checking filter cap

[mark enger]

ive got a old pa i want to tear it down im worried about shock i checked filter cap to pin 7 of preamp got a 0 reading it ran down to zero i turned this thing on about a month a go for five miutes i double checked and tis thing has voltage i should probly have a tech discharge it

[selloutrr]

if you aren't comfortable discharging them yourself a tech is the safest way. If you have a solid respect and understanding of electricity you can use a bleeder resistor or an insulated screw driver and short the + side of the cap to ground. If you have any doubt take it to a tech!

[C Moore]

I am having a little trouble understanding your question. Are you saying that you would like to do some work on a PA amp, but the caps are still charged and you do not know how to discharge them? If so, you can easily make or buy a discharge wand from Ted Weber. How will you be able to work on your amp in the future? You can't take it to a tech every time you want to drain the caps.

[Structo]

Don't use a screwdriver.

It is unsafe and bad on caps to do that.

Do some research online to find out how to safely discharge caps.

This one is pretty good even though it is geared towards a Hot Rod Deluxe.

http://studentweb.eku.edu/justin_holton/caps.html

I usually use a lead that has insulated alligator clips at each end.

I clip one end to the bare chassis and clip the other end to pin 1 or 6 on a preamp tube.
This is on the 12**7 tubes.

This will drain the caps through the plate resistors.

But always check the voltage on the caps before reaching in and touching anything.
Also the caps can re-charge by themselves, usually not to a lethal level but enough to let you know it.
So I just leave the resistor bleeder in place while working inside.

Some amps have bleeder resistors in the power supply so that when the amp is turned off the caps will bleed down.
But you can never assume the amp has these without measuring the voltage on the filter caps.

[mark enger]

thanks for the replys. i read in a book about the bleeder resist it worked fine i like the idea of a wand you mean like a 1/4' copper wire with a thick insulated rubber handle and it will arce out the cap? a guy at work in repairs and maint said to run a - wire from my wood work bench in my basement outside to a couple of copper spikes in the ground. should i put rubber feet on the bench legs also?

[Structo]

no a discharge wand has a resistor with a lead and insulated alligator clip on it.

You clip the alligator clip to a bare spot (ground) on the chassis and touch the tip of the resistor to the cap.

But a better way would be to have alligator clips at each end of the resistor and then insulate the resistor and solder joints on it with heat shrink tubing.

But the way I described using the preamp plate resistor is a time tested method.

Never just short the cap to ground. It will arc and posibly ruin the cap, take a chunk out of the chassis and possibly throw slag into your eyes.


You are kind of worrying me.
Maybe you should take the amp to a tech because unless you understand what you are doing, you may end up electrocuting yourself.