Zapping Oscilloscope Phosphor

[Mike I.]

Thought I'd ask as probably anyone with an old-fashioned, non-LCD scope encounters this sooner or later...

Typical situation: beam is moderate-brightness and well-focussed on CRT giving a narrow trace for lots of resolution - could be running in x-y mode or normal swept mode. As long as the beam is moving there is no danger of burning the CRT phosphor. Now a test lead(s) pops off, or the "equipment under test" is turned off, or an LFO is turned off, or the sweep is accidentally turned off, and the next time you look, there is a brilliant focussed motionless spot sitting there on scope screen, trying to burn itself into the phosphor... now being alert helps, un-focussing the beam between measurements can help avoid this problem, but sooner or later it happens - any "tried & true" methods to avoid this (aside from turning scope off)?
Regards, Mike

[Rob Strand]

The problem is mainly on X-Y mode, with Y-T mode you at least get a line.  If there's any danger of a spot while I'm using it I usually just dial the brightness down to a "safe" level before I start.

Other than that, I just make sure I don't walk away from it in X-Y mode, and if I do, I turn down the brightness or switch to Y-T mode, or both.  It's pretty much built into me now not to leave a trace or spot on for too long.

[Mike I.]

Thanks Rob...hmmmm... maybe what is needed is to turn the Scope into a Stompbox - use a momentary-contact footswitch to enable the beam (perhaps using the z-input) when you are about to make the measurement - take foot off when measurements are done, turning off beam. BTW this problem mostly occurs when I'm checking quadrature oscillators or using Lissajous figures in x-y mode.

I have seen some CRT's withstand a minute or two of having a focussed brilliant spot on the screen, but don't want to take any chances here.
Regards, Mike

[Rob Strand]

I'm not sure if the Z-input will work, a lot of Z-inputs need a pulse (IIRC they increase the brightness as well - might depend on the CRO).  The stomp box switch is a good idea, perhaps switch between the test input and a dummy input signal.