Thursday, April 23, 2015

More MOSFET nonsense. I need help.

 Help!  One of those two mosfets in the top right hand corner of that board is blown.  They say Y1A1, which appears to be a lot number, not a part number.  

How can  I identify these things?  Alternatively, can you recommend a part to put in place instead? 

Tuesday, April 21, 2015

A blown MOSFET, replacement, and saving a reciever. Losi Micro Crawlers

What a lovely truck.   But there's a story behind it.  And some broken stuff too.  

So this story starts about a month ago.  I have a Losi Micro Trail Trekker.  It's a great little truck.  It seems to be able to climb anything.  I bought it .. back in 2011 I think.  At the time, I put something like five battery packs through it.  Then.. life happened.

Years pass, I pull the thing out of storage.  I spend some time, and I get the battery packs (I have a Micro Rock Crawler too..) fixed, as from sitting one or two cells reversed.  And I go to drive my truck.  I go from reverse, to forward.. there's a puff, and the smell of escaping electrosity.  After that, the truck would only go in reverse.  I had blown one of the mosfets.

So, frustrated, I called Horizon Hobby.  (Horizon owns Losi now.)  After a little begging, they found it in their hearts to replace my ESC, in spite of being so deeply out of warranty.  Thanks a lot guys!  My truck was running again within a week.

That left me with a "dead" $50 reciever/esc package.  $50 is a lot of encouragement to fix something.  The usual rule with electronics, is you look for something that looks wrong.  The truck was happy to deliver clues.

Here's what a seriously blown mosfet looks like.  Yes, that's a hole.
That package is tiny.  But between my camera and some good lighting, I was able to identify the chips.  They're mosfets, two in each chip.  It's also a SOIC-8 package.  Roughly the smallest pin pitch I'd try soldering without a magnifying setup.  But try I shall!

Using the 4205 number I was able to identify on the chip, I tracked down the Datasheets.  The datasheets seemed sane, so I ordered a few.  Sadly, the only decent source was in china, so a month passes....   

Removing a chip like that, is a matter of getting all the pins on one side hot enough to flow.. and lifting it up one side at a time.  So I added some solder, and got to work.  The "near" side was easy.  As it's pins are all normal.  The far side was a bit more tricky.  It's behind a whole bunch of wires.
 To get access, I unsoldered the pink wire.  It was soldered to a giant blob of solder, bridging two contacts.  Which.. is ok.  As those mosfets are designed to have their center channels bridged.
 The bottom of the chip really shows how big of a failure this was.  Big ba-da-boom.  Now once the chip is removed, you need to clear out the excess solder.  Solder wick is big and difficult to deal with.  I end up using some 22ga stranded wire as my solder wick.  I pull the insulation off, and dip it in rosin.

Here is the cleaned off pads, ready for the new chip.
Oh yes, while we're at it, the MOSFET is an APM4502.  They're about $3 each.  I bought five.

Here it is, soldered back into place.  And before you ask, yes it works.  Both forward, and reverse.
 Here's the MOSFET I removed.  I love how they "blow up".
 And here's the receiver, all buttoned up and ready to go.

I'm glad it worked.  Hopefully this might give you confidence to repair your ESC if you ever smoke an IC.  

Monday, April 13, 2015

Smaller and smaller and smaller. Losi Micro Desert Truck

First, lets set the mood.  Smaller and smaller - Faith No More.

The Losi Micro Desert Truck is one the remaining trucks in the 1/36 scale lineup.  So what makes it the Desert Truck (DT) instead of a Micro-T? The DT has bigger tires, with a different tread pattern.  They look scale.  The chassis is stretched in the middle, which makes the battery compartment larger.

Yes, that's the truck sitting on my battery charger.  In spite of being small, it's a very serious truck.  Fully independent suspension, slipper clutch, classic 3 gear transmission, fully proportional everything, with a brake and reverse.


Wheelbase - 3.5"
Length - 5.5"
Front Track - 3.5"
Rear Track - 3.5"
Rear Travel - 0.5"
Front Travel - 0.39"

So, while we are on the subject of travel.  Something I've noticed while reading reviews on the truck note that "the back end sags."  The proper setup for the rear end of a buggy, truck, whatever, is dogbones level, or nearly so.  The truck sags about 3/16" of an inch.. which puts those dogbones level under acceleration.

Now... the proper way of dealing with the back end slapping the ground is damping.  Which is something that this truck is sorely lacking.  Losi appears to sell some oil filled shocks... but at a cost of nearly $30.  Well.. that'll happen soon enough.

The lengthened chassis provides a much bigger battery compartment.  There's a little block of foam in there, to keep the battery forward.  If you pull out the foam, there's room to fit the 220mah battery from the High Roller, or the Trail Trekker.  I just so happen to have a Trail Trekker, and some reasonably accurate scales.

First off, lets compare batteries:
150mah - 15.9g (I have two of these, they were within .1g of each other)
220mah - 23.2g

Empty truck - 98.0g
Truck with 150mah - 115.0g
Truck with 220mah - 121.3g

The battery is between 14 and 19% of the total weight of the vehicle.  That's, a big lump of weight to move around.  I think that might make a significant difference in the handling.

150mah Forward: 39.9f / 60.1r
150mah Rearward: 36.3f / 63.6r
220mah Only:  37.8f / 62.1r

So far, I've driven the truck with the 150mah pack forward, and the 220mah pack.  I can't say I've developed a good feel for both setups yet.

Now, the upgrades have already begun.  I found out how bad dealing the e-clips are.  So I swapped out for the threaded axles.  That's a good excuse to examine how serious of a truck this is.

Ah, the classic R/C car pose.  Topless, and no shoes.  And e-clip axles.  That is about to change, at least in this narrative.

So we'll do the hard ones first.  Now, these i'm describing as "the hard ones."  To pull the front axles, you unscrew the kingpin, and pull it out.  The kingpin retains the axle in the hub.  So once the kingpin is out, you can pull, or push out the front axle.

And the back end is a whole lot easier.  You just unscrew the upper control arm, and when you pivot the hub down, the drive cup and axle can just be pushed out the back of the hub.  Sadly, I didn't take pictures of that.  However, here's what that rear a-arm assembly looks like.

Looks just like a big r/c car.

And here we are, after the conversion.

Now having threaded axles introduces some new things to be concerned with.  Since the front hubs have no steps in them, you can over tighten the nuts and bind against the bushings in the front wheels.  The case is similar in the rear.  You want to tighten the nuts just enough that the slop goes away.  My usual method for dealing with things like this is to over tighten them just a little, then back off 1/4 turn at a time until things spin freely.

Bearings for the truck are in the mail....  And I fully intend on destroying some batteries in this thing.  I'm also trying to find reasonably priced 180mah 2s LiPos to put in the truck.  Not that it needs to be any faster... but it's r/c.  "Need" is such a dirty word...

Now that I think of it... .I don't think I actually reviewed the truck.

The Losi Micro Desert Truck, feels like a 1/10 scale stadium truck.  Everything from the backend squat, to how it likes to be under power in corners, feels just like my old LXT.  The only thing that disappoints is the lack of damping.  

Monday, April 6, 2015

That 3d printed cover. Detail shots.

Loclhst asked me for some detail shots of the RC-305 back cover I had printed for my Quanam goggles. 

Here's what I got from the local 3d printer.