Fried receiver. Is this repairable?

[groundfx]

I'm not sure if this is the best forum to post this in so let me know if there's a better place.

Anyway, because of my stupidity I managed to fry my favourite Plantraco DSP4 receiver. :( See attached pictures.

I think what happenned is that the motor output got shorted out and I'm guess that the fried IC in the lower corner is a FET? Does anyone know what FET (part#) was on there or what would be a good replacement? Also, the negative motor lead (the black wire on the lower left corner seems to be in the wrong place... I'm guess that some solder melted and it moved. If someone has a DSP4 could you post a close-up picture of the top-lower-left corner of the circuit that I could use as reference?

Also, I'm wondering... could I just remove the FET and not use the built-in ESC? Would that work?

Any help would be appreciated. Thanks.

[jeffs555]

Not familiar with that receiver, but from the pictures, it appears to just be an SO-8 n-channel mosfet. There are many of them that would probably work. Here is one that is probably has better specs than the original. http://www.mouser.com/index.cfm?&handler=data.listcategory&D=*SI4876*&terms=SI4876&Ntt=*SI4876*&Dk=1&N=0&crc=true

[zagisrule!]

Drop me your addy and I'll send some replacements free.

Catch me sometime this evening if possible, I'll be leaving on vacation for a week tommorrow (Monday).



-Matt

[zagisrule!]

Oh, and the black motor lead goes to the top leads of the burnt MOSFET...all 4 of them, the ones right above the labeling.



-Matt

[groundfx]

Not familiar with that receiver, but from the pictures, it appears to just be an SO-8 n-channel mosfet. There are many of them that would probably work. Here is one that is probably has better specs than the original. http://www.mouser.com/index.cfm?&handler=data.listcategory&D=*SI4876*&terms=SI4876&Ntt=*SI4876*&Dk=1&N=0&crc=true

Okay, thanks.

I have another question, I just looked through my parts drawer and I found that I have a couple IRF7456's. Would one of those work? Also, since originally posting, I searched rcgroups and found that apparently it originally came with a IRF7413.

I've downloaded the parts sheet for each, and I think it would work... but I'm not really sure what parameters I should be looking at. For the 7413; Vdss=30V, Rds(on)=0.011ohms. For the 7456; Vdss=20V, Rds(on)=0.0065ohms. It looks like the 7456 has less resistance but can handle fewer volts... I think 20V is plenty... I am right or is it more complicated?

Also... I figured out why it fried. The Schottky diode (1N5819) was fried/shorted. I don't have spare... but I've noticed that many speed controls don't have one... do I need one?

[jeffs555]

The IRF7456 will probably work fine. The purpose of the schottky is to protect the mosfet from the back emf pulses from the motor. Most ESC's have them, but many use surface mounted schottky's. The IRF7456 and the IRF7413 have built in diodes, so you may get by without a schottky, but the original schottky probably wouldn't have been there if it wasn't needed.

[zagisrule!]

Looks like yours will work but your RDS is fairly high so current capacity would be reduced to a few amps.

Schottky's save the FET's from inductive flyback...so yes you need them to keep your MOSFET's from premature failure. Many smaller ESC's have them built onto the PCB, but the Plantraco unit apparently uses an external unit.



-Matt

[zagisrule!]

Jeff and I were typing at the same time. The internal diodes are quite small, so external units are a still a very good idea.



-Matt

[groundfx]

Looks like yours will work but your RDS is fairly high so current capacity would be reduced to a few amps.

Schottky's save the FET's from inductive flyback...so yes you need them to keep your MOSFET's from premature failure. Many smaller ESC's have them built onto the PCB, but the Plantraco unit apparently uses an external unit.



-Matt

I'm confused... isn't the RDS lower on the 7456 than the original 7413. Doesn't that mean it can handle more amps with less heating? The original was rated up to 10amps... so won't the 7456 be better?

[zagisrule!]

Sorry, I just read the specs backwards...the new part will indeed be better.

Oops :(



-Matt

[groundfx]

I can't believe it. I fixed it! (After seeing it glow bright orange with a lot of smoke just a couple days ago :eek: I wouldn't have believed it to be possible.)

I even pushed my lucky and used it to to fly my Tiny-X around my backyard. I'll still get a Schottky diode... but I couldn't stop myself from just testing it for a second (or two.) :D

For those who might need to perform the same surgery one day, I've attached a couple of pictures. One mid surgery and one post surgery. I found that getting the old part removed to be the trickest part of the whole operation. I ended up putting on safety goggles and twisting an Xacto under the part while heating all 4 pins on one side. Watch out, the solder will fly! Keep wiggling the X-acto as the solder re-cools... that keeps the solder from re-attaching itself.

This is the first time, I've soldered SMT. I found that it was dead simple, even with the big 1/4" tip I had. Just line up the part, hold it down with a screwdriver and then heat all 4 pins on one side while pushing down. Then do the other side. I'm not afraid of SMT anymore. :)

Anyway... I hope my descriptions help someone else out someday.

[JMP_blackfoot]

I ended up putting on safety goggles and twisting an Xacto under the part while heating all 4 pins on one side. Watch out, the solder will fly! Keep wiggling the X-acto as the solder re-cools... that keeps the solder from re-attaching itself.
Well done. :cool:
I would comment that you were lucky that the copper was still attached to the PCB. I would suggest a method which works for me and is probably safer:
1-Snip each pin of the part to be removed close to the body (use fine tipped cutters).
2-When all pins have been cut, the body can be removed.
3-Then each pin can be desoldered easily.

In addition, I would advise that you clean the general area of the repair with acetone and an old toothbrush, so as to remove any solder rosin left. I have found that rosin can absorb water and cause delayed problems.

[groundfx]

JMP_blackfoot,

Thanks for the advice... I'll try your desolder technique if I have to do this again. (That said, I was fairly careful to not apply twisting pressure until the solder was molten... but your technique sounds a little less dangerous.)

Also, I was wondering if and how to remove the flux... now I know how. Thanks.

By the way, today I was lucky enough to find a local source for the exact Schottky diode I needed, so that's installed now too. All is good.

Thanks to everyone who pointed me in the right direction.