Desoldering Obsolete Electronics

[alderbody]

I ran into some old (full of Ge and other "electro-mojo" stuff) radios...

For the time, i am "warming up" with the resistors,
but i think i need some more opinions on how to
MOST_safely desolder those heat sensitive devices.

What's the best way to "heat-sink"?

[markm]

I de-soldered a bunch of Ge trannies not too long ago with just the iron and some braid here & there.
You gotta be quick but, look at it this way....they were soldered in right! 

[R.G.]

OK, here it is - but you probably won't like it.

(1) chuck the board into a vise on a workbench
(2) use the widest flame spreader on your propane torch and play the flame across the solder side of the board from a few inches away. If you burn the board, you're too close.
(3) Using needle nosed pliers or a chip puller, pull gently on the component to be removed from the component side of the board. When the solder melts, all of the pins will melt at once. Drop the component into the bucket of water you've thoughtfully placed right under the component side of the board.

You can pull out parts as fast as you can work the puller. This is almost a perfect setup for desoldering if you want to save components and not the board. The board will be destroyed. But I have never lost a part to heat damage in doing this over decades.

[markm]

 
Dare I say......Cool!

[Harry]

Wow! 

What I've always done (definately not the best way) is to push the component leads out with the tip of the iron while pulling on it from the other side of the board with fingers or tweezers. If you really don't want to destroy the part you can clip the leads, but they most likely will be way short.

[Meanderthal]

 I recently got my hands on the pcbs from a 1965 thomas malibu organ(free)... loaded with Ge trannys. They were all standing up like 1" off the pcbs- more than enough to just snip em off. Now, all those foil caps... had to be desoldered. The carbon comp resistors went in the trash with the spent boards. I salvaged thousands of those years ago when I had the patience, and never use em anyway.

[Ronsonic]

I usually just heat with the iron and pull from the other side. Sometimes I'll just heat and slap the board on the bench and let inertia do the extraction.

I like the flamethrower approach. I rarely find more than a thing or two on a salvage board I want, but that looks like the most direct way of removing stuff in quantity.

Ron

[The Tone God]

I would wear a respirator if you plan on this "torch" method.

Andrew

[Big Red]

the blowtorch idea is good RG. I read something similar before involving a heat gun which I dont own, so I never got to try it.
http://www.uchobby.com/index.php/2006/09/01/scrounging-a-3com-super-stack-ii/

as for the water, is that safe for all components? I know my digital camera went for a swim last summer and it no longer works (it was marine water though, perhaps the salt is to blame?)

[R.G.]

as for the water, is that safe for all components?

It's safe for essentially all components. Some inductors or transformers would be hard to dry out, but otherwise, a dunk in fresh water will not hurt electronic components that are not already hurt. The only important thing is to then dry them off.

You might have saved your camera if you could have immediately removed the batteries, flushed it thoroughly with fresh water, flushed it in 100 proof vodka or pure grain alcohol, then blow-dried it with a cool blast of air.

It's not the water, it's the things dissolved in the water, and the corrosion it causes when the water is not removed quickly.

It used to be common to clean PCB assemblies in a diswasher converted for the purpose. If you get them wet, then get them dry, they are not usually hurt.

The exclusion of atmospheric water through heremetic seals was a giant first step forward in semiconductor packaging.

[bancika]

I just break board into small pieces, it's easier to desolder then when you have one piece of board per lug...even without braid (which I don't have)
Cheers

[rockgardenlove]

What temperature does solder melt at?  Anybody know?  I want to figure out how far away from the board I have to be with my heat gun to melt solder at the optimum temp.

[R.G.]

What temperature does solder melt at?  Anybody know?  I want to figure out how far away from the board I have to be with my heat gun to melt solder at the optimum temp.

Tin-lead solder melts at different temperatures depending on what the relative composition of the alloy is.

Pure lead melts at 327C (621F), pure tin melts at 232C (450F). A 63%-37% alloy of tin-lead melts at 183C (361F). As you change the proportion of tin and lead away from 63-37, the melting point goes up.

And here's why you don't want to do what you're trying to figure out.

Let's assume that you have a part soldered in with eutectic (63-37) solder. You also have a soldering iron heated to a few degrees more than the melting temperature, say 190C/374F. When you heat the solder, heat starts warming both the solder and the parts behind the solder. The part leads actually cool the solder until they get too hot to keep the solder solid. The solder takes a long time to heat up to melting temperature, and by that time heat has time to conduct into the center of the part. If the part is heat sensitive, it will be heated a long time because it takes a long time to get the solder up to molten.

If you instead have a soldering iron at 371C/700F, the solder melts almost instantly at the iron, and reaches melting temperature in a second or so while the leads are still heating up. The solder releases and you get the soldering iron off the joint quickly. Less total heat has been put into the joint than if you used a lower temperature, and MUCH less heat than if you took a good long time at a lower temperature goes into the body of the part.

The motto for soldering is - Get it clean, then get in quickly with high heat, and get out.

[alderbody]

First of all, thanks to all!...   

I'll give R.G's method a try, since i don't care about the boards.

But i would agree with bancika about (carefully) breaking the pcb into smaller pieces.
That method might be used, depending on the topology, etc...

And yes, Tone God, some sort of respirator must be used either way because the heated materials give off some strange smell,
which i don't quite intend to breathe for long.

The main objective is to remove as many components as possible (Ge gets top priority).

There are a lot of carbon comps, which i will remove but not necessarily re-use, a couple of diodes and many ceramic caps.

Also, some small transformers that might be useful and some old-style_and_value axial lead electros, mostly for historical reasons...
(i found 30, 50mF and other obsolete values... )

Thanks again for all your recommendations!

[alderbody]

I forgot to report about the Ge devices i found.

so here they are:

2SA100 (matsushita)
2SA101 (matsushita)
2SA202
2SA203
2SA330
2SB54 (toshiba)
2SB56 (toshiba)
2SB186
2SB187

I found that they are all PNP Ge, but there is no clear description on their pinouts.
There is only a 12A or 12B outline type... ()

any help on this?

[mac]

Topic Summary
Posted on: Today at 05:02:51 AMPosted by: alderbody
Insert Quote
I forgot to report about the Ge devices i found.
so here they are:
2SA100 (matsushita)
2SA101 (matsushita)
2SA202
2SA203
2SA330
2SB54 (toshiba)
2SB56 (toshiba)
2SB186
2SB187

100/101/54/56 and probably the others have the same pinout of a 2N3904, from botton flat side up e b c. Matsushitas have a circle on the collector side, Toshibas a triangle.
You can check the others with the ddm. Set the ddm to read diodes and put the black cable in the center leg and the red in one of the others legs. If you get near 300 then it is a std ebc pnp. If not, the center leg is not the base or it is a npn device.

mac

[alderbody]

Thanks mac!