H89 lives - but for how long?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Jan 14 16:44:18 2004

> Here's my question - after the machine was running for a minute or so,
> the screen got very bright - the "black" background became green and the
> screen characters were so over-driven that they became "fuzzy". The
> brightness control worked (i.e. changed brightness levels) but the
> available range only went from "too bright" to "really too bright".
> After another minute or so, the brightness level gradually subsided back
> to normal and everything seemed ok for the next 20 minutes of operation.
> What caused the "glow" and what incipient failure does it suggest?

Sounds like either you've got too low a voltage on the CRT cathode, or
too high a voltage on the first anode (I don't suspect the EHT,
fortunately, because that would make the picture change size).

What I would do at this point (I assume you have the schematics that came
with the machine) is measure the CRT base pin voltages (none is higher
than 1kV, so you can use a normal multimeter with care). Then, if the
fault comes back, quickly measure them again, and see what's changed.


-tony
Received on Wed Jan 14 2004 - 16:44:18 GMT