Re: Atari B&W: no HV

On Thu, 6 Dec 2001 16:56:13 -0600, "Mark Shostak" <shostak@augustmail.com>
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.

And since it's a direct short to ground, the current spike going through the
diode can be quite high.

Voltage doublers/triplers, etc. are pretty common in HV sections and use
capacitors which can damage the diode upon discharging.

10meg is not a bad number to use, since we're talking about voltages that can be
around 15,000v. And stringing a bunch of resistors together is a good idea since
15kv can easily arc around (or through) a single resistor. And there is also
wattage to be concerned with:

Ohms law:

 15,000v / 10,000,000 = 1.5 ma

Resistor dissipation wattage:

 15,000v * .0015 amps = 22.5 watts.

By stringing 10 1/2 watt resistors together (length wise, don't bunch them up)
you can consider it a 5 watt resistor. Which should be fine, since it will only
be 15,000v for a very short time. Still, each resistor sees 1500v, which is
still pretty high, but heck it's not like were designing this into a production
unit, we just want to "unzap" a CRT now and then.

Wrap the resistors in heat shrink, then wrap that in heat shrink, then wrap that
in heat shrink, etc. Or place the resistors in a small PVC pipe and fill it with
silicon sealant, etc. And even then I wouldn't hold 'em, I'd attach one side to
ground and the other side to a clip lead that is clipped onto a well insulated
screwdriver, and use that to stick under the CRT's cap.

BTW: Designs like the color WG monitors, which place the focus voltage divider
across the output of the HV (IIRC), and other designs which have purposely
placed a bleeder resister across their HV don't need discharging... well,
assuming there's nothing wrong with the HV section. Of course if that were the
case you wouldn't be messing with it right?

I discharge everything (I use an HV probe), I *hate* getting zapped!

-Zonn

> ---->>>Remember: NEVER, EVER EVER SHORT A B&W HV ANODE TO THE CHASSIS!!!!!!!!!!!<<<----
>
>You must use a HV meter or a series of 1 meg resistors (total 10Meg insulate
>the suckers!) to discharge the HV anode on the side of the tube. This is what
>is killing the HV diodes, getting the anode shorted by a jumper wire to the
>chassis common...
>
> John :-#)#
>

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Received on Thu Dec 6 17:58:47 2001