Subject:
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RE: RCX Dead??
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 18 Feb 2002 20:22:30 GMT
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Original-From:
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Kingsley Gifford <kgifford@thezoneNOMORESPAM.net>
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Viewed:
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910 times
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I applied power to the last capacitor towards the back...and it works!!!
Should I connect to this capacitor using a fuse or anything first or is
it safe to solder leads directly from this capacitor?
thanks
-----Original Message-----
From: news-gateway@lugnet.com [mailto:news-gateway@lugnet.com] On Behalf
Of Matthias Jetleb
Sent: Sunday, February 17, 2002 4:13 AM
To: lego-robotics@crynwr.com
Subject: Re: RCX Dead??
You didn't indicate whether it is an old-style 1.0 RCX (external
adapter) or not, so I'll asume it's a batteries-only unit.
First of all, do you have any experience with electronics?
The first thing I'd be looking at after a mechanical shock would be
the tabs for the batteries. Make sure they are all making contact with
the batteries, and if not, bend them outward slightly. If you have a
power supply (9 - 12V DC), connect the positive and negative terminals
appropriately and see if you have life. If there is, it's the tabs
that connect to the batteries. If not, then you'll have to take the
unit apart.
With the cover off, you'll observe a large (usually dark blue or black
- could be grey) cylindrical object near the back of the unit (treat
the IR diodes as being at the front of the RCX). This is an
electrolytic capacitor and there will be a stripe down one of it's
sides alongside one of the terminals. This is the capacitor's negative
terminal. Connect a power supply (or a group of batteries) to the
positive and negative terminals of the capacitor and try switching the
RCX on. You can do this by removing one of the buttons from the RCX
lid and pressing the bottom of it against the "power" switch pad (near
the lower left corner of the display). If it works now, you either
have a bad connection to the battery clips of the RCX (the tabs at the
front-right and rear-left corners of the circuit board) or I'd be
wondering about the fuse* again. If it's still dead, measure the
voltage at pin 3 of the LM7805M voltage regulator (a three terminal
component to the left of the capacitor and up closer to the power
switch). It should be 5V. If it is, and the unit is still dead, you're
in trouble - you've probably got a cracked circuit board or one of the
soldered connections, most likely on the MPU itself has come off.
The last ditch effort would be to remove the circuit board from the
battery case, hook up an external supply to either the capacitor leads
or the battery box contact points and try gently flexing the RCX
circuit board while pushing the power button (and whistling dixie).
The best this will confirm for you (if it works at all) is that you
have a cracked board. I once repaired a cordless phone that had this
problem. Once I found the crack by flexing the board and looking
*very* closely (we're talking thinner-than-hairline here) I was able
to scrape the insulation of the trace on either side of the crack and
solder wire-wrap wire (the thinnest wire I could find at the time)
across it. It was hard enough fixing the phone - I'd hate to be the
one to have to do it on the RCX.
*With regard to my comment about it being the "fuse again", my RCX 2.0
from an RIS 1.5 just plain died on me while my son was playing with it
(actually just turning it on and off to listen to the beep) without
even dropping it. I didn't have a schematic of the RCX (still don't)
so I didn't feel too comfortable poking around in it, other than to
check battery connections. Without a schematic, I wasn't sure how the
capacitor was connected, and whether it would be safe to apply power
to it. I called Lego's Mindstorm's technical support and reported my
problem. It turns out, they don't take them back for repairs (even
under warranty - 1 Year BTW) and they can't/won't supply a schematic
for the thing either. Instead they simply replaced the unit, free of
charge and without requiring the old one to be sent back. Since I
still had the old one after getting a replacement, I decided I
couldn't kill it any more that it already was, so I went through it
with a meter and traced the power supply route. That's how I
determined that it was safe to apply power to the capacitor. When that
worked, it was simply a matter of tracing back from the capacitor to
the positive battery terminal wereupon I found a dead fuse (at least I
assume it's a fuse - I can't see what else it could be) along the way.
The device said "1.5" on it, so I presumed that represented 1.5 Amps.
I simply jumpered it and it's been working ever since. Frankly, I
don't understand why they'd have a fuse in there in the first place.
The motor controllers are protected against shorts, and will shut down
if the load gets too high. I've yet to see anyone start a fire with 6
AA batteries and I don't see the point in trying to protect the RCX
from an overload if you end up having to throw the unit out after one
occurs anyway since you can't replace the fuse yourself (easily) and
Lego doesn't service it. Whether the RCX is destroyed by an overload
or a fuse blows to protect it, it's all the same same in the long run.
I should also point out that the unit died with no load being applied
(except the speaker and LCD). This was very weird. I can't see it
drawing more than a few mA on power-up. Certainly not enough to blow
the fuse. My son was sitting beside me when it happened, so I know he
didn't do anything to it. That's what makes me wonder how flakey this
component actually is, and if it isn't a problem for you too.
Matthias Jetleb
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Message is in Reply To:
 | | Re: RCX Dead??
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| You didn't indicate whether it is an old-style 1.0 RCX (external adapter) or not, so I'll asume it's a batteries-only unit. First of all, do you have any experience with electronics? The first thing I'd be looking at after a mechanical shock would (...) (23 years ago, 17-Feb-02, to lugnet.robotics)
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