He says, " It is best to use a 100 ohm 5 watt wire wound resistor, with a clip lead to short capacitors. The 100 ohm resistor limits current enough, to prevent a damaging spark but will quickly discharge the capacitor. "
But unclejed613 says, " a 100K 5 watt would work better."
As stated earlier, a screwdriver across the capacitor terminals is a quick, easy and sometimes spectacular method of discharging a capacitor.
I remember one instance where I shorted a capacitor, accidentally I think, it was 50 years ago, the equipment was powered at the time but the capacitor was internally damaged, it appeared to go open circuit.
The equipment, a Marconi CR100 receiver instantly developed a very loud mains hum which was cured by replacing the capacitor.
If I wanted to be very precise about it, I would:
1 switch off.
2 unplug from the mains.
3 measure with a multimeter.
4 short the capacitor with a 1k Ohm resistor for 10 seconds.
5 measure with a multimeter.
6 get on and do the job.
Consider, the time constant of an RC circuit is given by
t= CR
t in seconds
C in Farads
R in Ohms
A typical HT supply in a valve amplifier may be 100uF
Our discharge resistor is 1k Ohm
time constant = 100 x 10^-6 x 1 x 10^3 = 100 x 10^3 = 100mS
So if we discharge our capacitor for 1 second, that is 10 time constants, so all the charge will be gone.
But beware, discharged capacitors can recharge themselves due to a property called dielectric absorption.
When the discharge path is removed, energy "comes back out of the dielectric" and the voltage across the capacitor terminals rises a bit.
JimB