Discharging CRT anode and capacitors on a VT52 DECscope

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu May 27 18:05:16 2004

> John> You may or may not hear a snap, or see a small blue flash when
> John> you get the tip of the screwdriver close enough to the anode
> John> connection - it depends on how well the CRT in the terminal is
> John> retaining it's charge.
>

Please don't do it this way -- without some kind of limiting resistor.
When the screwdriver makes contact a high current will flow which (a) can
damage the CRT (it can burn the connection between the internal final
anode coating and the connector socket) an (b) can induce currents in
other parts of the instrument and blow semiconductors all over the place.

It's proablt OK to discharge a CRT this way if you're going to throw it
out anyway, and if it's removed from the device, but even then I'd not
risk it.

Use a limiting resistor!

> One small point:



>
> In this process, be sure you keep your fingertips WELL away from the
> metal shaft of the screwdriver. The most natural position for the

Considering the shaft is connected to ground through a very low
resistance in your set-up, what's the harm in touching it ?

> fingers is close to the metal, occasionally slipping right onto it.
> That's NOT where you want to be when you're poking around high
> voltages.
>
> There are special "safety" screwdrivers (designed/approved for work in
> live mains power systems). Those have shafts that are insulated right
> up to the tip.

And are often only rated to 1000V. That's enough for working on live
mains (which is where I use them), but CRT anodes run at 10-25 times that
voltage.

>
> You could give yourself a little extra safety by wrapping a few layers
> of electrician's tape around the top 2-3 inches of the screwdriver

I am not sure what the breakdown voltage of insulting tape is (yes, I
always call it that!), but it's not that high. I doubt that a layer would
stand 10kV (which is what you're asking for here, at least on a colour CRT).

-tony
Received on Thu May 27 2004 - 18:05:16 BST