VT101 screen adjustment

Brent Hilpert hilpert at cs.ubc.ca
Mon May 9 21:17:39 CDT 2016

On 2016-May-09, at 5:20 PM, Tapley, Mark wrote:
> On May 9, 2016, at 4:21 PM, Mouse <mouse at Rodents-Montreal.ORG> wrote:
>> I would tend do something like inserting a 100K or
>> 1M resistor in the path to ground. ...
>> If I'm off base, I'm sure someone will correct me!
> 	I strongly second Mouse’s recommendation (and to you too, Adrian). Suddenly placing a large charge anyplace on your ground plane (the pipe is as good a choice as I can think of, but ….) could invert diodes, back-bias transistors, etc. etc. and it’s just so easy to avoid by having a large high-voltage resistor in series with your clip. No loud snap, no pyrotechnics, your hand won’t jump in involuntary reaction to the loud snap - it’s just a much more controlled and gentle effect on the system. 
> 	I have not done it myself (either way) but have seen it done, and it works well. Resistors are really cheap insurance in this context.

Resistors to limit the initial discharge current are a good practice for all those reasons, I would additionally suggest the place to put the 'ground' clip is on the ground straps over the CRT bell, or the ground wire coming from those straps, so the discharge currents are localised to the CRT.

The CRT doesn't discharge to 'earth' or 'ground' (earth/ground is not some inherent vast expanse of charge sink), it discharges 'to' the outside of the bell. The bell forms a capacitor: one plate is a coating on the inside, one plate a coating on the outside, and the glass the dielectric.
(Pedantically, at discharge, electrons flow from the outside plate of the bell to the inside plate.)

Another good practice is to use several carbon-composition resistors in series, to increase the physical path length to avoid arcing across/within resistor(s). 
1 or 2W resistors are all the better, not for power-handling but for the greater distance between the leads.
Common film resistors with an internal spiral resistive element present a very short path for arcs to jump over.

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