[sampson_smith]

Here's a simple question for all of you more-experienced DIY'ers: I am currently trying to change the LED of my Gilmore Lite dedicated power supply (DPS). The reasons at first were simply aesthetic; I wanted to match the LED of the GLite amp itself with darker blue LED, as they were not matching beforehand. Upon opening the DPS, it appeared that there was visible accumulation the solder on both sides of the board where each of the LED's end wires went through the holes in the board. I assumed that it was a simple matter of heating the solder, freeing the LED, and then reinserting the new one with an addition of solder. It appears that when and if I free the LED, there will be excess remaining solder that will block the insertion holes. The LED is currently still attached but due to my loosening of its contact by brief heating, the LED is 'permanently' off. I am cautious about adding too much heat because I do not want to damage the board, so it currently remains that way.

I have read into soldering much and thought that a solder wick (copper filament) would adequately remove excess solder, but it is clear that the contacts are so close together that it would be difficult to remove it without merging the two contacts. I have thought about simply shearing the old LED off of the board with wire cutters, and then placing the new one on top. I have not tried this, as I speculate that this is not an effective way to make contact (wire on top of solder vs. properly inserting the wires through the board).

I assume that the above is actually a simple matter, and I'll ascribe my reluctance to get the job done to being a nervous first-timer. Any advice on what I or somebody with better experience should do would be very much appreciated!

 

[nikongod]

Solder wick

Dont worry about momentarily soldering adjacent holes together when removing/swapping parts as long as you are 100% sure to remove the bridge before you finish, which solder wick should do!

 

[m1abrams]

Ok first off I would not apply power to that device until you figure out exactly why that LED is now off. I suspect you shorted the contacts of the LED and that is why it is off, this most likely is not a huge problem since the circuit powering the LED has a current limiter in it somewhere so worse case is that circuit is pushing a little more current than it normally would.

Second, you need to remove that solder to insert a new LED. Many ways to do this, my favorite is using solder wick. Wick is just some copper braid that you lay on the solder to remove and heat it with your iron and the solder wicks into the braid. I highly recommend practicing this on something other than your amp though because if you do it wrong you can easily lift the PCB traces.

I would not worry too much about applying too much heat to an LED, they can handle a good bit of heat, in fact you will most likely damage the traces before you damage the LED. Which is why I think the reason behind your current LED not working is because it is shorted. If you fix the short i bet that LED will light again.

 

[m1abrams]

Quote:

Originally Posted by sampson_smith /img/forum/go_quote.gif I have thought about simply shearing the old LED off of the board with wire cutters, and then placing the new one on top. I have not tried this, as I speculate that this is not an effective way to make contact (wire on top of solder vs. properly inserting the wires through the board).

Do not do this. It will be very difficult to make the new LED have a good physical connection and will be very susceptible to failure.

 

[tomb]

It usually takes a fine touch to clean the solder out of pads that are meant for LED's. You can't do it with the LED in there - the leads of the LED sink too much of the heat and you will not be able to suck the solder out of the internal hole plating, besides the fact that the LED's leads block the hole for flow from the wicked solder on the other side.

1. Remove the LED - this is best done by melting a blob of solder on the bottom of the PCB that's big enough to cover both pads/leads at once. Yes, that most likely means adding solder. Once the single blob of solder covering both leads is melted, the LED should drop out, or at the very least - a slight pull and it will come out. You can tie a string around the LED to use to pull it, if that will help. Don't force it! You can pull the pads and traces right off the PCB. It should practically fall out, with no force.

2. Clean up the solder - wick can remove the excess very easily and quickly. Do this with most of the extra solder you added in #1. However, it's a bit tougher to get it out of the plated holes. Again, adding solder - make certain you have a slightly convex finish of solder on the pad's hole (all on the bottom of the PCB). This is a kind of "tweaking" solder addition - nothing very big. Make certain when adding this solder that the hole is filled and completely melted through and through. The idea is that you want one contiguous blob of solder from one side of the PCB, through the hole, and out to the other side. If you don't do this, the wick will suck up only one part of it.

Prime the wick by using flux on the end (even if the wick is flux-impregnated) and prime the soldering tip by having a small blob (very small) of solder on the tip. Place the wick with flux on top of the solder blob, then apply the soldering iron with the solder on its tip. It should immediately suck up the solder from the soldering tip and create a wicking action that will suck up the solder from the pad and hole, too. It should wick out everything out of the hole, but will leave some on the other side of the PCB if it's outside the hole. That's OK - you can go back and get that later. If this doesn't work the first time, clean up and repeat (starting at #2). Be sure you start with a clean wick each and every time. Every time solder goes into the wick, it hampers it's ability to suck up more. So prime, wick, and trim - start over - and prime, wick, and trim, etc., etc.

Seems complicated, but I've never failed to clear out a hole this way. Other methods using drilling may damage the through-hole plating and ruin the solderability of the pads.


P.S. Be sure you orient the leads correctly for polarity when you install the new LED.

 

[sampson_smith]

Wow. So much info, so fast. I appreciate it, guys. Rest assured that I want to do this the right way and do not intend to slap the new LED on top of the old one. I appears that solder wick is the way to go. I assume that the local hardware/building supply store may have it.

I have checked thoroughly to see if I have created a short anywhere and it appears that this is not the case. At no point in the area of the board that I had very briefly worked, had there been any crossing. The extent of heating was just enough to slide one side of the LED out partially. The solder hardened before I could finish the process. Again, I am rather timid about this stuff, I have to admit, and stopped for safety's sake. The DPS has been plugged in a couple of times and it works with the GLite just as well as ever. There is no evidence of overheating to note.

Just out of curiosity, is it not likely that an LED can be burned out, or damaged by a short? If it is not a matter of a loose contact, as I expect, is it possible that the old LED (which needs to be replaced, anyway) is simply damaged and incapable of generating light? I am sure that a household light bulb and an LED are very different by design, but wonder if they also have a 'lifetime'.

Again, thanks very much for the info. Very helpful stuff!

 

[sampson_smith]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif P.S. Be sure you orient the leads correctly for polarity when you install the new LED.

Thanks for the detailed info, tomb! I didn't get a chance to read it before leaving my last post. That is a very involved process - thanks for explaining it so clearly. The chipboard lead-free solder that I purchased has both flux within it and also came with a tube of it, which is white and viscous. I assume that I can apply this with a brush to the copper wick, once I find one. How does one determine the polarity/way in which to align the new LED? There is no markings on it that indicate '+/-'. One length is slightly longer than the other, however, if that is supposed to help.

 

[Pars]

I wouldn't use lead-free solder, especially for a newbie to soldering. Too hard to work with and requires more heat than leaded solders.

 

[m1abrams]

Well it could be a loose contact after reading what you have done so far. LEDs are generally damaged by very excessive heat and over current. A short across the leads of an LED would not cause either of those conditions so if you did short the LED it would still most likely function once the short was removed.

Their actually are 2 standard markings on an LED to indicate polarity. You discovered one, one lead is longer Anode (+). The other is on the case of the LED, one side has a flat on the cathode (-) side.

Light-emitting diode - Wikipedia, the free encyclopedia

 

[sampson_smith]

Thanks again, everyone! With a bit more determination and the good advice, listed above, I was able to change the LED! Removing the excess solder and freeing the old LED wasn't much of a trying process once I had applied the copper solder wick. (This, of course, was done after practicing for some time on some old circuit boards.) The installation of the new LED was very easy, once the holes were cleared.

Only one predicament remains: The LED is much too faint, compared to the Gilmore Lite amp. I was aware that this may be the case, and have been told by Justin at Headamp that I may have to change the resistor as well. Thankfully, the one that needs replacing is clearly marked. Also, I am plenty glad that I now know how to do such things. I assume that I can either ask him what sort the most recent LED's have been requiring to achieve the proper brightness, or I can check the resistor that the GLite uses. Goody-goody! Sounds like I have work to do!

 

[m1abrams]

Good job.

As for the resistor, you probably could just see what value it currently is and go to a slightly lower value. That is a bit of trial and error way. Or you can find out what voltage is being sent to the LED. Then use this equation to find the right resistor.

(Vs-Vf)/Rled = I

Vs is the supply voltage, Vf is the forward voltage drop of your LED which should be part of the LED spec. I is the amount of current you want to go through the LED. Note the LED spec will have an Imax. This is the max current the LED can handle, depending on the LED you most likely do not want to run it at Imax because it may be extremely bright.

 

[dorkvader]

keep in mind that the smaller value resistor might have to be a higher wattage class, depending on how the maths work out. I found that out when building a CKKIII, and I looked up on digikey what current my LED could take. I put those values into the online calculator and found out I needed a 1W resistor. I decided I didn't need that bright an LED.

You probably won't have to worry about this, but it's something to look out for, just in case. The CKKIII's LED section is 30v, and by led is 2.2, so if I want a full 30 mW, I needed a 1k, 1W resistor. it was at this point, I decided I did'nt need to light up the world to show my amp was on.

When I build a plexiglas-topped case, I'll light the clear panel with two (probably green or blue) LEDs (probably in parallel? I'll lookup how best to do this later) and I'll probably make them pretty bright.

 

[sampson_smith]

Thanks very much for the advice.

I am actually thinking of opening up my Gilmore Lite (not the DPS, but the amp) and checking what resistor is in there. Both LED's are the exact same, and I assume that if I then match the DPS resistor to the resistor of the GLite, then I will get a comparable brightness. I assume, when Justin is able to get back to me, that he will suggest that I match the resistors. Or am I missing something here? Could the DPS circuit board be running on a higher/lower level of current than the GLite. Hmmm... I'm obviously out of my element and need to read up on this more once I get some beauty rest.

 

[m1abrams]

Quote:

Originally Posted by sampson_smith /img/forum/go_quote.gif Thanks very much for the advice. I am actually thinking of opening up my Gilmore Lite (not the DPS, but the amp) and checking what resistor is in there. Both LED's are the exact same, and I assume that if I then match the DPS resistor to the resistor of the GLite, then I will get a comparable brightness. I assume, when Justin is able to get back to me, that he will suggest that I match the resistors. Or am I missing something here? Could the DPS circuit board be running on a higher/lower level of current than the GLite. Hmmm... I'm obviously out of my element and need to read up on this more once I get some beauty rest.

Only if Vs (Supply voltage) is the same!

 

[sampson_smith]

Yes... there's a chance that the Vs is different between the GLite and the DPS. I figured that I would be able to look at the blown-up pic's of the GLite chip boards on the Headamp website to see what the color indicators are on the resistors. I'm starting to think that I am colorblind, because I cannot decide what the colors are of each resistor's bands.

For the GLite: http://www.headamp.com/home_amps/lit...board_high.jpg (click to 'zoom')

The resistor, which is just to the top-right of the LED looks to be violet ['7'] - green ['5'] - blue ['6'] - red [x 10^2] - violet [+/- 0.1%] = 75600 ohms?! (This seems extremely high, especially since the GLite LED is the brighter of the two.)

For the DPS: http://www.headamp.com/home_amps/lit...board_high.jpg (click to 'zoom')

The resistor, which is to the bottom-right of the LED looks to be violet ['7'] - green ['5'] - black ['0'] - brown [x 10^1] - brown [+/- 1%] = 7500 ohms.

***

Again, I am fairly sure that I am not seeing the colors for what they are truly supposed to indicate. If you have a second and know your way around resistor nomenclature, feel free to check and validate/dismiss my measurements. Thanks in advance!