John Robertson wrote:





** Total stupidity.

Scope CRTs (unlike picture tubes) never become magnetised,
and even if one did somehow it would deflect the trace not de-focus it,





However the surrounding metal could possibly get magnetized. While I
agree it is very unlikely, it can't hurt to run a demagnetizer over the
setup.


** A demagnetiser for tools is not likely to work on a steel box.

In any case the earth's magnetic field deflects the trace on a CRT
screen - unless the CRT is fully mu-metal shielded.

That is why most decent scopes have a "trace rotate" control that applies a correcting field.


I can't see it affecting the focus though, that does sound more like a
voltage issue, a stray magnetic field would simply deflect the beam to
some extent, but it would not make it fuzzy.


** Correct.

On our old B&W picture tube monitors when the HV at the anode (does this
'scope have an anode connection?) drops the image gets fuzzy and the
image blooms in all directions...


** Yep, the spot on the face of the picture tube de-focusses and the ( magnetic ) deflection becomes excessive if even some EHT is lost.

The simple test for a weak HV rectifier valve was to turn up the brightness, thereby increasing electron beam current and hence loading the EHT supply.

OTOH colour sets and monitors usually had regulated EHT supplies.



..... Phil