Re: Danger ! Risk of Life !

From: Stuart Bell <stuartsmacs_at_email.domain.hidden>
Date: Wed, 31 Aug 2005 07:56:31 +0100

On 31 Aug 2005, at 07:16, macmoni wrote:

> or carry your Lisa to a TV-repair store or somebody who is an
> HV-expert and knows which parts are high-voltage !
> The tube is powered by voltages from 13.000 up to 17.000 Volts for
> acceleration the electrons emitted by the inside orange glowing wire
> near the connector at the back.
> TVs (with a tube) use up to 21.000 Volts. So: fingers away from
> running HV-electronics... and wait min. 5 minutes after plugging the
> power plug, before you intend to put your fingers near HV-electronic
> components inside.
> The Lisa List cannot afford to loose members being killed ... by a
> Lisa !

This issue used to come up repeatedly on the Compacts list; here's the considered response of someone who should know what he's talking about:

For the record:

  1. Don't discharge the crt unless you really have to disconnect the HV lead for some reason (like swapping out the crt or analog board). For a detailed description of why you have little to fear from a compact mac's crt, see
  2. If you do have to discharge the crt, don't go to the trouble of building the resistor/alligator clip device. Unless your resistor is rated for HV operation, the HV will simply jump across it. This so-called "arc-over voltage" is only a few hundred volts for most resistors, so going to the trouble of including it makes no sense. To do it right, you'd need to string dozens of them in series, and carefully arrange them, say, in a helix around an insulator (this is in fact how high voltage resistors are constructed). It's more trouble than it's worth. Just make sure that the discharge is dumped well away from the logic board, or else you risk frying something there (another reason for not routinely discharging the crt just for the heck of it). My favorite method is to just to let it sit unplugged overnight. What little charge might remain will truly be inconsequential.
Prof. Thomas H. Lee
Center for Integrated Systems, CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
650-725-3709 voice, -3383 fax



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Received on 2005-08-31 08:28:01

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