Re: Atari B&W: no HV

Alrighty, you all knew I couldn't resist to nitpick this!

The cathode end of the diode is connected to the CRT, which IS the
filter capacitor.

I don't personally see a problem with directly discharging the CRT, but
John seems to think there is a link, so let's assume this somehow DOES
damage the diode. How could that happen?

The diode is actually quite a few diodes in series. Each one has
capacitance (not much, but it is there). Under normal operation the
stack is switched from conduction to reverse in a controlled fashion.
This allows the reverse voltage time to evenly distribute across each
diode in the stack. That way nothing gets broken down (avalanched) and
the stack is protected.

But, even with the power off, let's say you short the CRT anode to
ground. You have an EXTREMELY fast and uncontrolled change of voltage
(technically called dv/dt). One or more of the junctions could be
avalanched. Not a good thing and could cause that part of the stack to
short. That would then reduce to breakdown voltage of the entire stack,
which ups the possibility of failure of the entire stack.

(Of course this is IF John is right about discharging the CRT causing
diode stack failures).

In any case, if you are working on games in the first place you should
buy yourself a high voltage probe, if for no other reason than to make
sure high voltage supplies on color games are set correctly. Once you
have a probe, it makes a very handy way to discharge a CRT without more
than a slight crackle. And it also shows you exactly HOW discharged the
tube is.

Mark Shostak wrote:
>
> > >I'm curious how shorting the anode to the chassis will damage the HV
> rectifier.
> > >(Assuming the unit is powered down at the time of the short).
> >
> > I haven't looked into the HV section of the B&W monitors, but if there is
> a HV
> > filter capacitor, of any type, attached to the cathode side of the diode,
> then
> > discharging the CRT will also discharge the filter capacitor through the
> diode.
>
> Ok, to qualify the question further; assuming:
> A) The unit is powered down
> B) There are no filters on the cathode side of the HV rectifier*
>
> * - I can't recall ever seeing a filter on the cathode side of the HV
> rectifier
> on any of the B&W vector monitors or any of the golden age B&W rasters.
>
> In fact, I was under the impression that the second anode filter was the
> CRT's
> carbon coating, itself.
>
> Roger, any thoughts?
>
> -Mark
>
> <snip>
>
> >
> > -Zonn
> >
> > > ---->>>Remember: NEVER, EVER EVER SHORT A B&W HV ANODE TO THE
> CHASSIS!!!!!!!!!!!<<<----
> > >
> > >
> > > John :-#)#
> > >
>
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Received on Fri Dec 7 02:00:07 2001