Photo devices repairs: Soldering and desoldering

Long ago, my retired EE neighbor gave me his 1970s-era Weller iron with one temperature -- for lead/tin. It still works well. But SMD has taken over, so I'd like to get a hot-air unit. I notice the cheap chinese ones are around US$150, and Hakko is about US$2000. A close look at feedback suggest the chinese ones are unreliable. Does anyone here have that Hakko model? Or have you extensively used a chinese model?

Also, I notice Antex has a hot-air model for around US$500 in newark.com, but it is "UK stock", and appears to be sold only in the UK (and Europe?), and not exported to the US. Anyone know more about these, and whether they can be purchased in the US?

Mark

albada said:

A close look at feedback suggest the chinese ones are unreliable.

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I've not noticed anything along those lines. The Chinese hot air station I have cost considerably less than $150 (I think it was more like €40 or so) and it works just fine. I've been using it for one or two years now, regularly, without any issue. No way I'd drop $2k on something like this!

koraks said:

I've not noticed anything along those lines. The Chinese hot air station I have cost considerably less than $150 (I think it was more like €40 or so) and it works just fine. I've been using it for one or two years now, regularly, without any issue. No way I'd drop $2k on something like this!

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Could you tell us the brand and model you're using? I'm sure not all Chinese models are alike, and since you found a good one, we're all ears.

I did a quick search, but can't find the particular one I'm using. It's probably superseded by some other generic model. Don't attribute much value to specific Chinese brands or models. In my experience, it all turns out to be pretty much the same, which ends up pretty decent especially considering the modest price you pay. Besides, those hot air stations are simple and straightforward technology and they're produced by the hundreds of thousands. It's mature technology by now.

I think the problem with hot air, is just that, it's just hot air. Anybody who knows how to solder also knows how to break the 'rules' of soldering and hot air doesn't allow for taking the solder to the job on the tip when needed. It's the same as trying to use a small hammer tapped gently ten times instead of using a larger hammer and hitting the nail twice, twice is always better. If a solder joint is small and delicate and in an awkward place a quick zap with a hot tip primed with solder is far better than lengthy heating for the surrounding area and then applying the solder. Hot air is more appropriate for soldering large areas or original manufacturing where solder is already printed onto a circuit board ready to be connected to a wire.

Hot air is virtually indispensable for rework and repairs on smd parts, especially IC's. When done properly, the quality of the joints is excellent.

It is not suitable at all for through hole parts and point to point wiring.

Hot air is also very useful for desoldering TTH (through the hole) components.

For example, you can heat the three connections of a transistor at the same time, thereby liquefying the solder and pulling the transistor out of the circuit board. With the soldering iron you can only do this one soldering point at a time.

You're right; I didn't think of that. I don't do much through-hole anymore...

koraks said:

You're right; I didn't think of that. I don't do much through-hole anymore...

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We could also talk about how to handle electron tubes when soldering. You can still find such components in flash units from the 80s.

Andreas Thaler said:

We could also talk about how to handle electron tubes when soldering. You can still find such components in flash units from the 80s.

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Components of a Minolta 132X

Left to right:

Three glow lamps, the first is for illuminating the disc on the back of the flash.

The other two provide the flash ready and FDC display (confirmation of correct exposure).

I will still research the two tubes on the right. One of these should be the quench tube to eliminate the lightning energy (no storing the energy with a later thyristor circuit).

Above that is the flash tube.

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I asked @forest bagger regarding function of the parts:

The black-coated one is a small discharge tube to dissipate some of the lightning electrolytic energy.
The bright one next to it is the ignition coil - the primary winding at the bottom is the parallel feet, the secondary winding at the bottom right is the thin wire to the ground and at the top the contact to the flash tube.

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Unsoldered for the spare parts box or for viewing.

The flash electrolytic condensator is the dominator of the ensemble.

A potent charge storage with 350 volts and 850 uF.


Calculations/considerations (comments/corrections welcome!)

This means that the electrolytic capacitor can do the maximum

Q = C * U

Q = 850 uF * 350 V

Q = 297.5 mC (millicoulombs)

save.


For comparison:

This corresponds to approximately 1/3 ampere if this charge were to flow through the conductor cross-section in one second.

This creates a power loss of

P = U * I

P = 350 V * 297.5 mA

P = 104.13 W


However, an electrolytic capacitor discharges via an exponential function, meaning that most of the current flows at the very beginning.

If you were to short-circuit the electrolytic capacitor with your finger, for example, the electrolytic capacitor would discharge from 350 V to 47.37 V within 1.7 s (= 2 Tau) - assuming a skin resistance of 1 kOhm.

That should be enough for a burn. Definitely for a powerful electric shock.

If you short-circuit the electrolytic capacitor with one finger each, the current path goes diagonally through the body.

This is bad for the heart.

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To ignite the lightning, a high-voltage pulse is also induced.

Therefore, I strongly (!) advise against opening a flash unit if you are not sure about it!

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I did this in this case because I assumed that the large electrolytic capacitor had not been charged for a long time due to a defect and had lost its charge.

Of course, I was careful when dismantling it and checked the voltage on the electrolytic capacitor with the multimeter. That was 18 mV, so it was harmless.

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