[sdiy] Integral scope on VCO

[harrybissell]

Hi Thomas (et al)

>  You really think so? Anything going on in the VCO is at ultra low
> impedances compared to the CRT and the deflection amps. In any case
> there's not much of a magnetic field to worry about (I'd worry more about
> the fields from the HV power supply transformer affecting the CRT...)

That's what I'd worry about... as well as magnetic fields interefeing with the
scope.

I would not call VCO "ultra low impedance" with the usual comparators for the
reset...
they can be very touchy. Unless the HV supply was very clean they could
interfere...and the
horizontal sweep and RESET of the scope trace will have the harmonic content from
hell...
(soft sync etc ???)

>
> and the CRT can be shielded easily from the electrostatic fields.
>  Even in precision oscilloscopes there's always a mumetal shield around
> the CRT - to keep magnetic fields in the rest of the instrument from
> distorting the beam deflection in the CRT.

Unless your tube HAS a Mu-metal shield you will find that solution VERY
expensive, and probably still desirable. Forming such a shield yourself will
prlly require a heat treat (anneal) process after the bending.  DIY induction
heating furnace anyone ???

>  I'll probably wire a prototype for the basic scope PSU and drivers (and
> the CRT itself) anyway and see if I can provoke it to interfere with the VCO
> when brought into close proximity. That will certainly settle the question...

That's an excellent idea, I bet you'd do that anyway :^P

So I'll tell you a funny story about an instrument I built.  It had a cooling fan
package located
at the top, above a monitor.  As soon as the fans started, the magnetic fields
fluttered the monitor.  No problem, I just moved the fans to the bottom instead.

Imagine my surprise when I started the chemical process... including a magnetic
stirrer in the
back.  If a fan rotating at some multiple of 60hz makes a monitor
flicker...imagine what a
HORSESHOE MAGNET does rotating at low rpms right behind the monitor. YIKES

Moving the monitor to the bottom was not a solution (program on your knees?)...
and the
chemical equipment was fixed. The best solution was to make a steel box around
the whole
chemical section... even in 12ga steel the attenuation was barely acceptable

Goes to show... it don't work until after you make it work.

Maybe the scope project will go easy.  It could y'know...

>  What kind of interference scenario do you picture anyway.

Horrible, unimaginable interference.  The kind that could derail your
most carefully laid plans :^P

H^) harry