Discharging Caps - At what values do you need to do it?

[mremic01]

I just salvaged an old printer from the garbage. I opened it up and the circuit boards have plenty of components I can cannibalize for future projects, but there are some big nasty looking capacitors in there. There's a 400V 150uf cap, a 63V 470uf cap, and a 10V 1500uf cap. I've only worked on small 9V DC powered circuits, so I've never done any discharging before, but I just ordered an AX84 so I figure this printer might be a good opportunity to learn to do this safely. Which ones need to get discharged? Is there a cut off point with the values where you don't really need to? Some rule of thumb? 

[petemoore]

  Probably not charged to full rated capacity, what does the power supply read, or better what does it measure ?
  Whatever unloaded then add some...probably will read a bit over the output voltage it is rated at.
  I'd measure the caps to see what's in 'em.
  Probably something like 0.3 volts...@@ rate.
  Use a 22k [or 10k, I suppose it might matter if discharging rather large caps holding very high voltage].
  For caps that store large capacity and voltage...anyway...use a large 2watt resistor or larger, this way when you start with the amplifier, you'll have a honker drain resistor ready.
  Familiarizing yourself with the schematic and B+ should involve a routine that is followed 1rst time every time.
  It is up to you to decide what that is.
  I like my yellow wire that drains to be flexible, high voltage/current carrying capacity, and very well insulated [>3 layers tape works but shrink insulation is recommended].
  Make sure you at least drain, and then carefully measure the B+ Voltage is down and held down by the drain resistance [caps can regrow charge like batteries do]. If not take additional steps such as verify your DMM testwires are indeed in good shape [measure some known value] before critical tests.

[merlinb]

Is there a cut off point with the values where you don't really need to? Some rule of thumb? 

You are unlikely to feel anything below 50V, so I just ignore 63V rated cap and under. If this thing has been lounging in the garbage for days then the caps are probably empty anyway.

[Paul Marossy]

If this thing has been lounging in the garbage for days then the caps are probably empty anyway.

+1