Replacement capacitor, another thread on this, sorry..

Discussion in 'General Electronics Chat' started by hspalm, Jul 14, 2010.

  1. hspalm

    Thread Starter Active Member

    Feb 17, 2010
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    Hi all
    I have a 2 channel transmitter/receiver set for RC hobbies which worked fine until yesterday, when I think I managed to mount the battery the wrong way on the receiver. The brand is "Brilliant electronics" and the receiver model number is BRC-23IRL (or BRC-231RL) which I cannot find anything about on google.

    Anyways, I let the smoke out of a capacitor, which is now impossible for me to identify. Will this just be a guessing game?

    The crystal that sits next to it is a 27.045MHz.

    Tried to take some photos, this was the best I got
    http://hpstuff.no/bilgalleri/IMAG0017.jpg

    When looking at the underside of the PCB, I see that flux has overheated on the + and - pins of the power supply, and on the pins of the XTAL socket. I think the overheated socket pins was there from the factory, as they have used lots of solder on those pins compared to the other components.
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    .....ouch.....

    You blew that sucker away.

    It is probably a .33uF for the crystal.

    Can you get a pic of the bottom? I would like to see where the traces go for that cap.
     
  3. hspalm

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    Feb 17, 2010
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  4. jpanhalt

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    Looks like a tantalum electrolytic capacitor. They do flame when exposed to reverse voltage. If you insert a new one, you have to watch polarity. With tantalums, the POSITIVE lead is marked, as opposed to aluminum electrolytics (can type) where the negative lead is marked.

    John
     
  5. hspalm

    Thread Starter Active Member

    Feb 17, 2010
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    jpanhalt, thank you also for your answer. Can you tell the purpose of this capacitor, or maybe make a guess? Also, how accurate must the replacement value be? If capacitor is 0.33uF, can I replace it with 0.22uF? Or 0.47uF? Which are the only values I have similar to .33uF.
     
  6. jpanhalt

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    Try it. The tolerance on such capacitors is quite large. Be sure to get the polarity right!

    John
     
  7. hspalm

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    Looks as if my capacitors are not tantulum type, as they are not polarized. I guess they wont work then?
     
  8. jpanhalt

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    Yours are probably mylar/polyester and are probably a bit larger than the one that blew. I would go ahead and try one temporarily. It won't hurt anything more than is likely already, and it might just work.

    Be sure to get before and after pic's, so if something else happens, we will have more information to base our guesses on. :D

    John
     
  9. retched

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    Remember to use a BLAST SHIELD when experimenting.


    ;) Just kidding.

    The .47 should do the trick. Remember to duck when you power it up...

    ..and take pictures...or video :D
     
  10. hspalm

    Thread Starter Active Member

    Feb 17, 2010
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    Thank you all for your confidence in me experimenting with electronics :D
    I was actually not sure whether you were kidding about all this before/after pics and videotaping stuff, so I did videotape the startup. But nothing blew up, so I connected a servo to both the outputs and turned on the transmitter also.

    What happens is that when I turn on the power for the receiver, the servo starts jiggling a bit. When I turn on the transmitter the servo re-position. As this is an old 2-channel system I found in the trash, I do not wish to spend a whole lot of time trying to fix this, I have a more modern 6 channel system available. So my last question on this to you guys, are
    Is it possible to say what is the most likely thing that have happened here? Is it just the fried capacitor, or may it just as well be another component on the board? You have been a big help, thanks a bunch.
     
  11. jpanhalt

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    That is normal when there is no signal from the transmitter. In fact, in some systems, the servos will swing to the stops when there is no signal. Thus, it is usually a good idea to turn the Tx on first. No harm here, though.


    Again, that is normal too. The BIG question is whether when you jiggle the Tx controls does the servo move to a new position? If yes, then the Rx may be OK. If not, further trouble shooting is needed. To start that, what is the behavior with the new capacitor removed? Can you see any evidence whether one of the PCB tracks on the back side got damaged in the accident?

    It is hard to know. If you look at your 6 channel receiver, probably near the servo connection pins, you will see a decoder/logic chip. When you applied reverse polarity, you may have killed one or more such chips. However, many radios have protective diodes in case of such accidents. In other words, it is very hard to tell. If it were on my bench, I would trace a signal with an oscilloscope. I suspect you do not have that access nor want to spend that much time with it.

    There is nothing a 2-channel radio can do that a 6-channel cannot do. Is this for aircraft or land? Are you in North America?

    John
     
  12. hspalm

    Thread Starter Active Member

    Feb 17, 2010
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    Forgot to mention, no jiggling or repositioning when i move the Tx controls.


    But it is the 2-channel receiver I destroyed... As you can see from the picture, there is no decoder chip, just a crowded PCB of transistors and passive components. Why on earth did not the manufacturers of this Rx make it fool-proof by adding the security diodes for reverse polarity? The pins and connectors fits both ways! The whole reason i managed to destroy this was because I used a battery pack with 9v-battery connectors and forgot that the big/small connectors are small/big on the battery pack, not the connector :D Excuses, I know...

    I actually got my new oscilloscope just 2 days ago, and experimenting with servo signals seemed to be just the perfect way to learn its usage!

    I have several home-made cars, made up from scrap electronics and other components I buy. My fascination covers mostly mechanics and the programming of microcontrollers, I just found out I enjoyed electronics after a 6 months course on my engineering program for automation this semester. Sorry to disappoint you, that I don't have a special interest for RC, but it seems to be a very enjoyable hobby! Considering buying a heli some time.

    I am situated in Oslo and Drammen in Norway. I am surprised you suggested North America, considering my english! Guess it's just that you haven't heard me speak english yet :p
     
  13. jpanhalt

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    OK, so my English is not as good as yours, but then I grew up in Los Angeles, California. That's just an excuse. ;) Your English is quite good, and the use of 27 MHz made me think you might be in the USA or Canada. I am aware that frequency allocations in Europe are different; I just didn't know what they were. You can edit your profile to show your time zone, which helps.

    Enough joking. What I should have said was that I didn't see a decoder on the 2-channel, so I suggested you look at the 6-channel to see what one looks like. More modern (i.e., much smaller) radios may not have discrete decoders either.

    Cheap!

    I agree. Tracing the servo signals in an RC receiver was one of the first things I did when I got a good scope several years ago. Modern transmitters (e.g., Multiplex Evo and JR9303) will give you a screen so you can actually correlate what you see in pulse width to what the transmitter is doing.

    Great! I am not disappointed RC is not your thing. MCU's and mechanics are pretty darn interesting and fun too. In fact, I have been able to integrate some MCU's into my RC toys.

    Definitely fun. I got a Blade for my 8-year-old grandson at Christmas. Great fun, and he quickly surpassed me in flying ability. That took about 5 minutes, no kidding. There is definitely some advantage to being young when flying helicopters.

    John
     
  14. hspalm

    Thread Starter Active Member

    Feb 17, 2010
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    Ok, so I have attached screenshots from the scope, directly from the output of one of the channels.

    This is interesting, and reveals several things.
    When I first picked this Tx/Rx set from the electronics dumpster i noticed that the servos didn't move very far, only like maximum 90 degrees. Now the servos don't move at all, as you know. But the funny thing is that it all seems "fine" when looking at it with the scope. When directioning the wheel on the Tx to the right, the pulse width shows about 1.4 ms. When its center it shows about 2ms, and exceeds that when positioned to the right. That explains why my servos only moved when the wheel turned one way, and only did one quarter of a full rotation :D

    But my servo still does not move when connected to this channel. Not even in a wrong way, like it used to before I fried the capacitor. Hmmm. I will try different voltages and check with another servo.

    Edit:
    Ok, now I get it. As the servo is directly connected to + and - pins on the receiver I must have fried the logic in the servo. Will test a healthy one soon!

    Edit2:
    Working again, probably just the servo that was ever hurt then!
    Thank you so much for your help jpanhalt!
     
    Last edited: Jul 15, 2010
  15. jpanhalt

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    I'll bet you a donut that is right. Servos are not very forgiving of reversed polarity, in my experience.

    Please let us know.

    John
     
  16. hspalm

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    Feb 17, 2010
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    Inside the servo I found several blown capacitors like the one I've already replaced. plus a burning hot motor which have probably been stalled for too long while I was experimenting here right now. Most of the insulation on the wires were melted, so I am glad the servo didn't short circuit anything back on the receiver! The signal grounding wire was hanging by one thread only, so easy to pull off :)
     
  17. jpanhalt

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    If it is a cheap servo, just save it for parts. If it is a high-quality servo, you may be able to order just the "amplifier" board as a replacement. That should be about 1/3 to 1/2 the price of a new servo. Since the motor/brushes may well be damaged too, I would lean toward not attempting any repair and just keep the damaged servio for its gears.

    John
     
  18. hspalm

    Thread Starter Active Member

    Feb 17, 2010
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    Plastic gears, not worth keeping for me. Anyway its a really old and SLOW servo with trough-hole components. I'll keep the potmeter and motor. Thanks for the advice!
     
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