CRT decay - test-drb@ccmp.vtda.org

2 Nov 1998

-----Original Message-----
From: Allison J Parent &lt;allisonp(a)world.std.com&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Tuesday, 3 November 1998 15:59
Subject: Re: CRT decay

...
 < Don't CRT filaments degrade with use? 
I've seen this on other
< workstation monitors from Sun and HP, typically ones that were used as

It the common characteristic of all valves (tubes) with heated cathodes
that over time emission will decay and gain(brightness) goes away with
it.

< big-ticket item, the local TV repairman had cylindrical devices called
< "boosters" that could be inserted in the circuit between the plug on
< the back of the CRT and the set's CRT socket to extend the life of a

It got back some emission for a while but the overvoltage generally
killed the tube. 

There is another method that is frequently employed on "tired' tubes.
It involves putting and exceptionally high pulsed + potential on the 1st
Grid of the tube,
whilst simultaneously running the heater at about 30% overvoltage.  This had
the effect
of blasting  layers of tired cathodic material off the cathode, exposing
less depleted stuff
underneath it.  Depending on the tube, this could restore performance for
days or years.
There doesn't seem to be any way to tell in advance.  I have seen both
extremes.
There is a very old Philips K9 chassis TV dating from about 1976 that had
this done some
3 or 4 years ago when it got too dark to watch, and cranking up the beam
current caused
flaring.   The tube came right up to very bright and sharp, and is still
working very well.
A similar set required the procedure to be repeated at progressively more
frequent intervals,
til the owner (and tech) got sick of it and declared it dead.
Not sure what the situation is in USA but here TV crts are generally not
replaced because
of the cost, it's usually better to get another TV.

Cheers
Geoff Roberts
Computer Systems Manager
Saint Marks College
Port Pirie  South Australia.
My ICQ# is 1970476
Ph. 61-411-623-978 (Mobile)
      61-8-8633-0619 (Home)
      61-8-8633-8834 (Work-Direct)
      61-8-8633-0104 (Fax)