working on PCBs

[(chrisdski)]

I just picked up off local CL a Fender BXR300 combo amp for $50. 80lbs of 300 watt solid state, 15 inch woofer, early 80s big hair goodness. It has a busted mid freq pot. I ordered one and the red knob. It is a "snap in" mini pot. What is the best way to desolder these guys? I have a whimpy soldering iron and a solder vacuum. Is a really hot iron better? The key is getting all 7 points clean without ruining the board. Any tips or trade secrets appreciated.

[(cjj)]

I used to use solder suckers... and they kind of work, but mostly just suck.

The best thing I've found is "Soder-Wick" (yes, it's really spelled that way) desoldering braid.
http://www.chemtronics.com/products/pro ... m=2&id=419

It comes in unfluxed and with several different types of flux. I've found the fluxed type to be more of a pain to use and it always seems to make a mess so I use the plain, unfluxed type. You DO need some sort of flux to get the solder to flow into the braid though. I like to use a liquid flux pen such as this:
http://www.kester.com/SideMenu/Products ... xPens.aspx
I happen to have a type 2331-ZX on my bench right now, it's water soluble and works for both lead and lead free solders.

To use this, you pull out a bit, maybe 2 to 3 inches of the braid from the roll (don't cut it off). Apply flux, the pen works like a felt pen, just push down a bit and lef some flow out onto the braid. Then put the braid over the pad/joint where you want to remove the solder, put your iron on top and let it heat and watch the solder get sucked up and flow into the braid. You may have to move to a new section of braid if there's a lot of solder. It's amazing how clean the solder joints are after this, almost no solder left at all.

By the way, the flux pen is great for other soldering tasks too. Apply a little to wires you want to solder or tin and the solder flown like magic. Much better than just relying on the flux core in the solder...

[(chrisdski)]

Thanks CJ. I see the solder wicks come in different sizes- is there a preferred size for most jobs, or do you have the assortment? Not to endorse any one distributor, but do you use a particular vendor you like to use for items such as these?

[(cjj)]

In general, the size to use is related to what you are trying to do. For tiny surface mount stuff (like what's on the board behind them in my pic), you use small/tiny stuff. But for most things you'd do, the bigger stuff is just fine, I mostly use #5 size. It's big enough for pots and such, and if you're careful, good for even small things like resistors and such on circuit boards.

The main thing is how much heat it takes to get the braid up to temp. Bigger braid takes more heat, but also soaks up more solder. But too much heat can ruin small, delicate things like IC's and such. For general guitar and amp work, you're not messing whth too many tiny things, so bigger sizes are fine.

As for where to get it, I can't really say, I've got a supply I bought 5 or so years ago at an auction that a big electronics manufacturer was having when shutting down. But, I've bought a lot of things from places like Digikey and Newark, they are good to deal with. Heck, they have it on Amazon...

[(chrisdski)]

Alright! I ordered supplies off Amazon. I will let you know how it comes out.

[(chrisdski)]

OK, repair completed today. That solder wick works as clean as snot- it was really good. The solder just lifted out of the hole into the wick - you could see it move. The old pot came out with a little extra heat and wiggling. The new pot fit right in and soldered up clean. I bought the variable iron shown- worked really good for the repair. Test drove the amp and the mid sweep pot worked as designed. Thanks for the help!

[(cjj)]

It is SO much better than solder suckers. Glad to hear it went well for you!

[(johnallg)]

It is SO much better than solder suckers. Glad to hear it went well for you!

Interesting. I have a whole lot better luck with a solder sucker. I am perpetually cursing Solder Wick whenever I try it, and we do have it here at work.

[(chrisdski)]

John, I had the iron really hot and used a lot of flux. I think the flux is the key to getting the solder to flow easy.

[(cjj)]

Yeah, when I first came across the stuff (at work), I had a hard time getting it to work too. That was with the pre-fluxed type. Somehow, the dry flux they put on it just doesn't work the same. Then I learned from the PC Board rework folks at work that using liquid flux was the way to do it, making sure the wick is good and saturated.

Yes, you do have to use a hot enough and big enough iron (you have to heat a fairly large amount of copper, the wick, as well as the joint) to get it to work.

The hand type solder suckers always seem to cool things off to much between when you remove the iron and fire them. Plus, the air movement seems to cool stuff down too. I have used desoldering tools that have a built-in vacuum system. Those work fairly well, but you have to keep the solder from plugging them up. The Hakko FM-204 works quite well, but at $700, it's hard to justify buying one...

[(chrisdski)]

OK, hiccup in the BXR 300 rehab- no sound out of the speaker. I've run the amp thru another cab- all ok. I've checked the speaker wire down to where it plugs into the speaker tabs- all checks out OK for continuity and I even plugged that wire into another cab- all OK for sound. So that leaves the speaker. Is there a way to check for a bad speaker beside "if it don't work when you plug it in, then it's bad"? I know there are only 2 wires going into the cone/magnet- they are all attached and I have continuity along each wire. Hooking up a 6 volt battery moves the speaker.When I check the resistance of the speaker the ohm meter jumps around between 3-7 ohms (earlier today it seemed pretty stable around 7.5). Any thoughts/tips?

[(cjj)]

Well, if you've got continuity, then the voice coil isn't open. Seeing it move when you put a battery on it means the coil isn't jammed. The only way to really know if it's got some kind of short in the coil is to use an "LCR" meter to measure it, and see if the impedance is correct.

Do you have another amp you can test the speaker with? Even hooking up the output from your stereo should give some sound...

[(chrisdski)]

so I plugged the speaker into my stereo and got sound so I guess I will stick the speaker back in the cab and play with the wires before tightening the screws. One thing I've noticed is the resistance seems to vary when checking with my ohm meter. It is an 8 ohm speaker- sometimes it will read steady around 7.5, or it will read around 6. Both with the same meter. I may notice the lower reading after putting a signal thru it. Not sure if that means anything?

[(cjj)]

My first guess as to why you get varying readings is a poor connection. Unless, of course, if you are moving the cone/coil while measuring. Moving the coil a bit will produce a voltage variation in the coil which will affect the meter reading.

The 8 ohm impedance is... an impedance value and not a resistance value. Without going into full-on Techno-Nerd™ mode, impedance is like resistance, but takes into account the effects of frequency. That's what the "LCR" meter does, they measure inductance, capacitance and resistance over frequency. Since your ohm meter is only measuring with DC, you see what is called the series resistance of the coil, which is not the same as the impedance. It's quite possible to get a series resistance that is a lot different than the AC impedance.

Anyway, if the cone & coil are not being moved, and you have good solid connections between your meter and the speaker leads, you should see a steady resistance reading...

[(chrisdski)]

Put it back into the cab, fired it up and sound. OK- old man brain fart excuse. So I crank it up and start playing and then it cuts out. I hook another cab up to the amp- works fine. Previously had hooked up another speaker and it worked so wiring is fine. Intermittent short in the speaker?