TRS-80 Model 12 crt substitution

From: Pete Turnbull <>
Date: Thu Mar 4 12:23:29 2004

On Mar 4, 9:34, Witchy wrote:

> > the CRT can hold a charge for a lot longer than 'overnight'.
> Ah, I didn't know that - I thought they discharged naturally.....
> > and it may cause you to drop the CRT. I alwayse discharge a
> > CRT with my
> > EHT probe before handling it.

I'll second that.

Not only do CRTs not discharge naturally (unless they're in circuit
with a bleed resistor), a disconnected and supposedly-discharged CRT
may develop a charge if left alone. The original EHT can force
electrons into the glass, and since glass is an excellent insulator,
they don't come out again immediately when you discharge the tube.
 They *may* come to the surface slowly later, and give the CRT a
significant charge of opposite to the original polarity. So leave a
bleed resistor across a tube you plan to store for a long period (tip
given to me by a TV repairman).

If you're discharging a recently-used tube, don't use a simple
resistor. The EHT on a CRT could be anything from 8kV to several tens
of kV in a fancy colour tube, and most common resistors have a
breakdown voltage of only 500-1000V. Besides, think about the power
dissipation if you discharge a 25kV tube through a 10M resistor: 2.5
milliamps, but 62 watts. Sure, that 62W won't last long, but neither
the tube nor your resistor will like it!

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Thu Mar 04 2004 - 12:23:29 GMT

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