Can you overcook carbon comps with soldering?

[tubeswell]

I was soldering a couple of 220k carbon comps on an eyelet board today and the metal surface got quite hot before the solder would flow properly onto the eyelets.

Now having cooled back down, one of them read at 318k and the other read at 279k, and they both read around 220k ish before I soldered them.

They are the grid load resistors for the output stage, so I put some fresh ones in to be on the safe side, and everything reads as it should. So evidently it is possible to overcook them (which I hadn't realised, because this is my first time building with them :-0)

[Steve Conner]

Interesting, are they NOS carbon comps you bought off Ebay or something? Old ones might be temperamental, but new production ones from Allen-Bradley or whoever should be a bit more stable.

You definitely should change them, because they might go completely open-circuit and wreck your power tubes.

I personally don't buy into the carbon comp mojo thing, I just use ordinary resistors. 1% metal film for small ones, 5% carbon film or metal oxide for the bigger ones.

[tubeswell]

I don't know if they were old stock or not - they came from the shop unmolested (unfortunately they had to deal with me)

Anyway I too have used metal film resistors up to now, and no doubt will again, but I thought I'd try the CCs just for the hell of it.

[dai h.]

might have taken too long. Make sure the leads and eyelet(turret, etc.) are clean(alcohol, naphtha). Joint should be made in around 2 seconds max. If you can't get it happen quick it's probably 1. dirty(oxidized) 2. not enough iron (go to a bigger tip, higher wattage--but not too much heat since the flux which helps make the soldering happen will evaporate way too quickly) or 3. something is wrong with your technique. As far as carbon comp, if you can get them not too expensively or real cheap surplus or whatever then go ahead and try them out but remember they do have drawbacks like being less stable, more susceptible to moisture(I have some NOS Japanese ones which came in sealed bags with a small pack of silicone dessicant inside).

[Enzo]

As a rule I don't worry about it. But it can happen, just depends on how goonish you were with the iron, I suppose. Got any clip leads around? Clip an alligator clip onto the resistor lead next to the resistor body - instant heat sink.

[Chuck H]

As a rule I don't worry about it. But it can happen, just depends on how goonish you were with the iron, I suppose. Got any clip leads around? Clip an alligator clip onto the resistor lead next to the resistor body - instant heat sink.

Good tip with the alligator clip.

I'm not the most graceful performer with an iron. And there have been times when I've been...Ahem..."goonish". Modding old amps with five leads jammed into one eyelet and the like. It can take awhile to melt the joint if you fail to bump up the heat on your iron. There have been a few times when I felt compelled to check the CC resistor values afterwards but I've never had one go out of spec.

Just for the education I would save a couple of those 220k Rs from that batch and test them side by side with another batch or brand for value shift after heating. It's valuable to know what NOT to use.

Chuck

[Enzo]

Ever notice a dry sponge doesn't suck up water very well at first? In fact if you wet the sponge and squeeze it out it will then pick up water a lot better.

If you have a large joint to heat, I find it often helps to "wet" the joint. The reason the joint doesn;t heat easily is that you can't make good thermal contact with the old solder.

Put the iron on the joint and melt some fresh solder onto the tip at the spot of contact. This new molten solder will flow into the old solder, speeding up the melt process.

Likewise clearing solder from holes. If you have trouble clearing the hole, you get halfway and it stops clearing, refill the hole with fresh solder, then heat and extract. It will clear easier. That is more a pc board trick, but it works on eyelets too.

And keep your tip clean!

[tubeswell]

Yeah I did all of that - I just think I didn't tin the first few eyelets enough before I started soldering the components on, then I tried to melt more solder over the whole caboodle - teach me for being lazy.

Anyway swapped them for fresh parts and they are okay now.

Incidentally Chuck H, I always check the circuit with my meter afterwards to make sure I've got everything connected (carry-over from my pc board modding) - that's why I spotted it.

I'm not sure what brand they are. I have two maunfacturers, and the ones that went west are slightly lighter coloured tan. I replaced them with ones that are more dark-chocolatey coloured.

[Chuck H]

Put the iron on the joint and melt some fresh solder onto the tip at the spot of contact. This new molten solder will flow into the old solder, speeding up the melt process.

I've done that longer than I can remember. But I figured it was just something that worked that we all come up with incidentally. Basically increasing the heated contact surface area. Didn't know it was a trick of the trade. I do know that you almost can't melt some power supply eyelet joints without wetting them with a little new solder first.

Chuck

[Enzo]

It is amazing how many things we assume everyone knows that everyone don't.

My favorite shop tip, and the one that the most people thank me for, is pretty simple. WHen replacing wood or sheet metal screws in a hole, always turn them "backwards" until they drop into the old threads. THEN turn them on in tight. That way you don't cut new threads in the material each time. Cut new ones too many times, and no material will be left. Seems obvious to me, but so many people never think of it.

[Borg54]

It's always a good practice to pre-tin all surfaces that you are soldering on. You will find that it takes less solder and far less heat. And this really doesn't take any longer than it appears to, and the solder will flow so much smother.

Now you can get away with not using a heat sink on resistors if you like burnt fingers and after awhile they will be as tough as leather, so who needs a heat sink any way? Now on caps it's a different story I always use my dual purpose hemostats as a heat sink to keeps things a little cool. Nothing will destroy a cap faster than heat. I fact when a amp comes in for repair I always look for the caps with the most solder on them, invariably those are the dead ones, and this helps speed up the process.