 

As I said when I started this thread, it is not intended to be a tutorial of any kind- I don’t know what I’m doing half the time. I’m learning as I go. My goal for this thread is to chronicle my smd repair journey, for others to follow along with my adventures and mishaps, and for those who are more experienced to hopefully chime in and correct me when I’m wrong and/or give me some guidance when they can.

By all means, please correct me if I'm wrong, or explain it in less words than I am if possible.

Bear with me, this may get a bit convoluted. I don’t have a schematic of a flight controller or an esc- if I did, it would certainly help me wrap my head around things. Since I don't, I'm trying to logic my way through it.

I’ve been thinking about my probing and how I identified the bad mosfet a couple posts back. I was able to identify the bad one because drain had failed shorted to gate? I was only testing for shorts to ground. 

A mosfet has 3 leads- A gate, a source, and a drain. The gate is like Gandalf- it says to the source, "you may not pass" or "you may pass" depending on the voltage the gate is being fed. If the gate says, "you may pass", then power can flow from the source to the drain. Do I have that backwards?

We've got 3 motor pads per motor. To me, that makes me think that they must be reversible polarity. We can't have them all only be able to pass to positive or all be able to pass to negative without a way to switch them, right? From what I remember about mosfets from audio amplifiers, mosfets have a built in diode- which means they are one way unless they are dual-channel (two in one package). These aren't dual channel.

Okay, so that would explain why there is two mosfets per motor pad. I'd expect the second of the pair to be installed in the opposite direction- because this would mean that depending on which of the two mosfets was allowing power to pass, that motor pad could be ground or positive.

Checked the board- the bottom mosfets are installed in the opposite direction. Okay, maybe this is starting to make sense, at least to me.

With the mosfets on the top of the board, source was connected to ground. So with the mosfets on the bottom of the board, I should expect one side of it to be connected to positive. Source? no, that can't be right, because then whenever the top mosfets were allowing passage, it'd be a dead short. Drain? I'm not sure. But I think one side of the bottom mosfets should be connected to positive. I'll need to do some probing.

I was probing with a probe only connected to ground. I think I also need to switch that up and probe with one connected to positive. Pretty sure if I do that I'll probably find which mosfet on the other board is bad, because I'm wiling to bet that on that one it failed with a motor pad shorted to positive.

Maybe. Some more probing will happen later tonight, after I fix my tx16s and get some flights in.


LCSC has the JMSL0302AU mosfets for around $0.50 each. Might take a while to get here from China.

https://www.lcsc.com/product-detail/MOSFETs_Jiangsu-JieJie-Microelectronics-JMSL0302AU_C2890409.html

Hi Lemony,

Attached is a picture of the JHEMCU 600mW 20x20 MMCX VTX
without the metal plate. 

After doing some googling, I found a 6s 30A ST ESC reference design based off of a stm32f303, L6398 gate drivers, and STL160NS3LLH7 mosfets.
https://www.mouser.com/datasheet/2/389/e...107609.pdf

No, this is not the same esc design as these JHEMCU boards I'm working on, but, it does (or will) give me a much better understanding of an esc circuit.

I went back and probed both boards to see if there was a short to positive- no.

So, I've got one board with a known bad and removed mosfet- replacement mosfet cost about 50 cents and is on its way from China, along with a few spares.

The other board- doesn't have a mosfet that is obviously bad in circuit, unless I'm testing wrong.

So, I'm going back to my original plan of stress testing that board to identify the failing mosfet.

I'm going to think this through over the next few days before I try anything- if what I'm about to propose is a bad idea and there is a better way to do things, please let me know. My plan at this point is to pick a sacrificial motor- probably an Xing 2207.5 2555kv because I have two spares for a 5" quad that I rarely fly. I think I'll wire that up to one set of motor pads at a time, connect a watt meter in line with my power supply, and run up the motor. 

My thinking here is that either one mosfet will properly fail, or if not, I'll be able to see how much power was being pulled at each motor position and if one is significantly different than the others, at least I know where to focus- and if all work as they should and measure as they should, then I can safely assume that the board is okay to fly.

Does that sound like a good plan?

I’ve dealt with and reached out to a number of manufacturers trying to get information on a component- from back in my IT days to now with quad bits.

Dealing with a manufacturer and trying to get accurate information on something is usually like pulling teeth… (Hey JHEMCU, I’m still waiting for a response to my email, even though I’ve already figured out what those bits I asked you about are.)

But dude, Foxeer is awesome. I’ve had a burned out Foxeer Toothless 2 Micro on my shelf for a while now. I haven’t properly probed it to see what’s wrong, but based on the melted bit of the back plate (circled in red), it’s that inductor (also circled in red).

I reached out to Foxeer earlier this evening to ask what the value of that inductor was- I got a response within an hour. They not only told me the value, they also told me the tolerance and straight up gave me a manufacturer’s part number.

The response I got was:
inductor/22uH/±20%/3015/0.52A/SWPA3015S220MT
Best regards

Dude. That makes life so much easier. With that kind of customer service, I’ll buy Foxeer bits anytime they make a thing I need. (Yeah, no affiliation. I don’t get free stuff and I’m not sponsored by anyone. I’m just really impressed with their tech support.)

Without probing and solely based on the melted back panel bit, this is what I need:

https://www.lcsc.com/product-detail/Powe...83438.html
It’s about $0.09 each, with a minimum order of ten- so, one dollar, with 9 spares.

Most likely I’ll also need the rest of that power supply section (voltage regulator, diode, maybe a resistor or cap or two), but that inductor was the only thing I couldn’t easily identify the value of- and if I need to know what another bit is, apparently they are are happy to tell me. So, props to them.

I’ve got a hell of a tremor in my hands, so I needed a hands free way to do at least a two point probe with my lcr/esr meter to measure capacitors and whatnot in circuit, as well as be able to measure smd caps and resistors that I’ve removed.

I’m all about using what you’ve got, or making what you have work for you.

So, say hello to my removable mini banana plug tip tweezer probes. This would work better with “normally closed” tweezers, but I don’t have those. I do have rubber bands.

Ideally, I’d put proper male banana plugs on the wire end so that I can also plug them into my multimeter, frequency generator, etc.- but, my esr/lcr meter has this little lever action dip pin socket, and I’d have to open it up and wire in some female banana plugs to make it work. I probably will, but not tonight. In the mean time, it’s got little pin connectors similar to my tweezer ends on the far side of the wires.

The replacement mosfets for the JHEMCU GH743 AIO boards arrived at my local post office from China yesterday, but they didn't get delivered today. Boo.

That's alright. I've identified and removed the dead mosfet from one of these boards, but I still need to stress test the other.

I decided to use an Xing 2207.5 2555kv motor for stress testing. I decided on this motor for a couple of reasons. With a 5" prop on 4s, it pulls right around 40A, which is what these boards are rated for max. Also, I have two spares of these motors for my 5" Source One build- I rarely fly that build because it's just too big for my property, and when I do fly it, I'm not pushing it to the limit because of its size, so I'm unlikely to burn out a motor, and if I somehow did, I'm good with only one spare.

I mounted it to some random arm. I'm not sure where it came from, it doesn't match any of my builds.

Other than that Source One build, none of my builds pull more than around 25A max, so I don't actually need these boards to pull 40A- but that is what they're rated for, and if a mosfet is going to fail, I want it to fail on my bench and not in the air, so that is what I am testing them for.

This isn’t a “proper” stress test, because I’m not pushing it until it fails- I just want to push it to its rated limit and make sure it can handle what I’m likely to put through it.

Anyhow. On the board that doesn't have any mosfets that test bad:
I plugged in and made sure all esc's were present and readable- yes! I went ahead and flashed Bluejay, because that's what I'll be flying on.
I wired up my sacrificial stress test motor to one set of motor pads. Ran it up to 100%. My watt meter showed that it was only pulling around 3 to 4 amps. That can't be right. Wired it into another set of motor pads, same result.

Derp. I was running the motor with no load (no prop). I was also powering it off of my 13.8vdc bench power supply that is rated for 2.5A constant, 3A surge.

I put a Gemfan Hurricane 51433-3 prop on the motor, and switched to a proper 4s battery for power.

Alright, now I'm pulling 39+ amps when I run the motor up to full throttle.

Swapped the motor around to each position- Ack! One sounds horrible! No wait, never mind, that noise was because it was spinning the prop backwards.

Well hell, this board can handle multiple stationary motor runups to 100%, each held for 5+ seconds, pulling around 40A.

I think I'm comfortable putting this board in a bird that is going to pull 25A max at 100% throttle. Awesome. I’ll need to properly solder the main power wires on, I just tacked them down quickly to run some tests.

Hopefully by this weekend the replacement mosfets will arrive and I'll be able to get the other GH743 board airworthy.