Discharging CRT anode and capacitors on a VT52 DECscope

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu May 27 00:03:20 2004

>
> I recently acquired two VT52 DECScopes and I am
> ready to begin the troubleshooting process. I've
> never attempted to troubleshoot a CRT device
> before, and I don't want to get blasted across
> the room if I do something wrong. Has anyone here
> worked on these types of terminals before? I'm

Yes...


The power supply capacitors are the big electrolytics on the board across
the back. Nonoe of them charge to that high a voltage, although I think
there are some that charge to 10's of volts. I would discharge them by
connecting a 10k (or so) resistor across each one in turn (I think they
have screw terminals, which makes it easy to work out where to connect
the resistor.

To discharge the CRT final anode, I use my EHT probe. This contains an
800M high-voltage resistor. The ground lead goes on the CRT earthing
braid (across the back of the CRT flare), then you stick the probe under
the EHT connector on the side of the CRT and watch the reading on the
voltmeter decay.

> finding conflicting info on the proper method of
> discharging the CRT anode. I have turned both
> of them on. One lights up with random patterns,
> no cursor, and a clicking sound. The VT52 maintenance
> manual indicates that this is likely a problem with
> the anode clip or anode cap. I'd like to move on

This sounds much more like a logic problem to me.

> to the next step after I make sure that both the
> clip and cap are secure.
>
> Can an "expert" here provide some proper directions
> on how to discharge the anode and capacitors? How
> long do these things on a charge?

For the PSU electrolytics, not that long (hours, at most). For the CRT, a
long time (days or weeks!).

-tony
Received on Thu May 27 2004 - 00:03:20 BST