Electronic tinkering - difficulty, no death
May 13, 2019 8:58 AM   Subscribe

Are there any small electronics that are dangerous to dismantle or try to repair? What should I avoid entirely, what should I be careful around, how can I mitigate the danger.

I recently took apart a scanner and someone mentioned that you can get a good zap out of an old scanner. One of my younger charges (badumtish) wants to try and repair his guitar amp. I might want to get some buttons from inside an old desk phone, etc, etc.
What are good safety practices?
posted by J.R. Hartley to Technology (17 answers total) 7 users marked this as a favorite
Learn what capacitors look like, and how to safely discharge them.
posted by jon1270 at 9:05 AM on May 13, 2019 [12 favorites]

Tube based guitar amps contain high voltages in the plate circuits that can persist for a while after they are unplugged, so be careful when probing around in them.

Guitar amps and related tube audio gear is a great place to start because the circuits are usually very simple and have frequently been analyzed to death on various guitar websites so there's lots of information on them.
posted by Dr. Twist at 9:05 AM on May 13, 2019 [1 favorite]

Anything with a lithium-ion battery can be dangerous if the case around the pack is nicked or punctured. A very violent fire can result (example).
posted by JoeZydeco at 9:11 AM on May 13, 2019 [2 favorites]

The best safety practice when working with electricity is to know exactly what you're doing, not "trying" anything. Yes working on amplifier circuits without knowing exactly what you're doing, should be avoided until you do.
posted by humboldt32 at 9:29 AM on May 13, 2019 [6 favorites]

Tantalum capacitors can get all fiery if connected the wrong way round.
posted by scruss at 9:30 AM on May 13, 2019 [2 favorites]

Yes, capacitors and vacuum tubes are the two most dangerous things to look out for in (unplugged/battery removed) electronics. Old cathode ray tube televisions are particularly dangerous to service and disassemble.

If you want to tinker for tinkering's sake, I feel like amateur radio is a good place to start. Receivers especially don't require a lot of dangerous components or large amounts of stored energy, you don't need licenses for home-built receivers, you can do interesting stuff like listening to shortwave broadcasts from around the world or doing radio astronomy or whatever, etc.
posted by tobascodagama at 10:11 AM on May 13, 2019 [6 favorites]

Big Clive is all about taking stuff apart and poking around in the innards, and his advice on what not to do is generally well-informed.
posted by flabdablet at 10:17 AM on May 13, 2019 [5 favorites]

Amateur radio is indeed a good place to learn about electronics generally (and RF in particular). Most everything (excepting amplifiers) runs on 12V DC. There are also tons and tons of DIY kit companies out there that are perfect for learning basic circuits, how to solder, and so on. My two favorites are Adafruit and Sparkfun.

If amateur radio begins to tickle your interest (and it's a hobby that covers interests ranging from person-to-person communications, high-altitude ballooning, software-defined-radio, and about a zillion other niches), /r/amateurradio is a good community of helpful folks that can point the way to getting licensed and started.
posted by jquinby at 10:26 AM on May 13, 2019 [4 favorites]

Old cathode ray tube televisions are particularly dangerous to service and disassemble.

Seconded. Picture tubes are bastard things.

The final anode on a color TV tube operates at well over 20,000 volts, and the anode, the glass body of the tube and the conductive coating on the outside form a high-voltage capacitor that can store that voltage for days after the TV is powered down. Not only that, but breaking the picture tube can cause it to implode (there's a large volume of very high vacuum inside) which makes a whole lot of rapidly shattering glass fragments rush toward each other at about the speed of sound in air and then bounce. Fortunately there are enough idiots on YouTube that you don't have to be one yourself.

The voltages inside a typical tube amp are two orders of magnitude lower, but can still deliver a nasty bite to the unwary. Awareness of which parts of a tube circuit are at high voltages is needed before poking around inside one while it's powered up, as is knowing how to discharge a high voltage capacitor without blowing shit up.

A couple of old multimeter probe leads connected together via a 470kΩ resistor, with the solder joints and the resistor itself covered with a couple of layers of heat shrink tubing, makes a fairly drama-free way to discharge tube-circuit capacitors before getting amongst them with the soldering iron.

Most transistor-based circuitry is much less fraught than tube-based circuitry because the voltages involved are generally lower, though you can still get a nasty belt off the power supply rails in any moderately grunty audio amplifier, and you need to watch out for localized high-voltage regions such as those associated with fluorescent lighting tubes (these will be where the scanner zap stories come from).
posted by flabdablet at 10:31 AM on May 13, 2019 [5 favorites]

I used these instructions for building a capacitor discharge tool. I built a couple lower-voltage ones and a couple higher-voltage ones with beefier resistors. You can get fancier and safer, but if you're just working on stereos and stuff (like me), I am told that something like this will be sufficient. I wouldn't use these on a TV, though.

Also, lots of people say it, but it bears repeating: don't discharge capacitors with a screwdriver, especially big ones. It'll damage them and/or you.
posted by shapes that haunt the dusk at 10:53 AM on May 13, 2019 [2 favorites]

I might want to get some buttons from inside an old desk phone

Nothing inside an old phone is going to bite you if the phone is unplugged.

If it is plugged in, don't stick your fingers inside it; the standing off-hook voltage for phone landlines is around 50V DC which can deliver a startling nip, but if the phone rings while your fingers are in contact with the line you will experience the considerable teeth of a 100V AC ring signal on top of that.
posted by flabdablet at 11:28 AM on May 13, 2019 [2 favorites]

In terms of good safety practices in general:

Capacitors have already been mentioned, and learning what they are, how they work, and what that means is pretty foundational to this hobby.
Don't work on anything that's plugged in.
Don't repair electronics around animals.
Put down a rough towel or mat under what you're working on, so if something falls out it doesn't roll away.
Wear gloves.
Make sure there's decent ventilation and consider a mask, particularly with anything older or tube-driven.
I always wear goggles, mostly to keep me from accidentally rubbing my eyes.
posted by aspersioncast at 3:02 PM on May 13, 2019

also: wash your hands afterwards. Lead poisoning from old solder isn't anything you want
posted by scruss at 4:03 PM on May 13, 2019

The human body is a 100KΩ 0.01W resistor. Don't test the limits of that spec.

Some good advice upthread, but a clarification regarding capacitors: They're everywhere, but you don't have to be scared of the tiny ones. A big old electrolytic cap in an old TV will kill you and it will hurt the whole time you're dying. An SMT 0402 jellybean isn't a threat.

Line voltages and vacuum tubes (which are both implied but not certain with a guitar amp) are both warning signs that say "intermediate to advanced electronics hackers only". Get well up the learning curve on things that run on 5VDC at fractions of an amp before you mess with stuff like that.
posted by sourcequench at 5:19 PM on May 13, 2019 [3 favorites]

Don't work on anything that's plugged in.

This is very difficult to avoid in many situations. You cannot set the bias on an amp without it running with the cover off, for example. Learn good practice around this, there will be plenty online to help. The most important for me is to whenever possible use one hand only, with the other in my pocket.
posted by deadwax at 5:41 PM on May 13, 2019

I'm not sure this falls under your category of "small electronics" but microwave ovens have extremely high internal voltages (kilovolts) and a capacitor which can store charge after shutdown. Definitely not something to mess with unless you really know what you're doing.
posted by Larry David Syndrome at 7:41 PM on May 13, 2019 [1 favorite]

A big old electrolytic cap in an old TV will kill you and it will hurt the whole time you're dying.

No need to be melodramatic. There are no capacitors in a TV that will kill you if the TV is unplugged. The electrolytics are going to bleed down very quickly through circuit paths. The CRT tube can give you a very nasty shock but it won't kill you. Color CRT TVs will hold a charge of about 35,000 volts, but a Taser is 50,000 volts. There isn't enough energy to kill you in either case.

For a frame of reference, a CRT might store about 100 millijoules, for one zap. A Taser will zap you with 500 millijoules about 10 times a second. A cardiac defibrillator puts out 360,000 millijoules. So, no, a TV CRT capacitor won't kill you.

the glass body of the tube and the conductive coating on the outside form a high-voltage capacitor that can store that voltage for days after the TV is powered down.

Well, maybe the really old TVs from the 1950s with vacuum tube rectifiers that don't leak. But any TV from the mid-60s or so would have silicon diode rectifiers on the flyback circuit which do have reverse leakage and will bleed off the CRT high voltage charge in a short time, not days.
posted by JackFlash at 8:36 PM on May 13, 2019

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