RC10Talk - The Net's Largest Vintage R/C Community

I have tried to upload a file picture but it hasn't seemed to work. Do I have to be certain level of membership before I can upload photos?

You can attach a photo from the first post. It is a bit different here. It is a 2 part affair.

1. you need to upload the file. (In the file name section find your file)

2. You need to attach it to the post. (press the add the file button)

That is it!

Don't get discouraged, try it again.

Ok seems as though I didn't reduce the file size enough. Hopefully there is a photo with this one.

Ok, I have seen what you have done. Most of your problem is because you did not remove the solder. Before you can get the part from the board you have to get as much solder away from the board as possible. You do this with using one of the following techniques: solder wick (good), solder vacuums (better), heated solder vacuum (best) like the tool I showed you in the link in a earlier post. You will need a good quality soldering iron. Not a crappy 25w job, more like a 40w, better still a 60w. Remember that these tools are a investment and they will save you lots of money not jut in RC but other things that need repair around your house. Having goods tools will make the job easier. The trace is only held on by a bit of epoxy and if you heat it up too much it will separate. All is not lost with the board. Remove as much solder as possible and press the traces back into place. It is bridge soldered there anyways so you can put the new components in and put a fat solder job and you should be good to go.

Hi urban hype,

My solder iron looks as if it is rated as 40w unit. My equipment may not be trade duty, but I don't think it's poor quality either. I think the problem is my lack of knowledge and skill/technique in this area. I can only learn by my own trial and error. I never did any electronics kits as a kid and I have had no training in any facet of this area.

I tried to remove as much solder as I could with my solder sucker and I thought I was going ok except for where the prong/leg of the FET was longer than the others and bent over on the board. I could not seem to get the solder sucker to remove the solder from under and around the long legs. I decided to try to heat these legs up and free them using the soldering iron, this is when the trace separated from the board.

If you think there is still hope, than I am prepared to continue with this. My point of view is, that if nothing else, somebody may benefit from reading this thread.

Can you make out the markings on the FET? Do you know a good place to buy new FETs from?

Thanks again for your continued help urban hype.

Sunseeker, I did not have and formal training till I was working in a electronics factory. My father taught me how to solder and take things apart when I was 6 years old. I just kept plugging away at it. Your iron is ok. The vacuum tool is the problem. They are hard to use. To use that tool you have to heat the board a lot which causes the trace to separate. If you plan to continue with electronic repair I suggest that you get the de-soldering tool that I suggested. It has a heated tip so you can take out the solder much more easily.

Can you make out the markings on the FET? Do you know a good place to buy new FETs from?

From the picture it looks like a old motorolla fet.

the G,D,S means Gate,Drain,Source that is the name of each pin.

The F721 is most likely the model number.

More likely that not that fet has been disconnected. In most cases there is an equivalent part.

As for buying these parts, I am sure that there are electronics manufacturing in OZ. Someone has to supply the parts. You can go to place like mouser but you will end up paying $30 for a $1 part because of the shipping.

I may have some sort of a solution for you but I will have to check once I get home.

Can you post a good sharp photo of the fet. I am having some trouble reading the numbers. I can see the deeper marking but the series numbers are debatable. Hopefully I can help you more.

Hi again,
I have been to a local electronics shop called "Jaycar Electronics". It's staff were quite helpful and have pointed out (as you have already said) that it is a Motorolla FET, but they do not have the product listed in there data base. I have a list of the FETs they carry. Hopefully we can use some of the ones they stock as a substitute?

I cannot seem to find the datasheet for that fet. Here is a part number that is in production. I have successfully used this replacement in my Novak T4. It may work for you.

Substitute - 70A 30V 0.022 Ω N Channel Power Mosfet - RFP70N03

It seems that there are 2 manufactures Intersil and Harvey. Harvey is the original

Please note that when ever you substitute parts there is always a possibility that you can burn the circuit. So keep that in mind if you elect to change the part.

Wow you guys pay a lot for electronic components in OZ.

Hi again,

I have a bit of bad news. I tried to replace the FETs today, it seemed to work for a few seconds but then smoked out . I think it's safe to say we've done as much as we can.

Thanks for your help urban hype if nothing else it was a good learning experience for me

Hey no problem sunseeker. I am glad you have gotten some sort of experience from this even if the result was the death of your speed control. I have had more than my fair share of electronics fires Don't let this failure put you off.

Maybe you had the same problem as me. The Ch 2 has to be reversed then both pots centered.

Typically, because of its age, I would recommend replacing all of the forward fets with all of the same new part. Reason is that all those guys are in parallel so if they aren't all the exact same one guy takes more than the others and can exceed the individual current capacity quickly leading to heat and smoke.

.