Newb here with dead MOSFET + CA glue = toasted PCB and repair after 8 years :( - Page 1
Forum section: electronics / beginners

author: KoroKoro
author_rank: Newbie
timestamp: December 02, 2014, 02:48:21 pm
content: I hope I am posting in the right section... and I hope my plea for help isn't breaking the forum rules but here I go...Back it 2006, I spent a large portion of my teenage savings on an expensive piece of radio control hardware. $575.85 was not chump change for an 18 year old.Quite literally, the week after the warranty expired, the magic smoke came outand 8 years later, I think I now have the skills to take on repairing the poor thing to restore some kind of functionality.The piece of hardware in question was the GTB2 made by Novak in the USA.I have since stripped it down, but it has been a struggle to take photos to show the detail, so I will upload an HD youtube video tomorrow. There are some things I will need help with such as component identification, verifying component damage and help choosing a new set of replacement MOSFETS. The part I am unsure about is with switching speeds. (The device apparently works from 1.5kHz to 11kHz.)Funnily enough, many years ago, I blew up another speed controller device but fortunately, it is much more simple in design. Again, I will post up a video and detailed photos when I have better light.Anyway, glad to be on board the EEVblog forums. From the video blogs I've been watching, it seems you lot should really know your stuff.So I'm looking forward to learning a lot

author: marshallh
author_rank: Unknown
timestamp: December 02, 2014, 08:59:52 pm
content: Wow, I remember that, and the Super Sport. As well as the Hacker brushless system. Back in their infancy. Now they are everywhere and beat the snot out of any brushed motor.

author: KoroKoro
author_rank: Newbie
timestamp: December 03, 2014, 12:07:07 pm
content: Thanks for the replyActually my old friend from across the road had a 10.5T Super Sport which which was what introduced me to the Novak brand. I thought they were great in terms of performance. I myself got a 4.5T motor and was pretty amazed at the power. After a few years had passed after it blew up, I was a little reluctant to get a replacement system. Luckily they became ALOT cheaper then. The new 3.5T motor is just insane.Anyway, I took some video in 1080p. I just uploaded it so it may take a few hours before it gets the 1080p setting. If you guys need, I can sort through some of the photos I took prior to cleaning the PCBs to show the amount of soot I cleaned off... or if the video quality is no good.Sorry in advance if I ramble. Thanks for reading/watching.Novak GTBSpeed Attack II

author: marshallh
author_rank: Unknown
timestamp: December 03, 2014, 07:12:27 pm
content: Looks like its the same "blown MOSFET vaporized the PCB" issue as the guy in the "Toasted 7950" thread.It's up to you if you think you can repair the PCB and put on new mosfets (if you can source them)

author: KoroKoro
author_rank: Newbie
timestamp: December 04, 2014, 02:02:34 am
content: Thanks for the infoI probed those resistors and they look to be fine.I will look into buying new mosfet controllers. I have a feeling that they are what caused the melt down in the first place. Quick question though - How do the fumes of CA glue affect these sealed chips. Not only that, but the case is water proof. Coincidence perhaps?As for sourcing the mosfets, I think that it would be impossible to solder on the surface mount types. Too fiddly I think.Those mosfets are these.http://www.irf.com/product-info/datasheets/data/irf6691.pdfI don't know if this is a good method but I just added the timings together (23-95-25-10 @ 180A continuous) and compared that with how many cycles the unit does. I guess if I wanted to get 1/100th throttle control, the chips would need to be a hundred times faster than the operating frequency no?The speed attack II uses these mosfets. (49A continuous 12-60-44-45 delay times)http://www.irf.com/product-info/datasheets/data/irfz44n.pdfAnd I was wondering if it would be ok to substitute with this kind:http://www.jaycar.com.au/products_uploaded/zt-2468.pdfWith 169A of current, I assume that this would then be able to run very high powered motors, correct?After all that is over, my next challenge would be working out how to mount adequate heat sinks to the tightly packed sets of mosfets. There doesn't seem to be anything on the market designed for attaching to them when packed in the vertical position.

author: KoroKoro
author_rank: Newbie
timestamp: December 04, 2014, 11:28:56 pm
content: Sorry, am I asking too many questions? (I know, it's another question...)

author: KoroKoro
author_rank: Newbie
timestamp: December 05, 2014, 12:28:34 am
content: Ah, now we are cooking with gas!I just naturally assumed that they would all operate much the same. To be honest, I don't have the experience to even look for that. Its all a learning experience I guess.This might be a silly question but... is there some kind of online component selector or something, i.e. something you can enter design parameters and it spits out possible components that would be suitable? Or maybe some kind of spreadsheet or database that can sort through device specifications?I'm only a mechanical engineer, but we have charts n things when we do pump selection and things like that. Is there anything similar in the electrical world?Thanks so much for your help so far

author: SL4P
author_rank: Super Contributor
timestamp: December 05, 2014, 01:08:34 am
content: You're certainly on the right track so far.Withliterally nothing to lose- you may be able to fire up the controller PCB inisolation, and check for drive pulses etc with a scope on the interboard connection pins.That will tell you if the CPU (probably) and drivers (maybe) are still ok.That's a big indicator as to how far you might want top follow through on more repairs.If the controller PCB is still working OK (or can be fixed with simple driver replacements)Then you can decide whether to rebuild, replace or fabricate your own power/output (MOSFET) board.It looks like it's worth some effort, but at those currents - it will emit more white smoke if you don't handle, mount, isolate, and cool the power components properly from end-to-end.

author: KoroKoro
author_rank: Newbie
timestamp: December 05, 2014, 02:04:04 pm
content: Good news every one.I powered on the main cpu chip and she boots, takes a signal and programs.One thing I noticed is that one of the green LEDs (next to the red one) is dead. strangeBut the bad news is, I am a lazy buffoon as I should have at least bare minimum soldered the power wires in... because you guessed it, the positive lead fell out and shorted on something. Plus I was stupid enough to use my over powered lipo battery which is 8.5v and can give 208 amps (40C at 5200mAh).From what I can see externally, the only damage was to that red led circuit... but unfortunately within the chip. The resistor and LED are fine... but the chip seems to no longer allow the same amount of current through. Bugger that was stupid of meSome one hit me over the head! >.<Tomorrow I plan to put my cheap chinese usb scope on the mosfet controllers and see what signals are coming through.

author: KoroKoro
author_rank: Newbie
timestamp: December 06, 2014, 11:01:04 am
content: I'm really not having much luck with thisI installed a new LED, soldered up the wires so no possibility of any shorting and all was going well... But for some strange reason, while playing with programming the cpu, there were a few features I wasn't able to access, turns out Novak included the wrong programming sheet, so the options were all different.So I pulled out my scope and I started probing things... the scope was working fine... but then all of a sudden, something shorted on the board with a zap, an LED blew and some other thing on the reverse side of the PCB had its magic smoke escapeNot sure what it was but it's marked with: A6 then a small sideways:2 Looks like a diode but I am not sure >.<Turned out that all this was caused by a faulty scope probe... it was shorting trough the probe and out the little alligator clip. Seriously chinese stuff is the worst.So I replaced the LED and fed it power from the fan power... and removed those mosfet drivers as they were making the board hot to the touch. Now, when powered on, the board only draws 0.09 amps when idling. I set the device to output a drive signal at 1.5kHz (My scope can only handle 8kHz max) made my own scope probe and took this reading being output directly from the cpu chip.Though what seemed strange to me is that not all phases were outputting a signal, but I am not sure if this is because the motor is unable to spin and knows it from the motor sensor harness).I looked online for a new cpu chip and finally something goes my way. $19 USD for a new MC68HC908MR8CFA FREECAL QFP32 ,original,produced in 2005.The only thing I guess I need to worry about is flashing the chip as my guess is they probably come blank? I have an Altera USB Blaster flashing tool, but no idea if it can be used or not...All of this is uncharted territory, so learning quick

author: SL4P
author_rank: Super Contributor
timestamp: December 06, 2014, 11:10:03 am
content: I don't want to sound negative - but two rounds of bad luck on top of a faulty power module - mayput this into the ''very unlikely to see a career revival''category.Round 1was probably a new power boardRound 2might have left you with a dead LED driver circuitRound 3really doesn't sound like you should help him up off the mat.Perhaps he can become a sparring partner or training assistant (i.e. odd parts for future repair)I know your time is 'free', but the effort for a very likely disappointment may not be worth it - even at that price.Good luck - still listening.You've learned a lot - maybe think about designing a completely new module from scratch- schematic, PCB, custom code for your purposes. Sell them and make some $cratch.