A friend gave me an old Boss ME-10 Instrument Effects processor recently that he hasn't used in years. It's not in great shape, as you can see below. Lots of rusting on the metallic casing and it's missing the covering for the main value knob. I want to take it apart and either clean up or replace the casing and fit it with new 1/4-inch sockets at the rear but I'm worried about the risk of shock from capacitors inside the device: The unit hasn't been plugged into a wall socket in a few months so I'd imagine that any capacitors inside would be discharged by now, but I don't have much experience with this kind of thing, so I'm hoping someone else could advise me on the matter? I looked for service manuals and electronic schematics in google but I couldn't find anything for this model. Thanks in advance.
heh, no risk it doesn't matter that caps get discharged - they just, uh, charge up again. although if they're old/damaged there's a risk of them blowing. but no risk of shock.
When I have to play with mains caps, I use a metal spoon with a plastic handle to quickly discharge them (if there is charge at all).
... ooh, you were gonna take it apart? i should have read that more thoroughly. i'm glad you're not dead. if you do die feel free to bash me over the head in the afterlife. but yeah i've taken apart PSUs that had been unplugged for less than a day and gotten a nasty shock from caps. nothing life threatening or even serious, just an 'ouch' TVs and monitors, now THERE'S some serious caps in there.
No worries, I found some pretty big capacitors (in physical-size terms) in there (2200uF capacitance) but it hasn't been plugged into the mains in months so I think I was safe enough anyway. Just to be on the safe side, I didn't touch anything on the PCB Thanks for the tip, Profqwerty, I'll keep that one in mind. I'll just have to try and remember not to use that particular method when I go to paint my CRT in a few weeks. I have a feeling that a spoon may not offer enough protection from the caps in a CRT.
FYI, afrotechmods.com used a pair of oven gloves and some insulated earthing wire to discharge their monitor in this... uh, thing: http://www.afrotechmods.com/reallycheap/Jacob/jacob.htm
Just a normal screwdriver with a plastic handle - touch it on the bottom of the caps and there we go. Not necessarily the safest way, but there we go. If you're doing any work on this kind of thing whilst holding a metal object then really it's worth discharging the big caps, especially if they're high-voltage.
I wouldnt worry about the caps inside of that. In a monitor it isnt really a capacitor you're discharging, your discharging the capacitance of the CRT, if that makes any sense. And I defintiely wouldnt short out caps in audio equipment, best way to start breaking them, or at least start changing how they sound if its in a signal path.
after a few months all capacitors will be discharged. Even after a few mins they should be well on the way to been discharged.
Nice cpemma - myth dispelled. I'll tell people not to do it to audio equipment, and just use a power resistor instead.
There's a spreadsheet here to pick resistor value with time needed for safe discharge, so if you're using ordinary resistors you can check the wattage won't explode them.
Not true at all. Caps of any significant size can often hold a considerable charge for months or years.
FYI peeps ... If you don't have high-power resistors knocking around, you can get the same resistance (discharge time) by putting two of the same resistor value in series (say by twisting the legs together). Do this twice. Now twist the outermost legs together on both sets so you have two strings of resistors connected by wrapped legs. This will create a diamond sort of a shape. Use opposite ends of this diamond to discharge your capacitor. You've just multiplied the power the network will take before blowing up by a factor of four (and the difficulty of handling it by about 4000, but that's another story).
>MaximumShow : Every capacitor will exhibit a DC resistance, even it's very high. If it's a high-quality capacitor this will be very, very high, meaning long discharge times (think tants). Most capacitors in these sorts of supplies will not be tants though, they will be electrolytics with relatively low DC resistance values. They will self-discharge relatively quickly (still possibly a week or two). There's no way to tell unless you can get a datasheet. The one caveat is that there's currently one tantalum mine left in the world (South Africa, in case you were wondering) but replacement techs are queueing up to take over. This makes tants very, very pricey at the moment and therefore seldom used. Tants tend to be the big yellow caps (the yellow is the usual fire-resistant coating). Electrolytics are the "classic" long cylindrical type through-hole jobs.
About the only thing around with really high voltage caps in it are CRT monitors. Truth be told, even CRT monitors aren't really deadly, unless you have arrythmia or a pacemaker. If you shock yourself on a cap, it actually defilibrates your heart. I'll hurt like hell, but you'll live. It takes AC to drive your heart into filibration.
Hopefully you'll only bridge the cap with one hand and the current will arc through your finger. If you touch one side with one hand, and the current arc through to the other side of your body, hoo boy if it passes through your heart...
The LV electrolytics after the mains rectifier discharge quickly as they always have bleeder resistors in parallel. The only capacitor in a TV which retains it's charge is the tube itself. (It's the only high voltage capacitor on the HT side of the flyback)
