Re: Atari B&W: no HV

I have a write-up on this problem on a page
http://www.flippers.com/vid-tips.html#BWXY resulting from a discussion from
an electrical engineer. This would explain the common failure of these HV
diodes....

John :-#)#

At 01:43 AM 12/07/01, you wrote:
>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 :-#)#
> > > >
> >
> > ---------------------------------------------------------------------------
> > ** To UNSUBSCRIBE from vectorlist, send a message with "UNSUBSCRIBE" in the
> > ** message body to vectorlist-request@synthcom.com. Please direct other
> > ** questions, comments, or problems to neil@synthcom.com.
>
>---------------------------------------------------------------------------
>** To UNSUBSCRIBE from vectorlist, send a message with "UNSUBSCRIBE" in the
>** message body to vectorlist-request@synthcom.com. Please direct other
>** questions, comments, or problems to neil@synthcom.com.

---------------------------------------------------------------------------
** To UNSUBSCRIBE from vectorlist, send a message with "UNSUBSCRIBE" in the
** message body to vectorlist-request@synthcom.com. Please direct other
** questions, comments, or problems to neil@synthcom.com.
Received on Fri Dec 7 10:35:49 2001